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<!-- doc/src/sgml/config.sgml -->
<chapter id="runtime-config">
<title>Server Configuration</title>
<indexterm>
<primary>configuration</primary>
<secondary>of the server</secondary>
</indexterm>
<para>
There are many configuration parameters that affect the behavior of
the database system. In the first section of this chapter we
describe how to interact with configuration parameters. The subsequent sections
discuss each parameter in detail.
</para>
<sect1 id="config-setting">
<title>Setting Parameters</title>
<sect2 id="config-setting-names-values">
<title>Parameter Names and Values</title>
<para>
All parameter names are case-insensitive. Every parameter takes a
value of one of five types: boolean, string, integer, floating point,
or enumerated (enum). The type determines the syntax for setting the
parameter:
</para>
<itemizedlist>
<listitem>
<para>
<emphasis>Boolean:</emphasis>
Values can be written as
<literal>on</literal>,
<literal>off</literal>,
<literal>true</literal>,
<literal>false</literal>,
<literal>yes</literal>,
<literal>no</literal>,
<literal>1</literal>,
<literal>0</literal>
(all case-insensitive) or any unambiguous prefix of one of these.
</para>
</listitem>
<listitem>
<para>
<emphasis>String:</emphasis>
In general, enclose the value in single quotes, doubling any single
quotes within the value. Quotes can usually be omitted if the value
is a simple number or identifier, however.
(Values that match a SQL keyword require quoting in some contexts.)
</para>
</listitem>
<listitem>
<para>
<emphasis>Numeric (integer and floating point):</emphasis>
Numeric parameters can be specified in the customary integer and
floating-point formats; fractional values are rounded to the nearest
integer if the parameter is of integer type. Integer parameters
additionally accept hexadecimal input (beginning
with <literal>0x</literal>) and octal input (beginning
with <literal>0</literal>), but these formats cannot have a fraction.
Do not use thousands separators.
Quotes are not required, except for hexadecimal input.
</para>
</listitem>
<listitem>
<para>
<emphasis>Numeric with Unit:</emphasis>
Some numeric parameters have an implicit unit, because they describe
quantities of memory or time. The unit might be bytes, kilobytes, blocks
(typically eight kilobytes), milliseconds, seconds, or minutes.
An unadorned numeric value for one of these settings will use the
setting's default unit, which can be learned from
<structname>pg_settings</structname>.<structfield>unit</structfield>.
For convenience, settings can be given with a unit specified explicitly,
for example <literal>'120 ms'</literal> for a time value, and they will be
converted to whatever the parameter's actual unit is. Note that the
value must be written as a string (with quotes) to use this feature.
The unit name is case-sensitive, and there can be whitespace between
the numeric value and the unit.
<itemizedlist>
<listitem>
<para>
Valid memory units are <literal>B</literal> (bytes),
<literal>kB</literal> (kilobytes),
<literal>MB</literal> (megabytes), <literal>GB</literal>
(gigabytes), and <literal>TB</literal> (terabytes).
The multiplier for memory units is 1024, not 1000.
</para>
</listitem>
<listitem>
<para>
Valid time units are
<literal>us</literal> (microseconds),
<literal>ms</literal> (milliseconds),
<literal>s</literal> (seconds), <literal>min</literal> (minutes),
<literal>h</literal> (hours), and <literal>d</literal> (days).
</para>
</listitem>
</itemizedlist>
If a fractional value is specified with a unit, it will be rounded
to a multiple of the next smaller unit if there is one.
For example, <literal>30.1 GB</literal> will be converted
to <literal>30822 MB</literal> not <literal>32319628902 B</literal>.
If the parameter is of integer type, a final rounding to integer
occurs after any units conversion.
</para>
</listitem>
<listitem>
<para>
<emphasis>Enumerated:</emphasis>
Enumerated-type parameters are written in the same way as string
parameters, but are restricted to have one of a limited set of
values. The values allowable for such a parameter can be found from
<structname>pg_settings</structname>.<structfield>enumvals</structfield>.
Enum parameter values are case-insensitive.
</para>
</listitem>
</itemizedlist>
</sect2>
<sect2 id="config-setting-configuration-file">
<title>Parameter Interaction via the Configuration File</title>
<para>
The most fundamental way to set these parameters is to edit the file
<filename>postgresql.conf</filename><indexterm><primary>postgresql.conf</primary></indexterm>,
which is normally kept in the data directory. A default copy is
installed when the database cluster directory is initialized.
An example of what this file might look like is:
<programlisting>
# This is a comment
log_connections = yes
log_destination = 'syslog'
search_path = '"$user", public'
shared_buffers = 128MB
</programlisting>
One parameter is specified per line. The equal sign between name and
value is optional. Whitespace is insignificant (except within a quoted
parameter value) and blank lines are
ignored. Hash marks (<literal>#</literal>) designate the remainder
of the line as a comment. Parameter values that are not simple
identifiers or numbers must be single-quoted. To embed a single
quote in a parameter value, write either two quotes (preferred)
or backslash-quote.
If the file contains multiple entries for the same parameter,
all but the last one are ignored.
</para>
<para>
Parameters set in this way provide default values for the cluster.
The settings seen by active sessions will be these values unless they
are overridden. The following sections describe ways in which the
administrator or user can override these defaults.
</para>
<para>
<indexterm>
<primary>SIGHUP</primary>
</indexterm>
The configuration file is reread whenever the main server process
receives a <systemitem>SIGHUP</systemitem> signal; this signal is most easily
sent by running <literal>pg_ctl reload</literal> from the command line or by
calling the SQL function <function>pg_reload_conf()</function>. The main
server process also propagates this signal to all currently running
server processes, so that existing sessions also adopt the new values
(this will happen after they complete any currently-executing client
command). Alternatively, you can
send the signal to a single server process directly. Some parameters
can only be set at server start; any changes to their entries in the
configuration file will be ignored until the server is restarted.
Invalid parameter settings in the configuration file are likewise
ignored (but logged) during <systemitem>SIGHUP</systemitem> processing.
</para>
<para>
In addition to <filename>postgresql.conf</filename>,
a <productname>PostgreSQL</productname> data directory contains a file
<filename>postgresql.auto.conf</filename><indexterm><primary>postgresql.auto.conf</primary></indexterm>,
which has the same format as <filename>postgresql.conf</filename> but
is intended to be edited automatically, not manually. This file holds
settings provided through the <xref linkend="sql-altersystem"/> command.
This file is read whenever <filename>postgresql.conf</filename> is,
and its settings take effect in the same way. Settings
in <filename>postgresql.auto.conf</filename> override those
in <filename>postgresql.conf</filename>.
</para>
<para>
External tools may also
modify <filename>postgresql.auto.conf</filename>. It is not
recommended to do this while the server is running, since a
concurrent <command>ALTER SYSTEM</command> command could overwrite
such changes. Such tools might simply append new settings to the end,
or they might choose to remove duplicate settings and/or comments
(as <command>ALTER SYSTEM</command> will).
</para>
<para>
The system view
<link linkend="view-pg-file-settings"><structname>pg_file_settings</structname></link>
can be helpful for pre-testing changes to the configuration files, or for
diagnosing problems if a <systemitem>SIGHUP</systemitem> signal did not have the
desired effects.
</para>
</sect2>
<sect2 id="config-setting-sql-command-interaction">
<title>Parameter Interaction via SQL</title>
<para>
<productname>PostgreSQL</productname> provides three SQL
commands to establish configuration defaults.
The already-mentioned <xref linkend="sql-altersystem"/> command
provides a SQL-accessible means of changing global defaults; it is
functionally equivalent to editing <filename>postgresql.conf</filename>.
In addition, there are two commands that allow setting of defaults
on a per-database or per-role basis:
</para>
<itemizedlist>
<listitem>
<para>
The <xref linkend="sql-alterdatabase"/> command allows global
settings to be overridden on a per-database basis.
</para>
</listitem>
<listitem>
<para>
The <xref linkend="sql-alterrole"/> command allows both global and
per-database settings to be overridden with user-specific values.
</para>
</listitem>
</itemizedlist>
<para>
Values set with <command>ALTER DATABASE</command> and <command>ALTER ROLE</command>
are applied only when starting a fresh database session. They
override values obtained from the configuration files or server
command line, and constitute defaults for the rest of the session.
Note that some settings cannot be changed after server start, and
so cannot be set with these commands (or the ones listed below).
</para>
<para>
Once a client is connected to the database, <productname>PostgreSQL</productname>
provides two additional SQL commands (and equivalent functions) to
interact with session-local configuration settings:
</para>
<itemizedlist>
<listitem>
<para>
The <xref linkend="sql-show"/> command allows inspection of the
current value of all parameters. The corresponding function is
<function>current_setting(setting_name text)</function>.
</para>
</listitem>
<listitem>
<para>
The <xref linkend="sql-set"/> command allows modification of the
current value of those parameters that can be set locally to a
session; it has no effect on other sessions.
The corresponding function is
<function>set_config(setting_name, new_value, is_local)</function>.
</para>
</listitem>
</itemizedlist>
<para>
In addition, the system view <link
linkend="view-pg-settings"><structname>pg_settings</structname></link> can be
used to view and change session-local values:
</para>
<itemizedlist>
<listitem>
<para>
Querying this view is similar to using <command>SHOW ALL</command> but
provides more detail. It is also more flexible, since it's possible
to specify filter conditions or join against other relations.
</para>
</listitem>
<listitem>
<para>
Using <xref linkend="sql-update"/> on this view, specifically
updating the <structname>setting</structname> column, is the equivalent
of issuing <command>SET</command> commands. For example, the equivalent of
<programlisting>
SET configuration_parameter TO DEFAULT;
</programlisting>
is:
<programlisting>
UPDATE pg_settings SET setting = reset_val WHERE name = 'configuration_parameter';
</programlisting>
</para>
</listitem>
</itemizedlist>
</sect2>
<sect2>
<title>Parameter Interaction via the Shell</title>
<para>
In addition to setting global defaults or attaching
overrides at the database or role level, you can pass settings to
<productname>PostgreSQL</productname> via shell facilities.
Both the server and <application>libpq</application> client library
accept parameter values via the shell.
</para>
<itemizedlist>
<listitem>
<para>
During server startup, parameter settings can be
passed to the <command>postgres</command> command via the
<option>-c</option> command-line parameter. For example,
<programlisting>
postgres -c log_connections=yes -c log_destination='syslog'
</programlisting>
Settings provided in this way override those set via
<filename>postgresql.conf</filename> or <command>ALTER SYSTEM</command>,
so they cannot be changed globally without restarting the server.
</para>
</listitem>
<listitem>
<para>
When starting a client session via <application>libpq</application>,
parameter settings can be
specified using the <envar>PGOPTIONS</envar> environment variable.
Settings established in this way constitute defaults for the life
of the session, but do not affect other sessions.
For historical reasons, the format of <envar>PGOPTIONS</envar> is
similar to that used when launching the <command>postgres</command>
command; specifically, the <option>-c</option> flag must be specified.
For example,
<programlisting>
env PGOPTIONS="-c geqo=off -c statement_timeout=5min" psql
</programlisting>
</para>
<para>
Other clients and libraries might provide their own mechanisms,
via the shell or otherwise, that allow the user to alter session
settings without direct use of SQL commands.
</para>
</listitem>
</itemizedlist>
</sect2>
<sect2 id="config-includes">
<title>Managing Configuration File Contents</title>
<para>
<productname>PostgreSQL</productname> provides several features for breaking
down complex <filename>postgresql.conf</filename> files into sub-files.
These features are especially useful when managing multiple servers
with related, but not identical, configurations.
</para>
<para>
<indexterm>
<primary><literal>include</literal></primary>
<secondary>in configuration file</secondary>
</indexterm>
In addition to individual parameter settings,
the <filename>postgresql.conf</filename> file can contain <firstterm>include
directives</firstterm>, which specify another file to read and process as if
it were inserted into the configuration file at this point. This
feature allows a configuration file to be divided into physically
separate parts. Include directives simply look like:
<programlisting>
include 'filename'
</programlisting>
If the file name is not an absolute path, it is taken as relative to
the directory containing the referencing configuration file.
Inclusions can be nested.
</para>
<para>
<indexterm>
<primary><literal>include_if_exists</literal></primary>
<secondary>in configuration file</secondary>
</indexterm>
There is also an <literal>include_if_exists</literal> directive, which acts
the same as the <literal>include</literal> directive, except
when the referenced file does not exist or cannot be read. A regular
<literal>include</literal> will consider this an error condition, but
<literal>include_if_exists</literal> merely logs a message and continues
processing the referencing configuration file.
</para>
<para>
<indexterm>
<primary><literal>include_dir</literal></primary>
<secondary>in configuration file</secondary>
</indexterm>
The <filename>postgresql.conf</filename> file can also contain
<literal>include_dir</literal> directives, which specify an entire
directory of configuration files to include. These look like
<programlisting>
include_dir 'directory'
</programlisting>
Non-absolute directory names are taken as relative to the directory
containing the referencing configuration file. Within the specified
directory, only non-directory files whose names end with the
suffix <literal>.conf</literal> will be included. File names that
start with the <literal>.</literal> character are also ignored, to
prevent mistakes since such files are hidden on some platforms. Multiple
files within an include directory are processed in file name order
(according to C locale rules, i.e. numbers before letters, and
uppercase letters before lowercase ones).
</para>
<para>
Include files or directories can be used to logically separate portions
of the database configuration, rather than having a single large
<filename>postgresql.conf</filename> file. Consider a company that has two
database servers, each with a different amount of memory. There are
likely elements of the configuration both will share, for things such
as logging. But memory-related parameters on the server will vary
between the two. And there might be server specific customizations,
too. One way to manage this situation is to break the custom
configuration changes for your site into three files. You could add
this to the end of your <filename>postgresql.conf</filename> file to include
them:
<programlisting>
include 'shared.conf'
include 'memory.conf'
include 'server.conf'
</programlisting>
All systems would have the same <filename>shared.conf</filename>. Each
server with a particular amount of memory could share the
same <filename>memory.conf</filename>; you might have one for all servers
with 8GB of RAM, another for those having 16GB. And
finally <filename>server.conf</filename> could have truly server-specific
configuration information in it.
</para>
<para>
Another possibility is to create a configuration file directory and
put this information into files there. For example, a <filename>conf.d</filename>
directory could be referenced at the end of <filename>postgresql.conf</filename>:
<programlisting>
include_dir 'conf.d'
</programlisting>
Then you could name the files in the <filename>conf.d</filename> directory
like this:
<programlisting>
00shared.conf
01memory.conf
02server.conf
</programlisting>
This naming convention establishes a clear order in which these
files will be loaded. This is important because only the last
setting encountered for a particular parameter while the server is
reading configuration files will be used. In this example,
something set in <filename>conf.d/02server.conf</filename> would override a
value set in <filename>conf.d/01memory.conf</filename>.
</para>
<para>
You might instead use this approach to naming the files
descriptively:
<programlisting>
00shared.conf
01memory-8GB.conf
02server-foo.conf
</programlisting>
This sort of arrangement gives a unique name for each configuration file
variation. This can help eliminate ambiguity when several servers have
their configurations all stored in one place, such as in a version
control repository. (Storing database configuration files under version
control is another good practice to consider.)
</para>
</sect2>
</sect1>
<sect1 id="runtime-config-file-locations">
<title>File Locations</title>
<para>
In addition to the <filename>postgresql.conf</filename> file
already mentioned, <productname>PostgreSQL</productname> uses
two other manually-edited configuration files, which control
client authentication (their use is discussed in <xref
linkend="client-authentication"/>). By default, all three
configuration files are stored in the database cluster's data
directory. The parameters described in this section allow the
configuration files to be placed elsewhere. (Doing so can ease
administration. In particular it is often easier to ensure that
the configuration files are properly backed-up when they are
kept separate.)
</para>
<variablelist>
<varlistentry id="guc-data-directory" xreflabel="data_directory">
<term><varname>data_directory</varname> (<type>string</type>)
<indexterm>
<primary><varname>data_directory</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the directory to use for data storage.
This parameter can only be set at server start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-config-file" xreflabel="config_file">
<term><varname>config_file</varname> (<type>string</type>)
<indexterm>
<primary><varname>config_file</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the main server configuration file
(customarily called <filename>postgresql.conf</filename>).
This parameter can only be set on the <command>postgres</command> command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-hba-file" xreflabel="hba_file">
<term><varname>hba_file</varname> (<type>string</type>)
<indexterm>
<primary><varname>hba_file</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the configuration file for host-based authentication
(customarily called <filename>pg_hba.conf</filename>).
This parameter can only be set at server start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-ident-file" xreflabel="ident_file">
<term><varname>ident_file</varname> (<type>string</type>)
<indexterm>
<primary><varname>ident_file</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the configuration file for user name mapping
(customarily called <filename>pg_ident.conf</filename>).
This parameter can only be set at server start.
See also <xref linkend="auth-username-maps"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-external-pid-file" xreflabel="external_pid_file">
<term><varname>external_pid_file</varname> (<type>string</type>)
<indexterm>
<primary><varname>external_pid_file</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the name of an additional process-ID (PID) file that the
server should create for use by server administration programs.
This parameter can only be set at server start.
</para>
</listitem>
</varlistentry>
</variablelist>
<para>
In a default installation, none of the above parameters are set
explicitly. Instead, the
data directory is specified by the <option>-D</option> command-line
option or the <envar>PGDATA</envar> environment variable, and the
configuration files are all found within the data directory.
</para>
<para>
If you wish to keep the configuration files elsewhere than the
data directory, the <command>postgres</command> <option>-D</option>
command-line option or <envar>PGDATA</envar> environment variable
must point to the directory containing the configuration files,
and the <varname>data_directory</varname> parameter must be set in
<filename>postgresql.conf</filename> (or on the command line) to show
where the data directory is actually located. Notice that
<varname>data_directory</varname> overrides <option>-D</option> and
<envar>PGDATA</envar> for the location
of the data directory, but not for the location of the configuration
files.
</para>
<para>
If you wish, you can specify the configuration file names and locations
individually using the parameters <varname>config_file</varname>,
<varname>hba_file</varname> and/or <varname>ident_file</varname>.
<varname>config_file</varname> can only be specified on the
<command>postgres</command> command line, but the others can be
set within the main configuration file. If all three parameters plus
<varname>data_directory</varname> are explicitly set, then it is not necessary
to specify <option>-D</option> or <envar>PGDATA</envar>.
</para>
<para>
When setting any of these parameters, a relative path will be interpreted
with respect to the directory in which <command>postgres</command>
is started.
</para>
</sect1>
<sect1 id="runtime-config-connection">
<title>Connections and Authentication</title>
<sect2 id="runtime-config-connection-settings">
<title>Connection Settings</title>
<variablelist>
<varlistentry id="guc-listen-addresses" xreflabel="listen_addresses">
<term><varname>listen_addresses</varname> (<type>string</type>)
<indexterm>
<primary><varname>listen_addresses</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the TCP/IP address(es) on which the server is
to listen for connections from client applications.
The value takes the form of a comma-separated list of host names
and/or numeric IP addresses. The special entry <literal>*</literal>
corresponds to all available IP interfaces. The entry
<literal>0.0.0.0</literal> allows listening for all IPv4 addresses and
<literal>::</literal> allows listening for all IPv6 addresses.
If the list is empty, the server does not listen on any IP interface
at all, in which case only Unix-domain sockets can be used to connect
to it.
The default value is <systemitem class="systemname">localhost</systemitem>,
which allows only local TCP/IP <quote>loopback</quote> connections to be
made. While client authentication (<xref
linkend="client-authentication"/>) allows fine-grained control
over who can access the server, <varname>listen_addresses</varname>
controls which interfaces accept connection attempts, which
can help prevent repeated malicious connection requests on
insecure network interfaces. This parameter can only be set
at server start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-port" xreflabel="port">
<term><varname>port</varname> (<type>integer</type>)
<indexterm>
<primary><varname>port</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The TCP port the server listens on; 5432 by default. Note that the
same port number is used for all IP addresses the server listens on.
This parameter can only be set at server start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-max-connections" xreflabel="max_connections">
<term><varname>max_connections</varname> (<type>integer</type>)
<indexterm>
<primary><varname>max_connections</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Determines the maximum number of concurrent connections to the
database server. The default is typically 100 connections, but
might be less if your kernel settings will not support it (as
determined during <application>initdb</application>). This parameter can
only be set at server start.
</para>
<para>
When running a standby server, you must set this parameter to the
same or higher value than on the master server. Otherwise, queries
will not be allowed in the standby server.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-superuser-reserved-connections"
xreflabel="superuser_reserved_connections">
<term><varname>superuser_reserved_connections</varname>
(<type>integer</type>)
<indexterm>
<primary><varname>superuser_reserved_connections</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Determines the number of connection <quote>slots</quote> that
are reserved for connections by <productname>PostgreSQL</productname>
superusers. At most <xref linkend="guc-max-connections"/>
connections can ever be active simultaneously. Whenever the
number of active concurrent connections is at least
<varname>max_connections</varname> minus
<varname>superuser_reserved_connections</varname>, new
connections will be accepted only for superusers, and no
new replication connections will be accepted.
</para>
<para>
The default value is three connections. The value must be less
than <varname>max_connections</varname>.
This parameter can only be set at server start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-unix-socket-directories" xreflabel="unix_socket_directories">
<term><varname>unix_socket_directories</varname> (<type>string</type>)
<indexterm>
<primary><varname>unix_socket_directories</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the directory of the Unix-domain socket(s) on which the
server is to listen for connections from client applications.
Multiple sockets can be created by listing multiple directories
separated by commas. Whitespace between entries is
ignored; surround a directory name with double quotes if you need
to include whitespace or commas in the name.
An empty value
specifies not listening on any Unix-domain sockets, in which case
only TCP/IP sockets can be used to connect to the server.
The default value is normally
<filename>/tmp</filename>, but that can be changed at build time.
This parameter can only be set at server start.
</para>
<para>
In addition to the socket file itself, which is named
<literal>.s.PGSQL.<replaceable>nnnn</replaceable></literal> where
<replaceable>nnnn</replaceable> is the server's port number, an ordinary file
named <literal>.s.PGSQL.<replaceable>nnnn</replaceable>.lock</literal> will be
created in each of the <varname>unix_socket_directories</varname> directories.
Neither file should ever be removed manually.
</para>
<para>
This parameter is irrelevant on Windows, which does not have
Unix-domain sockets.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-unix-socket-group" xreflabel="unix_socket_group">
<term><varname>unix_socket_group</varname> (<type>string</type>)
<indexterm>
<primary><varname>unix_socket_group</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the owning group of the Unix-domain socket(s). (The owning
user of the sockets is always the user that starts the
server.) In combination with the parameter
<varname>unix_socket_permissions</varname> this can be used as
an additional access control mechanism for Unix-domain connections.
By default this is the empty string, which uses the default
group of the server user. This parameter can only be set at
server start.
</para>
<para>
This parameter is irrelevant on Windows, which does not have
Unix-domain sockets.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-unix-socket-permissions" xreflabel="unix_socket_permissions">
<term><varname>unix_socket_permissions</varname> (<type>integer</type>)
<indexterm>
<primary><varname>unix_socket_permissions</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the access permissions of the Unix-domain socket(s). Unix-domain
sockets use the usual Unix file system permission set.
The parameter value is expected to be a numeric mode
specified in the format accepted by the
<function>chmod</function> and <function>umask</function>
system calls. (To use the customary octal format the number
must start with a <literal>0</literal> (zero).)
</para>
<para>
The default permissions are <literal>0777</literal>, meaning
anyone can connect. Reasonable alternatives are
<literal>0770</literal> (only user and group, see also
<varname>unix_socket_group</varname>) and <literal>0700</literal>
(only user). (Note that for a Unix-domain socket, only write
permission matters, so there is no point in setting or revoking
read or execute permissions.)
</para>
<para>
This access control mechanism is independent of the one
described in <xref linkend="client-authentication"/>.
</para>
<para>
This parameter can only be set at server start.
</para>
<para>
This parameter is irrelevant on systems, notably Solaris as of Solaris
10, that ignore socket permissions entirely. There, one can achieve a
similar effect by pointing <varname>unix_socket_directories</varname> to a
directory having search permission limited to the desired audience.
This parameter is also irrelevant on Windows, which does not have
Unix-domain sockets.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-bonjour" xreflabel="bonjour">
<term><varname>bonjour</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>bonjour</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables advertising the server's existence via
<productname>Bonjour</productname>. The default is off.
This parameter can only be set at server start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-bonjour-name" xreflabel="bonjour_name">
<term><varname>bonjour_name</varname> (<type>string</type>)
<indexterm>
<primary><varname>bonjour_name</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the <productname>Bonjour</productname> service
name. The computer name is used if this parameter is set to the
empty string <literal>''</literal> (which is the default). This parameter is
ignored if the server was not compiled with
<productname>Bonjour</productname> support.
This parameter can only be set at server start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-tcp-keepalives-idle" xreflabel="tcp_keepalives_idle">
<term><varname>tcp_keepalives_idle</varname> (<type>integer</type>)
<indexterm>
<primary><varname>tcp_keepalives_idle</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the amount of time with no network activity after which
the operating system should send a TCP keepalive message to the client.
If this value is specified without units, it is taken as seconds.
A value of 0 (the default) selects the operating system's default.
This parameter is supported only on systems that support
<symbol>TCP_KEEPIDLE</symbol> or an equivalent socket option, and on
Windows; on other systems, it must be zero.
In sessions connected via a Unix-domain socket, this parameter is
ignored and always reads as zero.
</para>
<note>
<para>
On Windows, setting a value of 0 will set this parameter to 2 hours,
since Windows does not provide a way to read the system default value.
</para>
</note>
</listitem>
</varlistentry>
<varlistentry id="guc-tcp-keepalives-interval" xreflabel="tcp_keepalives_interval">
<term><varname>tcp_keepalives_interval</varname> (<type>integer</type>)
<indexterm>
<primary><varname>tcp_keepalives_interval</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the amount of time after which a TCP keepalive message
that has not been acknowledged by the client should be retransmitted.
If this value is specified without units, it is taken as seconds.
A value of 0 (the default) selects the operating system's default.
This parameter is supported only on systems that support
<symbol>TCP_KEEPINTVL</symbol> or an equivalent socket option, and on
Windows; on other systems, it must be zero.
In sessions connected via a Unix-domain socket, this parameter is
ignored and always reads as zero.
</para>
<note>
<para>
On Windows, setting a value of 0 will set this parameter to 1 second,
since Windows does not provide a way to read the system default value.
</para>
</note>
</listitem>
</varlistentry>
<varlistentry id="guc-tcp-keepalives-count" xreflabel="tcp_keepalives_count">
<term><varname>tcp_keepalives_count</varname> (<type>integer</type>)
<indexterm>
<primary><varname>tcp_keepalives_count</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the number of TCP keepalive messages that can be lost before
the server's connection to the client is considered dead.
A value of 0 (the default) selects the operating system's default.
This parameter is supported only on systems that support
<symbol>TCP_KEEPCNT</symbol> or an equivalent socket option;
on other systems, it must be zero.
In sessions connected via a Unix-domain socket, this parameter is
ignored and always reads as zero.
</para>
<note>
<para>
This parameter is not supported on Windows, and must be zero.
</para>
</note>
</listitem>
</varlistentry>
<varlistentry id="guc-tcp-user-timeout" xreflabel="tcp_user_timeout">
<term><varname>tcp_user_timeout</varname> (<type>integer</type>)
<indexterm>
<primary><varname>tcp_user_timeout</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the amount of time that transmitted data may
remain unacknowledged before the TCP connection is forcibly closed.
If this value is specified without units, it is taken as milliseconds.
A value of 0 (the default) selects the operating system's default.
This parameter is supported only on systems that support
<symbol>TCP_USER_TIMEOUT</symbol>; on other systems, it must be zero.
In sessions connected via a Unix-domain socket, this parameter is
ignored and always reads as zero.
</para>
<note>
<para>
This parameter is not supported on Windows, and must be zero.
</para>
</note>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-connection-authentication">
<title>Authentication</title>
<variablelist>
<varlistentry id="guc-authentication-timeout" xreflabel="authentication_timeout">
<term><varname>authentication_timeout</varname> (<type>integer</type>)
<indexterm><primary>timeout</primary><secondary>client authentication</secondary></indexterm>
<indexterm><primary>client authentication</primary><secondary>timeout during</secondary></indexterm>
<indexterm>
<primary><varname>authentication_timeout</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Maximum amount of time allowed to complete client authentication. If a
would-be client has not completed the authentication protocol in
this much time, the server closes the connection. This prevents
hung clients from occupying a connection indefinitely.
If this value is specified without units, it is taken as seconds.
The default is one minute (<literal>1m</literal>).
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-password-encryption" xreflabel="password_encryption">
<term><varname>password_encryption</varname> (<type>enum</type>)
<indexterm>
<primary><varname>password_encryption</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When a password is specified in <xref linkend="sql-createrole"/> or
<xref linkend="sql-alterrole"/>, this parameter determines the algorithm
to use to encrypt the password. The default value is <literal>md5</literal>,
which stores the password as an MD5 hash (<literal>on</literal> is also
accepted, as alias for <literal>md5</literal>). Setting this parameter to
<literal>scram-sha-256</literal> will encrypt the password with SCRAM-SHA-256.
</para>
<para>
Note that older clients might lack support for the SCRAM authentication
mechanism, and hence not work with passwords encrypted with
SCRAM-SHA-256. See <xref linkend="auth-password"/> for more details.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-krb-server-keyfile" xreflabel="krb_server_keyfile">
<term><varname>krb_server_keyfile</varname> (<type>string</type>)
<indexterm>
<primary><varname>krb_server_keyfile</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the location of the Kerberos server key file. See
<xref linkend="gssapi-auth"/>
for details. This parameter can only be set in the
<filename>postgresql.conf</filename> file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-krb-caseins-users" xreflabel="krb_caseins_users">
<term><varname>krb_caseins_users</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>krb_caseins_users</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets whether GSSAPI user names should be treated
case-insensitively.
The default is <literal>off</literal> (case sensitive). This parameter can only be
set in the <filename>postgresql.conf</filename> file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-db-user-namespace" xreflabel="db_user_namespace">
<term><varname>db_user_namespace</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>db_user_namespace</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This parameter enables per-database user names. It is off by default.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
<para>
If this is on, you should create users as <replaceable>username@dbname</replaceable>.
When <replaceable>username</replaceable> is passed by a connecting client,
<literal>@</literal> and the database name are appended to the user
name and that database-specific user name is looked up by the
server. Note that when you create users with names containing
<literal>@</literal> within the SQL environment, you will need to
quote the user name.
</para>
<para>
With this parameter enabled, you can still create ordinary global
users. Simply append <literal>@</literal> when specifying the user
name in the client, e.g. <literal>joe@</literal>. The <literal>@</literal>
will be stripped off before the user name is looked up by the
server.
</para>
<para>
<varname>db_user_namespace</varname> causes the client's and
server's user name representation to differ.
Authentication checks are always done with the server's user name
so authentication methods must be configured for the
server's user name, not the client's. Because
<literal>md5</literal> uses the user name as salt on both the
client and server, <literal>md5</literal> cannot be used with
<varname>db_user_namespace</varname>.
</para>
<note>
<para>
This feature is intended as a temporary measure until a
complete solution is found. At that time, this option will
be removed.
</para>
</note>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-connection-ssl">
<title>SSL</title>
<para>
See <xref linkend="ssl-tcp"/> for more information about setting up SSL.
</para>
<variablelist>
<varlistentry id="guc-ssl" xreflabel="ssl">
<term><varname>ssl</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>ssl</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables <acronym>SSL</acronym> connections.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
The default is <literal>off</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-ssl-ca-file" xreflabel="ssl_ca_file">
<term><varname>ssl_ca_file</varname> (<type>string</type>)
<indexterm>
<primary><varname>ssl_ca_file</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the name of the file containing the SSL server certificate
authority (CA).
Relative paths are relative to the data directory.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
The default is empty, meaning no CA file is loaded,
and client certificate verification is not performed.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-ssl-cert-file" xreflabel="ssl_cert_file">
<term><varname>ssl_cert_file</varname> (<type>string</type>)
<indexterm>
<primary><varname>ssl_cert_file</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the name of the file containing the SSL server certificate.
Relative paths are relative to the data directory.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
The default is <filename>server.crt</filename>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-ssl-crl-file" xreflabel="ssl_crl_file">
<term><varname>ssl_crl_file</varname> (<type>string</type>)
<indexterm>
<primary><varname>ssl_crl_file</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the name of the file containing the SSL server certificate
revocation list (CRL).
Relative paths are relative to the data directory.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
The default is empty, meaning no CRL file is loaded.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-ssl-key-file" xreflabel="ssl_key_file">
<term><varname>ssl_key_file</varname> (<type>string</type>)
<indexterm>
<primary><varname>ssl_key_file</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the name of the file containing the SSL server private key.
Relative paths are relative to the data directory.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
The default is <filename>server.key</filename>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-ssl-ciphers" xreflabel="ssl_ciphers">
<term><varname>ssl_ciphers</varname> (<type>string</type>)
<indexterm>
<primary><varname>ssl_ciphers</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies a list of <acronym>SSL</acronym> cipher suites that are allowed to be
used on secure connections. See
the <citerefentry><refentrytitle>ciphers</refentrytitle></citerefentry> manual page
in the <application>OpenSSL</application> package for the syntax of this setting
and a list of supported values.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
The default value is <literal>HIGH:MEDIUM:+3DES:!aNULL</literal>. The
default is usually a reasonable choice unless you have specific
security requirements.
</para>
<para>
Explanation of the default value:
<variablelist>
<varlistentry>
<term><literal>HIGH</literal></term>
<listitem>
<para>
Cipher suites that use ciphers from <literal>HIGH</literal> group (e.g.,
AES, Camellia, 3DES)
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>MEDIUM</literal></term>
<listitem>
<para>
Cipher suites that use ciphers from <literal>MEDIUM</literal> group
(e.g., RC4, SEED)
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>+3DES</literal></term>
<listitem>
<para>
The OpenSSL default order for <literal>HIGH</literal> is problematic
because it orders 3DES higher than AES128. This is wrong because
3DES offers less security than AES128, and it is also much
slower. <literal>+3DES</literal> reorders it after all other
<literal>HIGH</literal> and <literal>MEDIUM</literal> ciphers.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>!aNULL</literal></term>
<listitem>
<para>
Disables anonymous cipher suites that do no authentication. Such
cipher suites are vulnerable to man-in-the-middle attacks and
therefore should not be used.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
Available cipher suite details will vary across OpenSSL versions. Use
the command
<literal>openssl ciphers -v 'HIGH:MEDIUM:+3DES:!aNULL'</literal> to
see actual details for the currently installed <application>OpenSSL</application>
version. Note that this list is filtered at run time based on the
server key type.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-ssl-prefer-server-ciphers" xreflabel="ssl_prefer_server_ciphers">
<term><varname>ssl_prefer_server_ciphers</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>ssl_prefer_server_ciphers</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies whether to use the server's SSL cipher preferences, rather
than the client's.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
The default is <literal>on</literal>.
</para>
<para>
Older PostgreSQL versions do not have this setting and always use the
client's preferences. This setting is mainly for backward
compatibility with those versions. Using the server's preferences is
usually better because it is more likely that the server is appropriately
configured.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-ssl-ecdh-curve" xreflabel="ssl_ecdh_curve">
<term><varname>ssl_ecdh_curve</varname> (<type>string</type>)
<indexterm>
<primary><varname>ssl_ecdh_curve</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the name of the curve to use in <acronym>ECDH</acronym> key
exchange. It needs to be supported by all clients that connect.
It does not need to be the same curve used by the server's Elliptic
Curve key.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
The default is <literal>prime256v1</literal>.
</para>
<para>
OpenSSL names for the most common curves are:
<literal>prime256v1</literal> (NIST P-256),
<literal>secp384r1</literal> (NIST P-384),
<literal>secp521r1</literal> (NIST P-521).
The full list of available curves can be shown with the command
<command>openssl ecparam -list_curves</command>. Not all of them
are usable in <acronym>TLS</acronym> though.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-ssl-min-protocol-version" xreflabel="ssl_min_protocol_version">
<term><varname>ssl_min_protocol_version</varname> (<type>enum</type>)
<indexterm>
<primary><varname>ssl_min_protocol_version</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the minimum SSL/TLS protocol version to use. Valid values are
currently: <literal>TLSv1</literal>, <literal>TLSv1.1</literal>,
<literal>TLSv1.2</literal>, <literal>TLSv1.3</literal>. Older
versions of the <productname>OpenSSL</productname> library do not
support all values; an error will be raised if an unsupported setting
is chosen. Protocol versions before TLS 1.0, namely SSL version 2 and
3, are always disabled.
</para>
<para>
The default is <literal>TLSv1.2</literal>, which satisfies industry
best practices as of this writing.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-ssl-max-protocol-version" xreflabel="ssl_max_protocol_version">
<term><varname>ssl_max_protocol_version</varname> (<type>enum</type>)
<indexterm>
<primary><varname>ssl_max_protocol_version</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the maximum SSL/TLS protocol version to use. Valid values are as
for <xref linkend="guc-ssl-min-protocol-version"/>, with addition of
an empty string, which allows any protocol version. The default is to
allow any version. Setting the maximum protocol version is mainly
useful for testing or if some component has issues working with a
newer protocol.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-ssl-dh-params-file" xreflabel="ssl_dh_params_file">
<term><varname>ssl_dh_params_file</varname> (<type>string</type>)
<indexterm>
<primary><varname>ssl_dh_params_file</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the name of the file containing Diffie-Hellman parameters
used for so-called ephemeral DH family of SSL ciphers. The default is
empty, in which case compiled-in default DH parameters used. Using
custom DH parameters reduces the exposure if an attacker manages to
crack the well-known compiled-in DH parameters. You can create your own
DH parameters file with the command
<command>openssl dhparam -out dhparams.pem 2048</command>.
</para>
<para>
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-ssl-passphrase-command" xreflabel="ssl_passphrase_command">
<term><varname>ssl_passphrase_command</varname> (<type>string</type>)
<indexterm>
<primary><varname>ssl_passphrase_command</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets an external command to be invoked when a passphrase for
decrypting an SSL file such as a private key needs to be obtained. By
default, this parameter is empty, which means the built-in prompting
mechanism is used.
</para>
<para>
The command must print the passphrase to the standard output and exit
with code 0. In the parameter value, <literal>%p</literal> is
replaced by a prompt string. (Write <literal>%%</literal> for a
literal <literal>%</literal>.) Note that the prompt string will
probably contain whitespace, so be sure to quote adequately. A single
newline is stripped from the end of the output if present.
</para>
<para>
The command does not actually have to prompt the user for a
passphrase. It can read it from a file, obtain it from a keychain
facility, or similar. It is up to the user to make sure the chosen
mechanism is adequately secure.
</para>
<para>
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-ssl-passphrase-command-supports-reload" xreflabel="ssl_passphrase_command_supports_reload">
<term><varname>ssl_passphrase_command_supports_reload</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>ssl_passphrase_command_supports_reload</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This parameter determines whether the passphrase command set by
<varname>ssl_passphrase_command</varname> will also be called during a
configuration reload if a key file needs a passphrase. If this
parameter is off (the default), then
<varname>ssl_passphrase_command</varname> will be ignored during a
reload and the SSL configuration will not be reloaded if a passphrase
is needed. That setting is appropriate for a command that requires a
TTY for prompting, which might not be available when the server is
running. Setting this parameter to on might be appropriate if the
passphrase is obtained from a file, for example.
</para>
<para>
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
</sect1>
<sect1 id="runtime-config-resource">
<title>Resource Consumption</title>
<sect2 id="runtime-config-resource-memory">
<title>Memory</title>
<variablelist>
<varlistentry id="guc-shared-buffers" xreflabel="shared_buffers">
<term><varname>shared_buffers</varname> (<type>integer</type>)
<indexterm>
<primary><varname>shared_buffers</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the amount of memory the database server uses for shared
memory buffers. The default is typically 128 megabytes
(<literal>128MB</literal>), but might be less if your kernel settings will
not support it (as determined during <application>initdb</application>).
This setting must be at least 128 kilobytes. However,
settings significantly higher than the minimum are usually needed
for good performance.
If this value is specified without units, it is taken as blocks,
that is <symbol>BLCKSZ</symbol> bytes, typically 8kB.
(Non-default values of <symbol>BLCKSZ</symbol> change the minimum
value.)
This parameter can only be set at server start.
</para>
<para>
If you have a dedicated database server with 1GB or more of RAM, a
reasonable starting value for <varname>shared_buffers</varname> is 25%
of the memory in your system. There are some workloads where even
larger settings for <varname>shared_buffers</varname> are effective, but
because <productname>PostgreSQL</productname> also relies on the
operating system cache, it is unlikely that an allocation of more than
40% of RAM to <varname>shared_buffers</varname> will work better than a
smaller amount. Larger settings for <varname>shared_buffers</varname>
usually require a corresponding increase in
<varname>max_wal_size</varname>, in order to spread out the
process of writing large quantities of new or changed data over a
longer period of time.
</para>
<para>
On systems with less than 1GB of RAM, a smaller percentage of RAM is
appropriate, so as to leave adequate space for the operating system.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-huge-pages" xreflabel="huge_pages">
<term><varname>huge_pages</varname> (<type>enum</type>)
<indexterm>
<primary><varname>huge_pages</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Controls whether huge pages are requested for the main shared memory
area. Valid values are <literal>try</literal> (the default),
<literal>on</literal>, and <literal>off</literal>. With
<varname>huge_pages</varname> set to <literal>try</literal>, the
server will try to request huge pages, but fall back to the default if
that fails. With <literal>on</literal>, failure to request huge pages
will prevent the server from starting up. With <literal>off</literal>,
huge pages will not be requested.
</para>
<para>
At present, this setting is supported only on Linux and Windows. The
setting is ignored on other systems when set to
<literal>try</literal>.
</para>
<para>
The use of huge pages results in smaller page tables and less CPU time
spent on memory management, increasing performance. For more details about
using huge pages on Linux, see <xref linkend="linux-huge-pages"/>.
</para>
<para>
Huge pages are known as large pages on Windows. To use them, you need to
assign the user right Lock Pages in Memory to the Windows user account
that runs <productname>PostgreSQL</productname>.
You can use Windows Group Policy tool (gpedit.msc) to assign the user right
Lock Pages in Memory.
To start the database server on the command prompt as a standalone process,
not as a Windows service, the command prompt must be run as an administrator or
User Access Control (UAC) must be disabled. When the UAC is enabled, the normal
command prompt revokes the user right Lock Pages in Memory when started.
</para>
<para>
Note that this setting only affects the main shared memory area.
Operating systems such as Linux, FreeBSD, and Illumos can also use
huge pages (also known as <quote>super</quote> pages or
<quote>large</quote> pages) automatically for normal memory
allocation, without an explicit request from
<productname>PostgreSQL</productname>. On Linux, this is called
<quote>transparent huge pages</quote><indexterm><primary>transparent
huge pages</primary></indexterm> (THP). That feature has been known to
cause performance degradation with
<productname>PostgreSQL</productname> for some users on some Linux
versions, so its use is currently discouraged (unlike explicit use of
<varname>huge_pages</varname>).
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-temp-buffers" xreflabel="temp_buffers">
<term><varname>temp_buffers</varname> (<type>integer</type>)
<indexterm>
<primary><varname>temp_buffers</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the maximum amount of memory used for temporary buffers within
each database session. These are session-local buffers used only
for access to temporary tables.
If this value is specified without units, it is taken as blocks,
that is <symbol>BLCKSZ</symbol> bytes, typically 8kB.
The default is eight megabytes (<literal>8MB</literal>).
(If <symbol>BLCKSZ</symbol> is not 8kB, the default value scales
proportionally to it.)
This setting can be changed within individual
sessions, but only before the first use of temporary tables
within the session; subsequent attempts to change the value will
have no effect on that session.
</para>
<para>
A session will allocate temporary buffers as needed up to the limit
given by <varname>temp_buffers</varname>. The cost of setting a large
value in sessions that do not actually need many temporary
buffers is only a buffer descriptor, or about 64 bytes, per
increment in <varname>temp_buffers</varname>. However if a buffer is
actually used an additional 8192 bytes will be consumed for it
(or in general, <symbol>BLCKSZ</symbol> bytes).
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-max-prepared-transactions" xreflabel="max_prepared_transactions">
<term><varname>max_prepared_transactions</varname> (<type>integer</type>)
<indexterm>
<primary><varname>max_prepared_transactions</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the maximum number of transactions that can be in the
<quote>prepared</quote> state simultaneously (see <xref
linkend="sql-prepare-transaction"/>).
Setting this parameter to zero (which is the default)
disables the prepared-transaction feature.
This parameter can only be set at server start.
</para>
<para>
If you are not planning to use prepared transactions, this parameter
should be set to zero to prevent accidental creation of prepared
transactions. If you are using prepared transactions, you will
probably want <varname>max_prepared_transactions</varname> to be at
least as large as <xref linkend="guc-max-connections"/>, so that every
session can have a prepared transaction pending.
</para>
<para>
When running a standby server, you must set this parameter to the
same or higher value than on the master server. Otherwise, queries
will not be allowed in the standby server.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-work-mem" xreflabel="work_mem">
<term><varname>work_mem</varname> (<type>integer</type>)
<indexterm>
<primary><varname>work_mem</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the maximum amount of memory to be used by a query operation
(such as a sort or hash table) before writing to temporary disk files.
If this value is specified without units, it is taken as kilobytes.
The default value is four megabytes (<literal>4MB</literal>).
Note that for a complex query, several sort or hash operations might be
running in parallel; each operation will be allowed to use as much memory
as this value specifies before it starts to write data into temporary
files. Also, several running sessions could be doing such operations
concurrently. Therefore, the total memory used could be many
times the value of <varname>work_mem</varname>; it is necessary to
keep this fact in mind when choosing the value. Sort operations are
used for <literal>ORDER BY</literal>, <literal>DISTINCT</literal>, and
merge joins.
Hash tables are used in hash joins, hash-based aggregation, and
hash-based processing of <literal>IN</literal> subqueries.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-maintenance-work-mem" xreflabel="maintenance_work_mem">
<term><varname>maintenance_work_mem</varname> (<type>integer</type>)
<indexterm>
<primary><varname>maintenance_work_mem</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the maximum amount of memory to be used by maintenance
operations, such as <command>VACUUM</command>, <command>CREATE
INDEX</command>, and <command>ALTER TABLE ADD FOREIGN KEY</command>.
If this value is specified without units, it is taken as kilobytes.
It defaults
to 64 megabytes (<literal>64MB</literal>). Since only one of these
operations can be executed at a time by a database session, and
an installation normally doesn't have many of them running
concurrently, it's safe to set this value significantly larger
than <varname>work_mem</varname>. Larger settings might improve
performance for vacuuming and for restoring database dumps.
</para>
<para>
Note that when autovacuum runs, up to
<xref linkend="guc-autovacuum-max-workers"/> times this memory
may be allocated, so be careful not to set the default value
too high. It may be useful to control for this by separately
setting <xref linkend="guc-autovacuum-work-mem"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-autovacuum-work-mem" xreflabel="autovacuum_work_mem">
<term><varname>autovacuum_work_mem</varname> (<type>integer</type>)
<indexterm>
<primary><varname>autovacuum_work_mem</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the maximum amount of memory to be used by each
autovacuum worker process.
If this value is specified without units, it is taken as kilobytes.
It defaults to -1, indicating that
the value of <xref linkend="guc-maintenance-work-mem"/> should
be used instead. The setting has no effect on the behavior of
<command>VACUUM</command> when run in other contexts.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-logical-decoding-work-mem" xreflabel="logical_decoding_work_mem">
<term><varname>logical_decoding_work_mem</varname> (<type>integer</type>)
<indexterm>
<primary><varname>logical_decoding_work_mem</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the maximum amount of memory to be used by logical decoding,
before some of the decoded changes are written to local disk. This
limits the amount of memory used by logical streaming replication
connections. It defaults to 64 megabytes (<literal>64MB</literal>).
Since each replication connection only uses a single buffer of this size,
and an installation normally doesn't have many such connections
concurrently (as limited by <varname>max_wal_senders</varname>), it's
safe to set this value significantly higher than <varname>work_mem</varname>,
reducing the amount of decoded changes written to disk.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-max-stack-depth" xreflabel="max_stack_depth">
<term><varname>max_stack_depth</varname> (<type>integer</type>)
<indexterm>
<primary><varname>max_stack_depth</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the maximum safe depth of the server's execution stack.
The ideal setting for this parameter is the actual stack size limit
enforced by the kernel (as set by <literal>ulimit -s</literal> or local
equivalent), less a safety margin of a megabyte or so. The safety
margin is needed because the stack depth is not checked in every
routine in the server, but only in key potentially-recursive routines.
If this value is specified without units, it is taken as kilobytes.
The default setting is two megabytes (<literal>2MB</literal>), which
is conservatively small and unlikely to risk crashes. However,
it might be too small to allow execution of complex functions.
Only superusers can change this setting.
</para>
<para>
Setting <varname>max_stack_depth</varname> higher than
the actual kernel limit will mean that a runaway recursive function
can crash an individual backend process. On platforms where
<productname>PostgreSQL</productname> can determine the kernel limit,
the server will not allow this variable to be set to an unsafe
value. However, not all platforms provide the information,
so caution is recommended in selecting a value.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-shared-memory-type" xreflabel="shared_memory_type">
<term><varname>shared_memory_type</varname> (<type>enum</type>)
<indexterm>
<primary><varname>shared_memory_type</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the shared memory implementation that the server
should use for the main shared memory region that holds
<productname>PostgreSQL</productname>'s shared buffers and other
shared data. Possible values are <literal>mmap</literal> (for
anonymous shared memory allocated using <function>mmap</function>),
<literal>sysv</literal> (for System V shared memory allocated via
<function>shmget</function>) and <literal>windows</literal> (for Windows
shared memory). Not all values are supported on all platforms; the
first supported option is the default for that platform. The use of
the <literal>sysv</literal> option, which is not the default on any
platform, is generally discouraged because it typically requires
non-default kernel settings to allow for large allocations (see <xref
linkend="sysvipc"/>).
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-dynamic-shared-memory-type" xreflabel="dynamic_shared_memory_type">
<term><varname>dynamic_shared_memory_type</varname> (<type>enum</type>)
<indexterm>
<primary><varname>dynamic_shared_memory_type</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the dynamic shared memory implementation that the server
should use. Possible values are <literal>posix</literal> (for POSIX shared
memory allocated using <literal>shm_open</literal>), <literal>sysv</literal>
(for System V shared memory allocated via <literal>shmget</literal>),
<literal>windows</literal> (for Windows shared memory),
and <literal>mmap</literal> (to simulate shared memory using
memory-mapped files stored in the data directory).
Not all values are supported on all platforms; the first supported
option is the default for that platform. The use of the
<literal>mmap</literal> option, which is not the default on any platform,
is generally discouraged because the operating system may write
modified pages back to disk repeatedly, increasing system I/O load;
however, it may be useful for debugging, when the
<literal>pg_dynshmem</literal> directory is stored on a RAM disk, or when
other shared memory facilities are not available.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-resource-disk">
<title>Disk</title>
<variablelist>
<varlistentry id="guc-temp-file-limit" xreflabel="temp_file_limit">
<term><varname>temp_file_limit</varname> (<type>integer</type>)
<indexterm>
<primary><varname>temp_file_limit</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the maximum amount of disk space that a process can use
for temporary files, such as sort and hash temporary files, or the
storage file for a held cursor. A transaction attempting to exceed
this limit will be canceled.
If this value is specified without units, it is taken as kilobytes.
<literal>-1</literal> (the default) means no limit.
Only superusers can change this setting.
</para>
<para>
This setting constrains the total space used at any instant by all
temporary files used by a given <productname>PostgreSQL</productname> process.
It should be noted that disk space used for explicit temporary
tables, as opposed to temporary files used behind-the-scenes in query
execution, does <emphasis>not</emphasis> count against this limit.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-resource-kernel">
<title>Kernel Resource Usage</title>
<variablelist>
<varlistentry id="guc-max-files-per-process" xreflabel="max_files_per_process">
<term><varname>max_files_per_process</varname> (<type>integer</type>)
<indexterm>
<primary><varname>max_files_per_process</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the maximum number of simultaneously open files allowed to each
server subprocess. The default is one thousand files. If the kernel is enforcing
a safe per-process limit, you don't need to worry about this setting.
But on some platforms (notably, most BSD systems), the kernel will
allow individual processes to open many more files than the system
can actually support if many processes all try to open
that many files. If you find yourself seeing <quote>Too many open
files</quote> failures, try reducing this setting.
This parameter can only be set at server start.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-resource-vacuum-cost">
<title>Cost-based Vacuum Delay</title>
<para>
During the execution of <xref linkend="sql-vacuum"/>
and <xref linkend="sql-analyze"/>
commands, the system maintains an
internal counter that keeps track of the estimated cost of the
various I/O operations that are performed. When the accumulated
cost reaches a limit (specified by
<varname>vacuum_cost_limit</varname>), the process performing
the operation will sleep for a short period of time, as specified by
<varname>vacuum_cost_delay</varname>. Then it will reset the
counter and continue execution.
</para>
<para>
The intent of this feature is to allow administrators to reduce
the I/O impact of these commands on concurrent database
activity. There are many situations where it is not
important that maintenance commands like
<command>VACUUM</command> and <command>ANALYZE</command> finish
quickly; however, it is usually very important that these
commands do not significantly interfere with the ability of the
system to perform other database operations. Cost-based vacuum
delay provides a way for administrators to achieve this.
</para>
<para>
This feature is disabled by default for manually issued
<command>VACUUM</command> commands. To enable it, set the
<varname>vacuum_cost_delay</varname> variable to a nonzero
value.
</para>
<variablelist>
<varlistentry id="guc-vacuum-cost-delay" xreflabel="vacuum_cost_delay">
<term><varname>vacuum_cost_delay</varname> (<type>floating point</type>)
<indexterm>
<primary><varname>vacuum_cost_delay</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The amount of time that the process will sleep
when the cost limit has been exceeded.
If this value is specified without units, it is taken as milliseconds.
The default value is zero, which disables the cost-based vacuum
delay feature. Positive values enable cost-based vacuuming.
</para>
<para>
When using cost-based vacuuming, appropriate values for
<varname>vacuum_cost_delay</varname> are usually quite small, perhaps
less than 1 millisecond. While <varname>vacuum_cost_delay</varname>
can be set to fractional-millisecond values, such delays may not be
measured accurately on older platforms. On such platforms,
increasing <command>VACUUM</command>'s throttled resource consumption
above what you get at 1ms will require changing the other vacuum cost
parameters. You should, nonetheless,
keep <varname>vacuum_cost_delay</varname> as small as your platform
will consistently measure; large delays are not helpful.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-cost-page-hit" xreflabel="vacuum_cost_page_hit">
<term><varname>vacuum_cost_page_hit</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_cost_page_hit</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The estimated cost for vacuuming a buffer found in the shared buffer
cache. It represents the cost to lock the buffer pool, lookup
the shared hash table and scan the content of the page. The
default value is one.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-cost-page-miss" xreflabel="vacuum_cost_page_miss">
<term><varname>vacuum_cost_page_miss</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_cost_page_miss</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The estimated cost for vacuuming a buffer that has to be read from
disk. This represents the effort to lock the buffer pool,
lookup the shared hash table, read the desired block in from
the disk and scan its content. The default value is 10.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-cost-page-dirty" xreflabel="vacuum_cost_page_dirty">
<term><varname>vacuum_cost_page_dirty</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_cost_page_dirty</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The estimated cost charged when vacuum modifies a block that was
previously clean. It represents the extra I/O required to
flush the dirty block out to disk again. The default value is
20.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-cost-limit" xreflabel="vacuum_cost_limit">
<term><varname>vacuum_cost_limit</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_cost_limit</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The accumulated cost that will cause the vacuuming process to sleep.
The default value is 200.
</para>
</listitem>
</varlistentry>
</variablelist>
<note>
<para>
There are certain operations that hold critical locks and should
therefore complete as quickly as possible. Cost-based vacuum
delays do not occur during such operations. Therefore it is
possible that the cost accumulates far higher than the specified
limit. To avoid uselessly long delays in such cases, the actual
delay is calculated as <varname>vacuum_cost_delay</varname> *
<varname>accumulated_balance</varname> /
<varname>vacuum_cost_limit</varname> with a maximum of
<varname>vacuum_cost_delay</varname> * 4.
</para>
</note>
</sect2>
<sect2 id="runtime-config-resource-background-writer">
<title>Background Writer</title>
<para>
There is a separate server
process called the <firstterm>background writer</firstterm>, whose function
is to issue writes of <quote>dirty</quote> (new or modified) shared
buffers. It writes shared buffers so server processes handling
user queries seldom or never need to wait for a write to occur.
However, the background writer does cause a net overall
increase in I/O load, because while a repeatedly-dirtied page might
otherwise be written only once per checkpoint interval, the
background writer might write it several times as it is dirtied
in the same interval. The parameters discussed in this subsection
can be used to tune the behavior for local needs.
</para>
<variablelist>
<varlistentry id="guc-bgwriter-delay" xreflabel="bgwriter_delay">
<term><varname>bgwriter_delay</varname> (<type>integer</type>)
<indexterm>
<primary><varname>bgwriter_delay</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the delay between activity rounds for the
background writer. In each round the writer issues writes
for some number of dirty buffers (controllable by the
following parameters). It then sleeps for
the length of <varname>bgwriter_delay</varname>, and repeats.
When there are no dirty buffers in the
buffer pool, though, it goes into a longer sleep regardless of
<varname>bgwriter_delay</varname>.
If this value is specified without units, it is taken as milliseconds.
The default value is 200
milliseconds (<literal>200ms</literal>). Note that on many systems, the
effective resolution of sleep delays is 10 milliseconds; setting
<varname>bgwriter_delay</varname> to a value that is not a multiple of 10
might have the same results as setting it to the next higher multiple
of 10. This parameter can only be set in the
<filename>postgresql.conf</filename> file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-bgwriter-lru-maxpages" xreflabel="bgwriter_lru_maxpages">
<term><varname>bgwriter_lru_maxpages</varname> (<type>integer</type>)
<indexterm>
<primary><varname>bgwriter_lru_maxpages</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
In each round, no more than this many buffers will be written
by the background writer. Setting this to zero disables
background writing. (Note that checkpoints, which are managed by
a separate, dedicated auxiliary process, are unaffected.)
The default value is 100 buffers.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-bgwriter-lru-multiplier" xreflabel="bgwriter_lru_multiplier">
<term><varname>bgwriter_lru_multiplier</varname> (<type>floating point</type>)
<indexterm>
<primary><varname>bgwriter_lru_multiplier</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The number of dirty buffers written in each round is based on the
number of new buffers that have been needed by server processes
during recent rounds. The average recent need is multiplied by
<varname>bgwriter_lru_multiplier</varname> to arrive at an estimate of the
number of buffers that will be needed during the next round. Dirty
buffers are written until there are that many clean, reusable buffers
available. (However, no more than <varname>bgwriter_lru_maxpages</varname>
buffers will be written per round.)
Thus, a setting of 1.0 represents a <quote>just in time</quote> policy
of writing exactly the number of buffers predicted to be needed.
Larger values provide some cushion against spikes in demand,
while smaller values intentionally leave writes to be done by
server processes.
The default is 2.0.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-bgwriter-flush-after" xreflabel="bgwriter_flush_after">
<term><varname>bgwriter_flush_after</varname> (<type>integer</type>)
<indexterm>
<primary><varname>bgwriter_flush_after</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Whenever more than this amount of data has
been written by the background writer, attempt to force the OS to issue these
writes to the underlying storage. Doing so will limit the amount of
dirty data in the kernel's page cache, reducing the likelihood of
stalls when an <function>fsync</function> is issued at the end of a checkpoint, or when
the OS writes data back in larger batches in the background. Often
that will result in greatly reduced transaction latency, but there
also are some cases, especially with workloads that are bigger than
<xref linkend="guc-shared-buffers"/>, but smaller than the OS's page
cache, where performance might degrade. This setting may have no
effect on some platforms.
If this value is specified without units, it is taken as blocks,
that is <symbol>BLCKSZ</symbol> bytes, typically 8kB.
The valid range is between
<literal>0</literal>, which disables forced writeback, and
<literal>2MB</literal>. The default is <literal>512kB</literal> on Linux,
<literal>0</literal> elsewhere. (If <symbol>BLCKSZ</symbol> is not 8kB,
the default and maximum values scale proportionally to it.)
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
</listitem>
</varlistentry>
</variablelist>
<para>
Smaller values of <varname>bgwriter_lru_maxpages</varname> and
<varname>bgwriter_lru_multiplier</varname> reduce the extra I/O load
caused by the background writer, but make it more likely that server
processes will have to issue writes for themselves, delaying interactive
queries.
</para>
</sect2>
<sect2 id="runtime-config-resource-async-behavior">
<title>Asynchronous Behavior</title>
<variablelist>
<varlistentry id="guc-effective-io-concurrency" xreflabel="effective_io_concurrency">
<term><varname>effective_io_concurrency</varname> (<type>integer</type>)
<indexterm>
<primary><varname>effective_io_concurrency</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the number of concurrent disk I/O operations that
<productname>PostgreSQL</productname> expects can be executed
simultaneously. Raising this value will increase the number of I/O
operations that any individual <productname>PostgreSQL</productname> session
attempts to initiate in parallel. The allowed range is 1 to 1000,
or zero to disable issuance of asynchronous I/O requests. Currently,
this setting only affects bitmap heap scans.
</para>
<para>
For magnetic drives, a good starting point for this setting is the
number of separate
drives comprising a RAID 0 stripe or RAID 1 mirror being used for the
database. (For RAID 5 the parity drive should not be counted.)
However, if the database is often busy with multiple queries issued in
concurrent sessions, lower values may be sufficient to keep the disk
array busy. A value higher than needed to keep the disks busy will
only result in extra CPU overhead.
SSDs and other memory-based storage can often process many
concurrent requests, so the best value might be in the hundreds.
</para>
<para>
Asynchronous I/O depends on an effective <function>posix_fadvise</function>
function, which some operating systems lack. If the function is not
present then setting this parameter to anything but zero will result
in an error. On some operating systems (e.g., Solaris), the function
is present but does not actually do anything.
</para>
<para>
The default is 1 on supported systems, otherwise 0. This value can
be overridden for tables in a particular tablespace by setting the
tablespace parameter of the same name (see
<xref linkend="sql-altertablespace"/>).
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-max-worker-processes" xreflabel="max_worker_processes">
<term><varname>max_worker_processes</varname> (<type>integer</type>)
<indexterm>
<primary><varname>max_worker_processes</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the maximum number of background processes that the system
can support. This parameter can only be set at server start. The
default is 8.
</para>
<para>
When running a standby server, you must set this parameter to the
same or higher value than on the master server. Otherwise, queries
will not be allowed in the standby server.
</para>
<para>
When changing this value, consider also adjusting
<xref linkend="guc-max-parallel-workers"/>,
<xref linkend="guc-max-parallel-workers-maintenance"/>, and
<xref linkend="guc-max-parallel-workers-per-gather"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-max-parallel-workers-per-gather" xreflabel="max_parallel_workers_per_gather">
<term><varname>max_parallel_workers_per_gather</varname> (<type>integer</type>)
<indexterm>
<primary><varname>max_parallel_workers_per_gather</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the maximum number of workers that can be started by a single
<literal>Gather</literal> or <literal>Gather Merge</literal> node.
Parallel workers are taken from the pool of processes established by
<xref linkend="guc-max-worker-processes"/>, limited by
<xref linkend="guc-max-parallel-workers"/>. Note that the requested
number of workers may not actually be available at run time. If this
occurs, the plan will run with fewer workers than expected, which may
be inefficient. The default value is 2. Setting this value to 0
disables parallel query execution.
</para>
<para>
Note that parallel queries may consume very substantially more
resources than non-parallel queries, because each worker process is
a completely separate process which has roughly the same impact on the
system as an additional user session. This should be taken into
account when choosing a value for this setting, as well as when
configuring other settings that control resource utilization, such
as <xref linkend="guc-work-mem"/>. Resource limits such as
<varname>work_mem</varname> are applied individually to each worker,
which means the total utilization may be much higher across all
processes than it would normally be for any single process.
For example, a parallel query using 4 workers may use up to 5 times
as much CPU time, memory, I/O bandwidth, and so forth as a query which
uses no workers at all.
</para>
<para>
For more information on parallel query, see
<xref linkend="parallel-query"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-max-parallel-workers-maintenance" xreflabel="max_parallel_maintenance_workers">
<term><varname>max_parallel_maintenance_workers</varname> (<type>integer</type>)
<indexterm>
<primary><varname>max_parallel_maintenance_workers</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the maximum number of parallel workers that can be
started by a single utility command. Currently, the parallel
utility commands that support the use of parallel workers are
<command>CREATE INDEX</command> only when building a B-tree index,
and <command>VACUUM</command> without <literal>FULL</literal>
option. Parallel workers are taken from the pool of processes
established by <xref linkend="guc-max-worker-processes"/>, limited
by <xref linkend="guc-max-parallel-workers"/>. Note that the requested
number of workers may not actually be available at run time.
If this occurs, the utility operation will run with fewer
workers than expected. The default value is 2. Setting this
value to 0 disables the use of parallel workers by utility
commands.
</para>
<para>
Note that parallel utility commands should not consume
substantially more memory than equivalent non-parallel
operations. This strategy differs from that of parallel
query, where resource limits generally apply per worker
process. Parallel utility commands treat the resource limit
<varname>maintenance_work_mem</varname> as a limit to be applied to
the entire utility command, regardless of the number of
parallel worker processes. However, parallel utility
commands may still consume substantially more CPU resources
and I/O bandwidth.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-max-parallel-workers" xreflabel="max_parallel_workers">
<term><varname>max_parallel_workers</varname> (<type>integer</type>)
<indexterm>
<primary><varname>max_parallel_workers</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the maximum number of workers that the system can support for
parallel operations. The default value is 8. When increasing or
decreasing this value, consider also adjusting
<xref linkend="guc-max-parallel-workers-maintenance"/> and
<xref linkend="guc-max-parallel-workers-per-gather"/>.
Also, note that a setting for this value which is higher than
<xref linkend="guc-max-worker-processes"/> will have no effect,
since parallel workers are taken from the pool of worker processes
established by that setting.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-backend-flush-after" xreflabel="backend_flush_after">
<term><varname>backend_flush_after</varname> (<type>integer</type>)
<indexterm>
<primary><varname>backend_flush_after</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Whenever more than this amount of data has
been written by a single backend, attempt to force the OS to issue
these writes to the underlying storage. Doing so will limit the
amount of dirty data in the kernel's page cache, reducing the
likelihood of stalls when an <function>fsync</function> is issued at the end of a
checkpoint, or when the OS writes data back in larger batches in the
background. Often that will result in greatly reduced transaction
latency, but there also are some cases, especially with workloads
that are bigger than <xref linkend="guc-shared-buffers"/>, but smaller
than the OS's page cache, where performance might degrade. This
setting may have no effect on some platforms.
If this value is specified without units, it is taken as blocks,
that is <symbol>BLCKSZ</symbol> bytes, typically 8kB.
The valid range is
between <literal>0</literal>, which disables forced writeback,
and <literal>2MB</literal>. The default is <literal>0</literal>, i.e., no
forced writeback. (If <symbol>BLCKSZ</symbol> is not 8kB,
the maximum value scales proportionally to it.)
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-old-snapshot-threshold" xreflabel="old_snapshot_threshold">
<term><varname>old_snapshot_threshold</varname> (<type>integer</type>)
<indexterm>
<primary><varname>old_snapshot_threshold</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the minimum amount of time that a query snapshot can be used
without risk of a <quote>snapshot too old</quote> error occurring
when using the snapshot. Data that has been dead for longer than
this threshold is allowed to be vacuumed away. This can help
prevent bloat in the face of snapshots which remain in use for a
long time. To prevent incorrect results due to cleanup of data which
would otherwise be visible to the snapshot, an error is generated
when the snapshot is older than this threshold and the snapshot is
used to read a page which has been modified since the snapshot was
built.
</para>
<para>
If this value is specified without units, it is taken as minutes.
A value of <literal>-1</literal> (the default) disables this feature,
effectively setting the snapshot age limit to infinity.
This parameter can only be set at server start.
</para>
<para>
Useful values for production work probably range from a small number
of hours to a few days. Small values (such as <literal>0</literal> or
<literal>1min</literal>) are only allowed because they may sometimes be
useful for testing. While a setting as high as <literal>60d</literal> is
allowed, please note that in many workloads extreme bloat or
transaction ID wraparound may occur in much shorter time frames.
</para>
<para>
When this feature is enabled, freed space at the end of a relation
cannot be released to the operating system, since that could remove
information needed to detect the <quote>snapshot too old</quote>
condition. All space allocated to a relation remains associated with
that relation for reuse only within that relation unless explicitly
freed (for example, with <command>VACUUM FULL</command>).
</para>
<para>
This setting does not attempt to guarantee that an error will be
generated under any particular circumstances. In fact, if the
correct results can be generated from (for example) a cursor which
has materialized a result set, no error will be generated even if the
underlying rows in the referenced table have been vacuumed away.
Some tables cannot safely be vacuumed early, and so will not be
affected by this setting, such as system catalogs. For such tables
this setting will neither reduce bloat nor create a possibility
of a <quote>snapshot too old</quote> error on scanning.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
</sect1>
<sect1 id="runtime-config-wal">
<title>Write Ahead Log</title>
<para>
For additional information on tuning these settings,
see <xref linkend="wal-configuration"/>.
</para>
<sect2 id="runtime-config-wal-settings">
<title>Settings</title>
<variablelist>
<varlistentry id="guc-wal-level" xreflabel="wal_level">
<term><varname>wal_level</varname> (<type>enum</type>)
<indexterm>
<primary><varname>wal_level</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
<varname>wal_level</varname> determines how much information is written to
the WAL. The default value is <literal>replica</literal>, which writes enough
data to support WAL archiving and replication, including running
read-only queries on a standby server. <literal>minimal</literal> removes all
logging except the information required to recover from a crash or
immediate shutdown. Finally,
<literal>logical</literal> adds information necessary to support logical
decoding. Each level includes the information logged at all lower
levels. This parameter can only be set at server start.
</para>
<para>
In <literal>minimal</literal> level, WAL-logging of some bulk
operations can be safely skipped, which can make those
operations much faster (see <xref linkend="populate-pitr"/>).
Operations in which this optimization can be applied include:
<simplelist>
<member><command>CREATE TABLE AS</command></member>
<member><command>CREATE INDEX</command></member>
<member><command>CLUSTER</command></member>
<member><command>COPY</command> into tables that were created or truncated in the same
transaction</member>
</simplelist>
But minimal WAL does not contain enough information to reconstruct the
data from a base backup and the WAL logs, so <literal>replica</literal> or
higher must be used to enable WAL archiving
(<xref linkend="guc-archive-mode"/>) and streaming replication.
</para>
<para>
In <literal>logical</literal> level, the same information is logged as
with <literal>replica</literal>, plus information needed to allow
extracting logical change sets from the WAL. Using a level of
<literal>logical</literal> will increase the WAL volume, particularly if many
tables are configured for <literal>REPLICA IDENTITY FULL</literal> and
many <command>UPDATE</command> and <command>DELETE</command> statements are
executed.
</para>
<para>
In releases prior to 9.6, this parameter also allowed the
values <literal>archive</literal> and <literal>hot_standby</literal>.
These are still accepted but mapped to <literal>replica</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-fsync" xreflabel="fsync">
<term><varname>fsync</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>fsync</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
If this parameter is on, the <productname>PostgreSQL</productname> server
will try to make sure that updates are physically written to
disk, by issuing <function>fsync()</function> system calls or various
equivalent methods (see <xref linkend="guc-wal-sync-method"/>).
This ensures that the database cluster can recover to a
consistent state after an operating system or hardware crash.
</para>
<para>
While turning off <varname>fsync</varname> is often a performance
benefit, this can result in unrecoverable data corruption in
the event of a power failure or system crash. Thus it
is only advisable to turn off <varname>fsync</varname> if
you can easily recreate your entire database from external
data.
</para>
<para>
Examples of safe circumstances for turning off
<varname>fsync</varname> include the initial loading of a new
database cluster from a backup file, using a database cluster
for processing a batch of data after which the database
will be thrown away and recreated,
or for a read-only database clone which
gets recreated frequently and is not used for failover. High
quality hardware alone is not a sufficient justification for
turning off <varname>fsync</varname>.
</para>
<para>
For reliable recovery when changing <varname>fsync</varname>
off to on, it is necessary to force all modified buffers in the
kernel to durable storage. This can be done while the cluster
is shutdown or while <varname>fsync</varname> is on by running <command>initdb
--sync-only</command>, running <command>sync</command>, unmounting the
file system, or rebooting the server.
</para>
<para>
In many situations, turning off <xref linkend="guc-synchronous-commit"/>
for noncritical transactions can provide much of the potential
performance benefit of turning off <varname>fsync</varname>, without
the attendant risks of data corruption.
</para>
<para>
<varname>fsync</varname> can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
If you turn this parameter off, also consider turning off
<xref linkend="guc-full-page-writes"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-synchronous-commit" xreflabel="synchronous_commit">
<term><varname>synchronous_commit</varname> (<type>enum</type>)
<indexterm>
<primary><varname>synchronous_commit</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies whether transaction commit will wait for WAL records
to be written to disk before the command returns a <quote>success</quote>
indication to the client. Valid values are <literal>on</literal>,
<literal>remote_apply</literal>, <literal>remote_write</literal>, <literal>local</literal>,
and <literal>off</literal>. The default, and safe, setting
is <literal>on</literal>. When <literal>off</literal>, there can be a delay between
when success is reported to the client and when the transaction is
really guaranteed to be safe against a server crash. (The maximum
delay is three times <xref linkend="guc-wal-writer-delay"/>.) Unlike
<xref linkend="guc-fsync"/>, setting this parameter to <literal>off</literal>
does not create any risk of database inconsistency: an operating
system or database crash might
result in some recent allegedly-committed transactions being lost, but
the database state will be just the same as if those transactions had
been aborted cleanly. So, turning <varname>synchronous_commit</varname> off
can be a useful alternative when performance is more important than
exact certainty about the durability of a transaction. For more
discussion see <xref linkend="wal-async-commit"/>.
</para>
<para>
If <xref linkend="guc-synchronous-standby-names"/> is non-empty, this
parameter also controls whether or not transaction commits will wait
for their WAL records to be replicated to the standby server(s).
When set to <literal>on</literal>, commits will wait until replies
from the current synchronous standby(s) indicate they have received
the commit record of the transaction and flushed it to disk. This
ensures the transaction will not be lost unless both the primary and
all synchronous standbys suffer corruption of their database storage.
When set to <literal>remote_apply</literal>, commits will wait until replies
from the current synchronous standby(s) indicate they have received the
commit record of the transaction and applied it, so that it has become
visible to queries on the standby(s).
When set to <literal>remote_write</literal>, commits will wait until replies
from the current synchronous standby(s) indicate they have
received the commit record of the transaction and written it out to
their operating system. This setting is sufficient to
ensure data preservation even if a standby instance of
<productname>PostgreSQL</productname> were to crash, but not if the standby
suffers an operating-system-level crash, since the data has not
necessarily reached stable storage on the standby.
Finally, the setting <literal>local</literal> causes commits to wait for
local flush to disk, but not for replication. This is not usually
desirable when synchronous replication is in use, but is provided for
completeness.
</para>
<para>
If <varname>synchronous_standby_names</varname> is empty, the settings
<literal>on</literal>, <literal>remote_apply</literal>, <literal>remote_write</literal>
and <literal>local</literal> all provide the same synchronization level:
transaction commits only wait for local flush to disk.
</para>
<para>
This parameter can be changed at any time; the behavior for any
one transaction is determined by the setting in effect when it
commits. It is therefore possible, and useful, to have some
transactions commit synchronously and others asynchronously.
For example, to make a single multistatement transaction commit
asynchronously when the default is the opposite, issue <command>SET
LOCAL synchronous_commit TO OFF</command> within the transaction.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-wal-sync-method" xreflabel="wal_sync_method">
<term><varname>wal_sync_method</varname> (<type>enum</type>)
<indexterm>
<primary><varname>wal_sync_method</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Method used for forcing WAL updates out to disk.
If <varname>fsync</varname> is off then this setting is irrelevant,
since WAL file updates will not be forced out at all.
Possible values are:
</para>
<itemizedlist>
<listitem>
<para>
<literal>open_datasync</literal> (write WAL files with <function>open()</function> option <symbol>O_DSYNC</symbol>)
</para>
</listitem>
<listitem>
<para>
<literal>fdatasync</literal> (call <function>fdatasync()</function> at each commit)
</para>
</listitem>
<listitem>
<para>
<literal>fsync</literal> (call <function>fsync()</function> at each commit)
</para>
</listitem>
<listitem>
<para>
<literal>fsync_writethrough</literal> (call <function>fsync()</function> at each commit, forcing write-through of any disk write cache)
</para>
</listitem>
<listitem>
<para>
<literal>open_sync</literal> (write WAL files with <function>open()</function> option <symbol>O_SYNC</symbol>)
</para>
</listitem>
</itemizedlist>
<para>
The <literal>open_</literal>* options also use <literal>O_DIRECT</literal> if available.
Not all of these choices are available on all platforms.
The default is the first method in the above list that is supported
by the platform, except that <literal>fdatasync</literal> is the default on
Linux. The default is not necessarily ideal; it might be
necessary to change this setting or other aspects of your system
configuration in order to create a crash-safe configuration or
achieve optimal performance.
These aspects are discussed in <xref linkend="wal-reliability"/>.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-full-page-writes" xreflabel="full_page_writes">
<term><varname>full_page_writes</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>full_page_writes</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When this parameter is on, the <productname>PostgreSQL</productname> server
writes the entire content of each disk page to WAL during the
first modification of that page after a checkpoint.
This is needed because
a page write that is in process during an operating system crash might
be only partially completed, leading to an on-disk page
that contains a mix of old and new data. The row-level change data
normally stored in WAL will not be enough to completely restore
such a page during post-crash recovery. Storing the full page image
guarantees that the page can be correctly restored, but at the price
of increasing the amount of data that must be written to WAL.
(Because WAL replay always starts from a checkpoint, it is sufficient
to do this during the first change of each page after a checkpoint.
Therefore, one way to reduce the cost of full-page writes is to
increase the checkpoint interval parameters.)
</para>
<para>
Turning this parameter off speeds normal operation, but
might lead to either unrecoverable data corruption, or silent
data corruption, after a system failure. The risks are similar to turning off
<varname>fsync</varname>, though smaller, and it should be turned off
only based on the same circumstances recommended for that parameter.
</para>
<para>
Turning off this parameter does not affect use of
WAL archiving for point-in-time recovery (PITR)
(see <xref linkend="continuous-archiving"/>).
</para>
<para>
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
The default is <literal>on</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-wal-log-hints" xreflabel="wal_log_hints">
<term><varname>wal_log_hints</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>wal_log_hints</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When this parameter is <literal>on</literal>, the <productname>PostgreSQL</productname>
server writes the entire content of each disk page to WAL during the
first modification of that page after a checkpoint, even for
non-critical modifications of so-called hint bits.
</para>
<para>
If data checksums are enabled, hint bit updates are always WAL-logged
and this setting is ignored. You can use this setting to test how much
extra WAL-logging would occur if your database had data checksums
enabled.
</para>
<para>
This parameter can only be set at server start. The default value is <literal>off</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-wal-compression" xreflabel="wal_compression">
<term><varname>wal_compression</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>wal_compression</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When this parameter is <literal>on</literal>, the <productname>PostgreSQL</productname>
server compresses a full page image written to WAL when
<xref linkend="guc-full-page-writes"/> is on or during a base backup.
A compressed page image will be decompressed during WAL replay.
The default value is <literal>off</literal>.
Only superusers can change this setting.
</para>
<para>
Turning this parameter on can reduce the WAL volume without
increasing the risk of unrecoverable data corruption,
but at the cost of some extra CPU spent on the compression during
WAL logging and on the decompression during WAL replay.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-wal-buffers" xreflabel="wal_buffers">
<term><varname>wal_buffers</varname> (<type>integer</type>)
<indexterm>
<primary><varname>wal_buffers</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The amount of shared memory used for WAL data that has not yet been
written to disk. The default setting of -1 selects a size equal to
1/32nd (about 3%) of <xref linkend="guc-shared-buffers"/>, but not less
than <literal>64kB</literal> nor more than the size of one WAL
segment, typically <literal>16MB</literal>. This value can be set
manually if the automatic choice is too large or too small,
but any positive value less than <literal>32kB</literal> will be
treated as <literal>32kB</literal>.
If this value is specified without units, it is taken as WAL blocks,
that is <symbol>XLOG_BLCKSZ</symbol> bytes, typically 8kB.
This parameter can only be set at server start.
</para>
<para>
The contents of the WAL buffers are written out to disk at every
transaction commit, so extremely large values are unlikely to
provide a significant benefit. However, setting this value to at
least a few megabytes can improve write performance on a busy
server where many clients are committing at once. The auto-tuning
selected by the default setting of -1 should give reasonable
results in most cases.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-wal-writer-delay" xreflabel="wal_writer_delay">
<term><varname>wal_writer_delay</varname> (<type>integer</type>)
<indexterm>
<primary><varname>wal_writer_delay</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies how often the WAL writer flushes WAL, in time terms.
After flushing WAL the writer sleeps for the length of time given
by <varname>wal_writer_delay</varname>, unless woken up sooner
by an asynchronously committing transaction. If the last flush
happened less than <varname>wal_writer_delay</varname> ago and less
than <varname>wal_writer_flush_after</varname> worth of WAL has been
produced since, then WAL is only written to the operating system, not
flushed to disk.
If this value is specified without units, it is taken as milliseconds.
The default value is 200 milliseconds (<literal>200ms</literal>). Note that
on many systems, the effective resolution of sleep delays is 10
milliseconds; setting <varname>wal_writer_delay</varname> to a value that is
not a multiple of 10 might have the same results as setting it to the
next higher multiple of 10. This parameter can only be set in the
<filename>postgresql.conf</filename> file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-wal-writer-flush-after" xreflabel="wal_writer_flush_after">
<term><varname>wal_writer_flush_after</varname> (<type>integer</type>)
<indexterm>
<primary><varname>wal_writer_flush_after</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies how often the WAL writer flushes WAL, in volume terms.
If the last flush happened less
than <varname>wal_writer_delay</varname> ago and less
than <varname>wal_writer_flush_after</varname> worth of WAL has been
produced since, then WAL is only written to the operating system, not
flushed to disk. If <varname>wal_writer_flush_after</varname> is set
to <literal>0</literal> then WAL data is always flushed immediately.
If this value is specified without units, it is taken as WAL blocks,
that is <symbol>XLOG_BLCKSZ</symbol> bytes, typically 8kB.
The default is <literal>1MB</literal>.
This parameter can only be set in the
<filename>postgresql.conf</filename> file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-commit-delay" xreflabel="commit_delay">
<term><varname>commit_delay</varname> (<type>integer</type>)
<indexterm>
<primary><varname>commit_delay</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Setting <varname>commit_delay</varname> adds a time delay
before a WAL flush is initiated. This can improve
group commit throughput by allowing a larger number of transactions
to commit via a single WAL flush, if system load is high enough
that additional transactions become ready to commit within the
given interval. However, it also increases latency by up to the
<varname>commit_delay</varname> for each WAL
flush. Because the delay is just wasted if no other transactions
become ready to commit, a delay is only performed if at least
<varname>commit_siblings</varname> other transactions are active
when a flush is about to be initiated. Also, no delays are
performed if <varname>fsync</varname> is disabled.
If this value is specified without units, it is taken as microseconds.
The default <varname>commit_delay</varname> is zero (no delay).
Only superusers can change this setting.
</para>
<para>
In <productname>PostgreSQL</productname> releases prior to 9.3,
<varname>commit_delay</varname> behaved differently and was much
less effective: it affected only commits, rather than all WAL flushes,
and waited for the entire configured delay even if the WAL flush
was completed sooner. Beginning in <productname>PostgreSQL</productname> 9.3,
the first process that becomes ready to flush waits for the configured
interval, while subsequent processes wait only until the leader
completes the flush operation.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-commit-siblings" xreflabel="commit_siblings">
<term><varname>commit_siblings</varname> (<type>integer</type>)
<indexterm>
<primary><varname>commit_siblings</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Minimum number of concurrent open transactions to require
before performing the <varname>commit_delay</varname> delay. A larger
value makes it more probable that at least one other
transaction will become ready to commit during the delay
interval. The default is five transactions.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-wal-checkpoints">
<title>Checkpoints</title>
<variablelist>
<varlistentry id="guc-checkpoint-timeout" xreflabel="checkpoint_timeout">
<term><varname>checkpoint_timeout</varname> (<type>integer</type>)
<indexterm>
<primary><varname>checkpoint_timeout</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Maximum time between automatic WAL checkpoints.
If this value is specified without units, it is taken as seconds.
The valid range is between 30 seconds and one day.
The default is five minutes (<literal>5min</literal>).
Increasing this parameter can increase the amount of time needed
for crash recovery.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-checkpoint-completion-target" xreflabel="checkpoint_completion_target">
<term><varname>checkpoint_completion_target</varname> (<type>floating point</type>)
<indexterm>
<primary><varname>checkpoint_completion_target</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the target of checkpoint completion, as a fraction of
total time between checkpoints. The default is 0.5.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-checkpoint-flush-after" xreflabel="checkpoint_flush_after">
<term><varname>checkpoint_flush_after</varname> (<type>integer</type>)
<indexterm>
<primary><varname>checkpoint_flush_after</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Whenever more than this amount of data has been
written while performing a checkpoint, attempt to force the
OS to issue these writes to the underlying storage. Doing so will
limit the amount of dirty data in the kernel's page cache, reducing
the likelihood of stalls when an <function>fsync</function> is issued at the end of the
checkpoint, or when the OS writes data back in larger batches in the
background. Often that will result in greatly reduced transaction
latency, but there also are some cases, especially with workloads
that are bigger than <xref linkend="guc-shared-buffers"/>, but smaller
than the OS's page cache, where performance might degrade. This
setting may have no effect on some platforms.
If this value is specified without units, it is taken as blocks,
that is <symbol>BLCKSZ</symbol> bytes, typically 8kB.
The valid range is
between <literal>0</literal>, which disables forced writeback,
and <literal>2MB</literal>. The default is <literal>256kB</literal> on
Linux, <literal>0</literal> elsewhere. (If <symbol>BLCKSZ</symbol> is not
8kB, the default and maximum values scale proportionally to it.)
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-checkpoint-warning" xreflabel="checkpoint_warning">
<term><varname>checkpoint_warning</varname> (<type>integer</type>)
<indexterm>
<primary><varname>checkpoint_warning</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Write a message to the server log if checkpoints caused by
the filling of WAL segment files happen closer together
than this amount of time (which suggests that
<varname>max_wal_size</varname> ought to be raised).
If this value is specified without units, it is taken as seconds.
The default is 30 seconds (<literal>30s</literal>).
Zero disables the warning.
No warnings will be generated if <varname>checkpoint_timeout</varname>
is less than <varname>checkpoint_warning</varname>.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-max-wal-size" xreflabel="max_wal_size">
<term><varname>max_wal_size</varname> (<type>integer</type>)
<indexterm>
<primary><varname>max_wal_size</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Maximum size to let the WAL grow to between automatic WAL
checkpoints. This is a soft limit; WAL size can exceed
<varname>max_wal_size</varname> under special circumstances, such as
heavy load, a failing <varname>archive_command</varname>, or a high
<varname>wal_keep_segments</varname> setting.
If this value is specified without units, it is taken as megabytes.
The default is 1 GB.
Increasing this parameter can increase the amount of time needed for
crash recovery.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-min-wal-size" xreflabel="min_wal_size">
<term><varname>min_wal_size</varname> (<type>integer</type>)
<indexterm>
<primary><varname>min_wal_size</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
As long as WAL disk usage stays below this setting, old WAL files are
always recycled for future use at a checkpoint, rather than removed.
This can be used to ensure that enough WAL space is reserved to
handle spikes in WAL usage, for example when running large batch
jobs.
If this value is specified without units, it is taken as megabytes.
The default is 80 MB.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-wal-archiving">
<title>Archiving</title>
<variablelist>
<varlistentry id="guc-archive-mode" xreflabel="archive_mode">
<term><varname>archive_mode</varname> (<type>enum</type>)
<indexterm>
<primary><varname>archive_mode</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When <varname>archive_mode</varname> is enabled, completed WAL segments
are sent to archive storage by setting
<xref linkend="guc-archive-command"/>. In addition to <literal>off</literal>,
to disable, there are two modes: <literal>on</literal>, and
<literal>always</literal>. During normal operation, there is no
difference between the two modes, but when set to <literal>always</literal>
the WAL archiver is enabled also during archive recovery or standby
mode. In <literal>always</literal> mode, all files restored from the archive
or streamed with streaming replication will be archived (again). See
<xref linkend="continuous-archiving-in-standby"/> for details.
</para>
<para>
<varname>archive_mode</varname> and <varname>archive_command</varname> are
separate variables so that <varname>archive_command</varname> can be
changed without leaving archiving mode.
This parameter can only be set at server start.
<varname>archive_mode</varname> cannot be enabled when
<varname>wal_level</varname> is set to <literal>minimal</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-archive-command" xreflabel="archive_command">
<term><varname>archive_command</varname> (<type>string</type>)
<indexterm>
<primary><varname>archive_command</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The local shell command to execute to archive a completed WAL file
segment. Any <literal>%p</literal> in the string is
replaced by the path name of the file to archive, and any
<literal>%f</literal> is replaced by only the file name.
(The path name is relative to the working directory of the server,
i.e., the cluster's data directory.)
Use <literal>%%</literal> to embed an actual <literal>%</literal> character in the
command. It is important for the command to return a zero
exit status only if it succeeds. For more information see
<xref linkend="backup-archiving-wal"/>.
</para>
<para>
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line. It is ignored unless
<varname>archive_mode</varname> was enabled at server start.
If <varname>archive_command</varname> is an empty string (the default) while
<varname>archive_mode</varname> is enabled, WAL archiving is temporarily
disabled, but the server continues to accumulate WAL segment files in
the expectation that a command will soon be provided. Setting
<varname>archive_command</varname> to a command that does nothing but
return true, e.g. <literal>/bin/true</literal> (<literal>REM</literal> on
Windows), effectively disables
archiving, but also breaks the chain of WAL files needed for
archive recovery, so it should only be used in unusual circumstances.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-archive-timeout" xreflabel="archive_timeout">
<term><varname>archive_timeout</varname> (<type>integer</type>)
<indexterm>
<primary><varname>archive_timeout</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The <xref linkend="guc-archive-command"/> is only invoked for
completed WAL segments. Hence, if your server generates little WAL
traffic (or has slack periods where it does so), there could be a
long delay between the completion of a transaction and its safe
recording in archive storage. To limit how old unarchived
data can be, you can set <varname>archive_timeout</varname> to force the
server to switch to a new WAL segment file periodically. When this
parameter is greater than zero, the server will switch to a new
segment file whenever this amount of time has elapsed since the last
segment file switch, and there has been any database activity,
including a single checkpoint (checkpoints are skipped if there is
no database activity). Note that archived files that are closed
early due to a forced switch are still the same length as completely
full files. Therefore, it is unwise to use a very short
<varname>archive_timeout</varname> &mdash; it will bloat your archive
storage. <varname>archive_timeout</varname> settings of a minute or so are
usually reasonable. You should consider using streaming replication,
instead of archiving, if you want data to be copied off the master
server more quickly than that.
If this value is specified without units, it is taken as seconds.
This parameter can only be set in the
<filename>postgresql.conf</filename> file or on the server command line.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-wal-archive-recovery">
<title>Archive Recovery</title>
<indexterm>
<primary>configuration</primary>
<secondary>of recovery</secondary>
<tertiary>of a standby server</tertiary>
</indexterm>
<para>
This section describes the settings that apply only for the duration of
the recovery. They must be reset for any subsequent recovery you wish to
perform.
</para>
<para>
<quote>Recovery</quote> covers using the server as a standby or for
executing a targeted recovery. Typically, standby mode would be used to
provide high availability and/or read scalability, whereas a targeted
recovery is used to recover from data loss.
</para>
<para>
To start the server in standby mode, create a file called
<filename>standby.signal</filename><indexterm><primary>standby.signal</primary></indexterm>
in the data directory. The server will enter recovery and will not stop
recovery when the end of archived WAL is reached, but will keep trying to
continue recovery by connecting to the sending server as specified by the
<varname>primary_conninfo</varname> setting and/or by fetching new WAL
segments using <varname>restore_command</varname>. For this mode, the
parameters from this section and <xref
linkend="runtime-config-replication-standby"/> are of interest.
Parameters from <xref linkend="runtime-config-wal-recovery-target"/> will
also be applied but are typically not useful in this mode.
</para>
<para>
To start the server in targeted recovery mode, create a file called
<filename>recovery.signal</filename><indexterm><primary>recovery.signal</primary></indexterm>
in the data directory. If both <filename>standby.signal</filename> and
<filename>recovery.signal</filename> files are created, standby mode
takes precedence. Targeted recovery mode ends when the archived WAL is
fully replayed, or when <varname>recovery_target</varname> is reached.
In this mode, the parameters from both this section and <xref
linkend="runtime-config-wal-recovery-target"/> will be used.
</para>
<variablelist>
<varlistentry id="guc-restore-command" xreflabel="restore_command">
<term><varname>restore_command</varname> (<type>string</type>)
<indexterm>
<primary><varname>restore_command</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The local shell command to execute to retrieve an archived segment of
the WAL file series. This parameter is required for archive recovery,
but optional for streaming replication.
Any <literal>%f</literal> in the string is
replaced by the name of the file to retrieve from the archive,
and any <literal>%p</literal> is replaced by the copy destination path name
on the server.
(The path name is relative to the current working directory,
i.e., the cluster's data directory.)
Any <literal>%r</literal> is replaced by the name of the file containing the
last valid restart point. That is the earliest file that must be kept
to allow a restore to be restartable, so this information can be used
to truncate the archive to just the minimum required to support
restarting from the current restore. <literal>%r</literal> is typically only
used by warm-standby configurations
(see <xref linkend="warm-standby"/>).
Write <literal>%%</literal> to embed an actual <literal>%</literal> character.
</para>
<para>
It is important for the command to return a zero exit status
only if it succeeds. The command <emphasis>will</emphasis> be asked for file
names that are not present in the archive; it must return nonzero
when so asked. Examples:
<programlisting>
restore_command = 'cp /mnt/server/archivedir/%f "%p"'
restore_command = 'copy "C:\\server\\archivedir\\%f" "%p"' # Windows
</programlisting>
An exception is that if the command was terminated by a signal (other
than <systemitem>SIGTERM</systemitem>, which is used as part of a
database server shutdown) or an error by the shell (such as command
not found), then recovery will abort and the server will not start up.
</para>
<para>
This parameter can only be set at server start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-archive-cleanup-command" xreflabel="archive_cleanup_command">
<term><varname>archive_cleanup_command</varname> (<type>string</type>)
<indexterm>
<primary><varname>archive_cleanup_command</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This optional parameter specifies a shell command that will be executed
at every restartpoint. The purpose of
<varname>archive_cleanup_command</varname> is to provide a mechanism for
cleaning up old archived WAL files that are no longer needed by the
standby server.
Any <literal>%r</literal> is replaced by the name of the file containing the
last valid restart point.
That is the earliest file that must be <emphasis>kept</emphasis> to allow a
restore to be restartable, and so all files earlier than <literal>%r</literal>
may be safely removed.
This information can be used to truncate the archive to just the
minimum required to support restart from the current restore.
The <xref linkend="pgarchivecleanup"/> module
is often used in <varname>archive_cleanup_command</varname> for
single-standby configurations, for example:
<programlisting>archive_cleanup_command = 'pg_archivecleanup /mnt/server/archivedir %r'</programlisting>
Note however that if multiple standby servers are restoring from the
same archive directory, you will need to ensure that you do not delete
WAL files until they are no longer needed by any of the servers.
<varname>archive_cleanup_command</varname> would typically be used in a
warm-standby configuration (see <xref linkend="warm-standby"/>).
Write <literal>%%</literal> to embed an actual <literal>%</literal> character in the
command.
</para>
<para>
If the command returns a nonzero exit status then a warning log
message will be written. An exception is that if the command was
terminated by a signal or an error by the shell (such as command not
found), a fatal error will be raised.
</para>
<para>
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-recovery-end-command" xreflabel="recovery_end_command">
<term><varname>recovery_end_command</varname> (<type>string</type>)
<indexterm>
<primary><varname>recovery_end_command</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This parameter specifies a shell command that will be executed once only
at the end of recovery. This parameter is optional. The purpose of the
<varname>recovery_end_command</varname> is to provide a mechanism for cleanup
following replication or recovery.
Any <literal>%r</literal> is replaced by the name of the file containing the
last valid restart point, like in <xref linkend="guc-archive-cleanup-command"/>.
</para>
<para>
If the command returns a nonzero exit status then a warning log
message will be written and the database will proceed to start up
anyway. An exception is that if the command was terminated by a
signal or an error by the shell (such as command not found), the
database will not proceed with startup.
</para>
<para>
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-wal-recovery-target">
<title>Recovery Target</title>
<para>
By default, recovery will recover to the end of the WAL log. The
following parameters can be used to specify an earlier stopping point.
At most one of <varname>recovery_target</varname>,
<varname>recovery_target_lsn</varname>, <varname>recovery_target_name</varname>,
<varname>recovery_target_time</varname>, or <varname>recovery_target_xid</varname>
can be used; if more than one of these is specified in the configuration
file, an error will be raised.
These parameters can only be set at server start.
</para>
<variablelist>
<varlistentry id="guc-recovery-target" xreflabel="recovery_target">
<term><varname>recovery_target</varname><literal> = 'immediate'</literal>
<indexterm>
<primary><varname>recovery_target</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This parameter specifies that recovery should end as soon as a
consistent state is reached, i.e. as early as possible. When restoring
from an online backup, this means the point where taking the backup
ended.
</para>
<para>
Technically, this is a string parameter, but <literal>'immediate'</literal>
is currently the only allowed value.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-recovery-target-name" xreflabel="recovery_target_name">
<term><varname>recovery_target_name</varname> (<type>string</type>)
<indexterm>
<primary><varname>recovery_target_name</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This parameter specifies the named restore point (created with
<function>pg_create_restore_point()</function>) to which recovery will proceed.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-recovery-target-time" xreflabel="recovery_target_time">
<term><varname>recovery_target_time</varname> (<type>timestamp</type>)
<indexterm>
<primary><varname>recovery_target_time</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This parameter specifies the time stamp up to which recovery
will proceed.
The precise stopping point is also influenced by
<xref linkend="guc-recovery-target-inclusive"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-recovery-target-xid" xreflabel="recovery_target_xid">
<term><varname>recovery_target_xid</varname> (<type>string</type>)
<indexterm>
<primary><varname>recovery_target_xid</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This parameter specifies the transaction ID up to which recovery
will proceed. Keep in mind
that while transaction IDs are assigned sequentially at transaction
start, transactions can complete in a different numeric order.
The transactions that will be recovered are those that committed
before (and optionally including) the specified one.
The precise stopping point is also influenced by
<xref linkend="guc-recovery-target-inclusive"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-recovery-target-lsn" xreflabel="recovery_target_lsn">
<term><varname>recovery_target_lsn</varname> (<type>pg_lsn</type>)
<indexterm>
<primary><varname>recovery_target_lsn</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This parameter specifies the LSN of the write-ahead log location up
to which recovery will proceed. The precise stopping point is also
influenced by <xref linkend="guc-recovery-target-inclusive"/>. This
parameter is parsed using the system data type
<link linkend="datatype-pg-lsn"><type>pg_lsn</type></link>.
</para>
</listitem>
</varlistentry>
</variablelist>
<para>
The following options further specify the recovery target, and affect
what happens when the target is reached:
</para>
<variablelist>
<varlistentry id="guc-recovery-target-inclusive"
xreflabel="recovery_target_inclusive">
<term><varname>recovery_target_inclusive</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>recovery_target_inclusive</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies whether to stop just after the specified recovery target
(<literal>on</literal>), or just before the recovery target
(<literal>off</literal>).
Applies when <xref linkend="guc-recovery-target-lsn"/>,
<xref linkend="guc-recovery-target-time"/>, or
<xref linkend="guc-recovery-target-xid"/> is specified.
This setting controls whether transactions
having exactly the target WAL location (LSN), commit time, or transaction ID, respectively, will
be included in the recovery. Default is <literal>on</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-recovery-target-timeline"
xreflabel="recovery_target_timeline">
<term><varname>recovery_target_timeline</varname> (<type>string</type>)
<indexterm>
<primary><varname>recovery_target_timeline</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies recovering into a particular timeline. The value can be a
numeric timeline ID or a special value. The value
<literal>current</literal> recovers along the same timeline that was
current when the base backup was taken. The
value <literal>latest</literal> recovers
to the latest timeline found in the archive, which is useful in
a standby server. <literal>latest</literal> is the default.
</para>
<para>
You usually only need to set this parameter
in complex re-recovery situations, where you need to return to
a state that itself was reached after a point-in-time recovery.
See <xref linkend="backup-timelines"/> for discussion.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-recovery-target-action"
xreflabel="recovery_target_action">
<term><varname>recovery_target_action</varname> (<type>enum</type>)
<indexterm>
<primary><varname>recovery_target_action</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies what action the server should take once the recovery target is
reached. The default is <literal>pause</literal>, which means recovery will
be paused. <literal>promote</literal> means the recovery process will finish
and the server will start to accept connections.
Finally <literal>shutdown</literal> will stop the server after reaching the
recovery target.
</para>
<para>
The intended use of the <literal>pause</literal> setting is to allow queries
to be executed against the database to check if this recovery target
is the most desirable point for recovery.
The paused state can be resumed by
using <function>pg_wal_replay_resume()</function> (see
<xref linkend="functions-recovery-control-table"/>), which then
causes recovery to end. If this recovery target is not the
desired stopping point, then shut down the server, change the
recovery target settings to a later target and restart to
continue recovery.
</para>
<para>
The <literal>shutdown</literal> setting is useful to have the instance ready
at the exact replay point desired. The instance will still be able to
replay more WAL records (and in fact will have to replay WAL records
since the last checkpoint next time it is started).
</para>
<para>
Note that because <filename>recovery.signal</filename> will not be
removed when <varname>recovery_target_action</varname> is set to <literal>shutdown</literal>,
any subsequent start will end with immediate shutdown unless the
configuration is changed or the <filename>recovery.signal</filename>
file is removed manually.
</para>
<para>
This setting has no effect if no recovery target is set.
If <xref linkend="guc-hot-standby"/> is not enabled, a setting of
<literal>pause</literal> will act the same as <literal>shutdown</literal>.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
</sect1>
<sect1 id="runtime-config-replication">
<title>Replication</title>
<para>
These settings control the behavior of the built-in
<firstterm>streaming replication</firstterm> feature (see
<xref linkend="streaming-replication"/>). Servers will be either a
master or a standby server. Masters can send data, while standbys
are always receivers of replicated data. When cascading replication
(see <xref linkend="cascading-replication"/>) is used, standby servers
can also be senders, as well as receivers.
Parameters are mainly for sending and standby servers, though some
parameters have meaning only on the master server. Settings may vary
across the cluster without problems if that is required.
</para>
<sect2 id="runtime-config-replication-sender">
<title>Sending Servers</title>
<para>
These parameters can be set on any server that is
to send replication data to one or more standby servers.
The master is always a sending server, so these parameters must
always be set on the master.
The role and meaning of these parameters does not change after a
standby becomes the master.
</para>
<variablelist>
<varlistentry id="guc-max-wal-senders" xreflabel="max_wal_senders">
<term><varname>max_wal_senders</varname> (<type>integer</type>)
<indexterm>
<primary><varname>max_wal_senders</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the maximum number of concurrent connections from standby
servers or streaming base backup clients (i.e., the maximum number of
simultaneously running WAL sender processes). The default is
<literal>10</literal>. The value <literal>0</literal> means
replication is disabled. Abrupt streaming client disconnection might
leave an orphaned connection slot behind until a timeout is reached,
so this parameter should be set slightly higher than the maximum
number of expected clients so disconnected clients can immediately
reconnect. This parameter can only be set at server start. Also,
<varname>wal_level</varname> must be set to
<literal>replica</literal> or higher to allow connections from standby
servers.
</para>
<para>
When running a standby server, you must set this parameter to the
same or higher value than on the master server. Otherwise, queries
will not be allowed in the standby server.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-max-replication-slots" xreflabel="max_replication_slots">
<term><varname>max_replication_slots</varname> (<type>integer</type>)
<indexterm>
<primary><varname>max_replication_slots</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the maximum number of replication slots
(see <xref linkend="streaming-replication-slots"/>) that the server
can support. The default is 10. This parameter can only be set at
server start.
Setting it to a lower value than the number of currently
existing replication slots will prevent the server from starting.
Also, <varname>wal_level</varname> must be set
to <literal>replica</literal> or higher to allow replication slots to
be used.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-wal-keep-segments" xreflabel="wal_keep_segments">
<term><varname>wal_keep_segments</varname> (<type>integer</type>)
<indexterm>
<primary><varname>wal_keep_segments</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the minimum number of past log file segments kept in the
<filename>pg_wal</filename>
directory, in case a standby server needs to fetch them for streaming
replication. Each segment is normally 16 megabytes. If a standby
server connected to the sending server falls behind by more than
<varname>wal_keep_segments</varname> segments, the sending server might remove
a WAL segment still needed by the standby, in which case the
replication connection will be terminated. Downstream connections
will also eventually fail as a result. (However, the standby
server can recover by fetching the segment from archive, if WAL
archiving is in use.)
</para>
<para>
This sets only the minimum number of segments retained in
<filename>pg_wal</filename>; the system might need to retain more segments
for WAL archival or to recover from a checkpoint. If
<varname>wal_keep_segments</varname> is zero (the default), the system
doesn't keep any extra segments for standby purposes, so the number
of old WAL segments available to standby servers is a function of
the location of the previous checkpoint and status of WAL
archiving.
This parameter can only be set in the
<filename>postgresql.conf</filename> file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-wal-init-zero" xreflabel="wal_init_zero">
<term><varname>wal_init_zero</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>wal_init_zero</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
If set to <literal>on</literal> (the default), this option causes new
WAL files to be filled with zeroes. On some file systems, this ensures
that space is allocated before we need to write WAL records. However,
<firstterm>Copy-On-Write</firstterm> (COW) file systems may not benefit
from this technique, so the option is given to skip the unnecessary
work. If set to <literal>off</literal>, only the final byte is written
when the file is created so that it has the expected size.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-wal-recycle" xreflabel="wal_recycle">
<term><varname>wal_recycle</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>wal_recycle</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
If set to <literal>on</literal> (the default), this option causes WAL
files to be recycled by renaming them, avoiding the need to create new
ones. On COW file systems, it may be faster to create new ones, so the
option is given to disable this behavior.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-wal-sender-timeout" xreflabel="wal_sender_timeout">
<term><varname>wal_sender_timeout</varname> (<type>integer</type>)
<indexterm>
<primary><varname>wal_sender_timeout</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Terminate replication connections that are inactive for longer
than this amount of time. This is useful for
the sending server to detect a standby crash or network outage.
If this value is specified without units, it is taken as milliseconds.
The default value is 60 seconds.
A value of zero disables the timeout mechanism.
</para>
<para>
With a cluster distributed across multiple geographic
locations, using different values per location brings more flexibility
in the cluster management. A smaller value is useful for faster
failure detection with a standby having a low-latency network
connection, and a larger value helps in judging better the health
of a standby if located on a remote location, with a high-latency
network connection.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-track-commit-timestamp" xreflabel="track_commit_timestamp">
<term><varname>track_commit_timestamp</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>track_commit_timestamp</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Record commit time of transactions. This parameter
can only be set in <filename>postgresql.conf</filename> file or on the server
command line. The default value is <literal>off</literal>.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-replication-master">
<title>Master Server</title>
<para>
These parameters can be set on the master/primary server that is
to send replication data to one or more standby servers.
Note that in addition to these parameters,
<xref linkend="guc-wal-level"/> must be set appropriately on the master
server, and optionally WAL archiving can be enabled as
well (see <xref linkend="runtime-config-wal-archiving"/>).
The values of these parameters on standby servers are irrelevant,
although you may wish to set them there in preparation for the
possibility of a standby becoming the master.
</para>
<variablelist>
<varlistentry id="guc-synchronous-standby-names" xreflabel="synchronous_standby_names">
<term><varname>synchronous_standby_names</varname> (<type>string</type>)
<indexterm>
<primary><varname>synchronous_standby_names</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies a list of standby servers that can support
<firstterm>synchronous replication</firstterm>, as described in
<xref linkend="synchronous-replication"/>.
There will be one or more active synchronous standbys;
transactions waiting for commit will be allowed to proceed after
these standby servers confirm receipt of their data.
The synchronous standbys will be those whose names appear
in this list, and
that are both currently connected and streaming data in real-time
(as shown by a state of <literal>streaming</literal> in the <link
linkend="pg-stat-replication-view"><literal>pg_stat_replication</literal></link>
view).
Specifying more than one synchronous standby can allow for very high
availability and protection against data loss.
</para>
<para>
The name of a standby server for this purpose is the
<varname>application_name</varname> setting of the standby, as set in the
standby's connection information. In case of a physical replication
standby, this should be set in the <varname>primary_conninfo</varname>
setting; the default is the setting of <xref linkend="guc-cluster-name"/>
if set, else <literal>walreceiver</literal>.
For logical replication, this can be set in the connection
information of the subscription, and it defaults to the
subscription name. For other replication stream consumers,
consult their documentation.
</para>
<para>
This parameter specifies a list of standby servers using
either of the following syntaxes:
<synopsis>
[FIRST] <replaceable class="parameter">num_sync</replaceable> ( <replaceable class="parameter">standby_name</replaceable> [, ...] )
ANY <replaceable class="parameter">num_sync</replaceable> ( <replaceable class="parameter">standby_name</replaceable> [, ...] )
<replaceable class="parameter">standby_name</replaceable> [, ...]
</synopsis>
where <replaceable class="parameter">num_sync</replaceable> is
the number of synchronous standbys that transactions need to
wait for replies from,
and <replaceable class="parameter">standby_name</replaceable>
is the name of a standby server.
<literal>FIRST</literal> and <literal>ANY</literal> specify the method to choose
synchronous standbys from the listed servers.
</para>
<para>
The keyword <literal>FIRST</literal>, coupled with
<replaceable class="parameter">num_sync</replaceable>, specifies a
priority-based synchronous replication and makes transaction commits
wait until their WAL records are replicated to
<replaceable class="parameter">num_sync</replaceable> synchronous
standbys chosen based on their priorities. For example, a setting of
<literal>FIRST 3 (s1, s2, s3, s4)</literal> will cause each commit to wait for
replies from three higher-priority standbys chosen from standby servers
<literal>s1</literal>, <literal>s2</literal>, <literal>s3</literal> and <literal>s4</literal>.
The standbys whose names appear earlier in the list are given higher
priority and will be considered as synchronous. Other standby servers
appearing later in this list represent potential synchronous standbys.
If any of the current synchronous standbys disconnects for whatever
reason, it will be replaced immediately with the next-highest-priority
standby. The keyword <literal>FIRST</literal> is optional.
</para>
<para>
The keyword <literal>ANY</literal>, coupled with
<replaceable class="parameter">num_sync</replaceable>, specifies a
quorum-based synchronous replication and makes transaction commits
wait until their WAL records are replicated to <emphasis>at least</emphasis>
<replaceable class="parameter">num_sync</replaceable> listed standbys.
For example, a setting of <literal>ANY 3 (s1, s2, s3, s4)</literal> will cause
each commit to proceed as soon as at least any three standbys of
<literal>s1</literal>, <literal>s2</literal>, <literal>s3</literal> and <literal>s4</literal>
reply.
</para>
<para>
<literal>FIRST</literal> and <literal>ANY</literal> are case-insensitive. If these
keywords are used as the name of a standby server,
its <replaceable class="parameter">standby_name</replaceable> must
be double-quoted.
</para>
<para>
The third syntax was used before <productname>PostgreSQL</productname>
version 9.6 and is still supported. It's the same as the first syntax
with <literal>FIRST</literal> and
<replaceable class="parameter">num_sync</replaceable> equal to 1.
For example, <literal>FIRST 1 (s1, s2)</literal> and <literal>s1, s2</literal> have
the same meaning: either <literal>s1</literal> or <literal>s2</literal> is chosen
as a synchronous standby.
</para>
<para>
The special entry <literal>*</literal> matches any standby name.
</para>
<para>
There is no mechanism to enforce uniqueness of standby names. In case
of duplicates one of the matching standbys will be considered as
higher priority, though exactly which one is indeterminate.
</para>
<note>
<para>
Each <replaceable class="parameter">standby_name</replaceable>
should have the form of a valid SQL identifier, unless it
is <literal>*</literal>. You can use double-quoting if necessary. But note
that <replaceable class="parameter">standby_name</replaceable>s are
compared to standby application names case-insensitively, whether
double-quoted or not.
</para>
</note>
<para>
If no synchronous standby names are specified here, then synchronous
replication is not enabled and transaction commits will not wait for
replication. This is the default configuration. Even when
synchronous replication is enabled, individual transactions can be
configured not to wait for replication by setting the
<xref linkend="guc-synchronous-commit"/> parameter to
<literal>local</literal> or <literal>off</literal>.
</para>
<para>
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-defer-cleanup-age" xreflabel="vacuum_defer_cleanup_age">
<term><varname>vacuum_defer_cleanup_age</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_defer_cleanup_age</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the number of transactions by which <command>VACUUM</command> and
<acronym>HOT</acronym> updates will defer cleanup of dead row versions. The
default is zero transactions, meaning that dead row versions can be
removed as soon as possible, that is, as soon as they are no longer
visible to any open transaction. You may wish to set this to a
non-zero value on a primary server that is supporting hot standby
servers, as described in <xref linkend="hot-standby"/>. This allows
more time for queries on the standby to complete without incurring
conflicts due to early cleanup of rows. However, since the value
is measured in terms of number of write transactions occurring on the
primary server, it is difficult to predict just how much additional
grace time will be made available to standby queries.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
<para>
You should also consider setting <varname>hot_standby_feedback</varname>
on standby server(s) as an alternative to using this parameter.
</para>
<para>
This does not prevent cleanup of dead rows which have reached the age
specified by <varname>old_snapshot_threshold</varname>.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-replication-standby">
<title>Standby Servers</title>
<para>
These settings control the behavior of a standby server that is
to receive replication data. Their values on the master server
are irrelevant.
</para>
<variablelist>
<varlistentry id="guc-primary-conninfo" xreflabel="primary_conninfo">
<term><varname>primary_conninfo</varname> (<type>string</type>)
<indexterm>
<primary><varname>primary_conninfo</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies a connection string to be used for the standby server
to connect with a sending server. This string is in the format
described in <xref linkend="libpq-connstring"/>. If any option is
unspecified in this string, then the corresponding environment
variable (see <xref linkend="libpq-envars"/>) is checked. If the
environment variable is not set either, then
defaults are used.
</para>
<para>
The connection string should specify the host name (or address)
of the sending server, as well as the port number if it is not
the same as the standby server's default.
Also specify a user name corresponding to a suitably-privileged role
on the sending server (see
<xref linkend="streaming-replication-authentication"/>).
A password needs to be provided too, if the sender demands password
authentication. It can be provided in the
<varname>primary_conninfo</varname> string, or in a separate
<filename>~/.pgpass</filename> file on the standby server (use
<literal>replication</literal> as the database name).
Do not specify a database name in the
<varname>primary_conninfo</varname> string.
</para>
<para>
This parameter can only be set at server start.
This setting has no effect if the server is not in standby mode.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-primary-slot-name" xreflabel="primary_slot_name">
<term><varname>primary_slot_name</varname> (<type>string</type>)
<indexterm>
<primary><varname>primary_slot_name</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Optionally specifies an existing replication slot to be used when
connecting to the sending server via streaming replication to control
resource removal on the upstream node
(see <xref linkend="streaming-replication-slots"/>).
This parameter can only be set at server start.
This setting has no effect if <varname>primary_conninfo</varname> is not
set.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-promote-trigger-file" xreflabel="promote_trigger_file">
<term><varname>promote_trigger_file</varname> (<type>string</type>)
<indexterm>
<primary><varname>promote_trigger_file</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies a trigger file whose presence ends recovery in the
standby. Even if this value is not set, you can still promote
the standby using <command>pg_ctl promote</command> or calling
<function>pg_promote</function>.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-hot-standby" xreflabel="hot_standby">
<term><varname>hot_standby</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>hot_standby</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies whether or not you can connect and run queries during
recovery, as described in <xref linkend="hot-standby"/>.
The default value is <literal>on</literal>.
This parameter can only be set at server start. It only has effect
during archive recovery or in standby mode.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-max-standby-archive-delay" xreflabel="max_standby_archive_delay">
<term><varname>max_standby_archive_delay</varname> (<type>integer</type>)
<indexterm>
<primary><varname>max_standby_archive_delay</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When Hot Standby is active, this parameter determines how long the
standby server should wait before canceling standby queries that
conflict with about-to-be-applied WAL entries, as described in
<xref linkend="hot-standby-conflict"/>.
<varname>max_standby_archive_delay</varname> applies when WAL data is
being read from WAL archive (and is therefore not current).
If this value is specified without units, it is taken as milliseconds.
The default is 30 seconds.
A value of -1 allows the standby to wait forever for conflicting
queries to complete.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
<para>
Note that <varname>max_standby_archive_delay</varname> is not the same as the
maximum length of time a query can run before cancellation; rather it
is the maximum total time allowed to apply any one WAL segment's data.
Thus, if one query has resulted in significant delay earlier in the
WAL segment, subsequent conflicting queries will have much less grace
time.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-max-standby-streaming-delay" xreflabel="max_standby_streaming_delay">
<term><varname>max_standby_streaming_delay</varname> (<type>integer</type>)
<indexterm>
<primary><varname>max_standby_streaming_delay</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When Hot Standby is active, this parameter determines how long the
standby server should wait before canceling standby queries that
conflict with about-to-be-applied WAL entries, as described in
<xref linkend="hot-standby-conflict"/>.
<varname>max_standby_streaming_delay</varname> applies when WAL data is
being received via streaming replication.
If this value is specified without units, it is taken as milliseconds.
The default is 30 seconds.
A value of -1 allows the standby to wait forever for conflicting
queries to complete.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
<para>
Note that <varname>max_standby_streaming_delay</varname> is not the same as
the maximum length of time a query can run before cancellation; rather
it is the maximum total time allowed to apply WAL data once it has
been received from the primary server. Thus, if one query has
resulted in significant delay, subsequent conflicting queries will
have much less grace time until the standby server has caught up
again.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-wal-receiver-create-temp-slot" xreflabel="wal_receiver_create_temp_slot">
<term><varname>wal_receiver_create_temp_slot</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>wal_receiver_create_temp_slot</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies whether a WAL receiver should create a temporary replication
slot on the remote instance when no permanent replication slot to use
has been configured (using <xref linkend="guc-primary-slot-name"/>).
The default is on. The only reason to turn this off would be if the
remote instance is currently out of available replication slots. This
parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line. Changes only take effect when the
WAL receiver process starts a new connection.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-wal-receiver-status-interval" xreflabel="wal_receiver_status_interval">
<term><varname>wal_receiver_status_interval</varname> (<type>integer</type>)
<indexterm>
<primary><varname>wal_receiver_status_interval</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the minimum frequency for the WAL receiver
process on the standby to send information about replication progress
to the primary or upstream standby, where it can be seen using the
<link
linkend="pg-stat-replication-view"><literal>pg_stat_replication</literal></link>
view. The standby will report
the last write-ahead log location it has written, the last position it
has flushed to disk, and the last position it has applied.
This parameter's
value is the maximum amount of time between reports. Updates are
sent each time the write or flush positions change, or at least as
often as specified by this parameter. Thus, the apply position may
lag slightly behind the true position.
If this value is specified without units, it is taken as seconds.
The default value is 10 seconds.
Setting this parameter to zero disables status updates completely.
This parameter can only be set in
the <filename>postgresql.conf</filename> file or on the server
command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-hot-standby-feedback" xreflabel="hot_standby_feedback">
<term><varname>hot_standby_feedback</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>hot_standby_feedback</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies whether or not a hot standby will send feedback to the primary
or upstream standby
about queries currently executing on the standby. This parameter can
be used to eliminate query cancels caused by cleanup records, but
can cause database bloat on the primary for some workloads.
Feedback messages will not be sent more frequently than once per
<varname>wal_receiver_status_interval</varname>. The default value is
<literal>off</literal>. This parameter can only be set in the
<filename>postgresql.conf</filename> file or on the server command line.
</para>
<para>
If cascaded replication is in use the feedback is passed upstream
until it eventually reaches the primary. Standbys make no other use
of feedback they receive other than to pass upstream.
</para>
<para>
This setting does not override the behavior of
<varname>old_snapshot_threshold</varname> on the primary; a snapshot on the
standby which exceeds the primary's age threshold can become invalid,
resulting in cancellation of transactions on the standby. This is
because <varname>old_snapshot_threshold</varname> is intended to provide an
absolute limit on the time which dead rows can contribute to bloat,
which would otherwise be violated because of the configuration of a
standby.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-wal-receiver-timeout" xreflabel="wal_receiver_timeout">
<term><varname>wal_receiver_timeout</varname> (<type>integer</type>)
<indexterm>
<primary><varname>wal_receiver_timeout</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Terminate replication connections that are inactive for longer
than this amount of time. This is useful for
the receiving standby server to detect a primary node crash or network
outage.
If this value is specified without units, it is taken as milliseconds.
The default value is 60 seconds.
A value of zero disables the timeout mechanism.
This parameter can only be set in
the <filename>postgresql.conf</filename> file or on the server
command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-wal-retrieve-retry-interval" xreflabel="wal_retrieve_retry_interval">
<term><varname>wal_retrieve_retry_interval</varname> (<type>integer</type>)
<indexterm>
<primary><varname>wal_retrieve_retry_interval</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies how long the standby server should wait when WAL data is not
available from any sources (streaming replication,
local <filename>pg_wal</filename> or WAL archive) before trying
again to retrieve WAL data.
If this value is specified without units, it is taken as milliseconds.
The default value is 5 seconds.
This parameter can only be set in
the <filename>postgresql.conf</filename> file or on the server
command line.
</para>
<para>
This parameter is useful in configurations where a node in recovery
needs to control the amount of time to wait for new WAL data to be
available. For example, in archive recovery, it is possible to
make the recovery more responsive in the detection of a new WAL
log file by reducing the value of this parameter. On a system with
low WAL activity, increasing it reduces the amount of requests necessary
to access WAL archives, something useful for example in cloud
environments where the amount of times an infrastructure is accessed
is taken into account.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-recovery-min-apply-delay" xreflabel="recovery_min_apply_delay">
<term><varname>recovery_min_apply_delay</varname> (<type>integer</type>)
<indexterm>
<primary><varname>recovery_min_apply_delay</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
By default, a standby server restores WAL records from the
sending server as soon as possible. It may be useful to have a time-delayed
copy of the data, offering opportunities to correct data loss errors.
This parameter allows you to delay recovery by a specified amount
of time. For example, if
you set this parameter to <literal>5min</literal>, the standby will
replay each transaction commit only when the system time on the standby
is at least five minutes past the commit time reported by the master.
If this value is specified without units, it is taken as milliseconds.
The default is zero, adding no delay.
</para>
<para>
It is possible that the replication delay between servers exceeds the
value of this parameter, in which case no delay is added.
Note that the delay is calculated between the WAL time stamp as written
on master and the current time on the standby. Delays in transfer
because of network lag or cascading replication configurations
may reduce the actual wait time significantly. If the system
clocks on master and standby are not synchronized, this may lead to
recovery applying records earlier than expected; but that is not a
major issue because useful settings of this parameter are much larger
than typical time deviations between servers.
</para>
<para>
The delay occurs only on WAL records for transaction commits.
Other records are replayed as quickly as possible, which
is not a problem because MVCC visibility rules ensure their effects
are not visible until the corresponding commit record is applied.
</para>
<para>
The delay occurs once the database in recovery has reached a consistent
state, until the standby is promoted or triggered. After that the standby
will end recovery without further waiting.
</para>
<para>
This parameter is intended for use with streaming replication deployments;
however, if the parameter is specified it will be honored in all cases
except crash recovery.
<varname>hot_standby_feedback</varname> will be delayed by use of this feature
which could lead to bloat on the master; use both together with care.
<warning>
<para>
Synchronous replication is affected by this setting when <varname>synchronous_commit</varname>
is set to <literal>remote_apply</literal>; every <literal>COMMIT</literal>
will need to wait to be applied.
</para>
</warning>
</para>
<para>
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-replication-subscriber">
<title>Subscribers</title>
<para>
These settings control the behavior of a logical replication subscriber.
Their values on the publisher are irrelevant.
</para>
<para>
Note that <varname>wal_receiver_timeout</varname>,
<varname>wal_receiver_status_interval</varname> and
<varname>wal_retrieve_retry_interval</varname> configuration parameters
affect the logical replication workers as well.
</para>
<variablelist>
<varlistentry id="guc-max-logical-replication-workers" xreflabel="max_logical_replication_workers">
<term><varname>max_logical_replication_workers</varname> (<type>int</type>)
<indexterm>
<primary><varname>max_logical_replication_workers</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies maximum number of logical replication workers. This includes
both apply workers and table synchronization workers.
</para>
<para>
Logical replication workers are taken from the pool defined by
<varname>max_worker_processes</varname>.
</para>
<para>
The default value is 4.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-max-sync-workers-per-subscription" xreflabel="max_sync_workers_per_subscription">
<term><varname>max_sync_workers_per_subscription</varname> (<type>integer</type>)
<indexterm>
<primary><varname>max_sync_workers_per_subscription</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Maximum number of synchronization workers per subscription. This
parameter controls the amount of parallelism of the initial data copy
during the subscription initialization or when new tables are added.
</para>
<para>
Currently, there can be only one synchronization worker per table.
</para>
<para>
The synchronization workers are taken from the pool defined by
<varname>max_logical_replication_workers</varname>.
</para>
<para>
The default value is 2.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
</sect1>
<sect1 id="runtime-config-query">
<title>Query Planning</title>
<sect2 id="runtime-config-query-enable">
<title>Planner Method Configuration</title>
<para>
These configuration parameters provide a crude method of
influencing the query plans chosen by the query optimizer. If
the default plan chosen by the optimizer for a particular query
is not optimal, a <emphasis>temporary</emphasis> solution is to use one
of these configuration parameters to force the optimizer to
choose a different plan.
Better ways to improve the quality of the
plans chosen by the optimizer include adjusting the planner cost
constants (see <xref linkend="runtime-config-query-constants"/>),
running <xref linkend="sql-analyze"/> manually, increasing
the value of the <xref
linkend="guc-default-statistics-target"/> configuration parameter,
and increasing the amount of statistics collected for
specific columns using <command>ALTER TABLE SET
STATISTICS</command>.
</para>
<variablelist>
<varlistentry id="guc-enable-bitmapscan" xreflabel="enable_bitmapscan">
<term><varname>enable_bitmapscan</varname> (<type>boolean</type>)
<indexterm>
<primary>bitmap scan</primary>
</indexterm>
<indexterm>
<primary><varname>enable_bitmapscan</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables or disables the query planner's use of bitmap-scan plan
types. The default is <literal>on</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-enable-gathermerge" xreflabel="enable_gathermerge">
<term><varname>enable_gathermerge</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>enable_gathermerge</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables or disables the query planner's use of gather
merge plan types. The default is <literal>on</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-enable-hashagg" xreflabel="enable_hashagg">
<term><varname>enable_hashagg</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>enable_hashagg</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables or disables the query planner's use of hashed
aggregation plan types. The default is <literal>on</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-enable-hashjoin" xreflabel="enable_hashjoin">
<term><varname>enable_hashjoin</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>enable_hashjoin</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables or disables the query planner's use of hash-join plan
types. The default is <literal>on</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-enable-indexscan" xreflabel="enable_indexscan">
<term><varname>enable_indexscan</varname> (<type>boolean</type>)
<indexterm>
<primary>index scan</primary>
</indexterm>
<indexterm>
<primary><varname>enable_indexscan</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables or disables the query planner's use of index-scan plan
types. The default is <literal>on</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-enable-indexonlyscan" xreflabel="enable_indexonlyscan">
<term><varname>enable_indexonlyscan</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>enable_indexonlyscan</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables or disables the query planner's use of index-only-scan plan
types (see <xref linkend="indexes-index-only-scans"/>).
The default is <literal>on</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-enable-material" xreflabel="enable_material">
<term><varname>enable_material</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>enable_material</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables or disables the query planner's use of materialization.
It is impossible to suppress materialization entirely,
but turning this variable off prevents the planner from inserting
materialize nodes except in cases where it is required for correctness.
The default is <literal>on</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-enable-mergejoin" xreflabel="enable_mergejoin">
<term><varname>enable_mergejoin</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>enable_mergejoin</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables or disables the query planner's use of merge-join plan
types. The default is <literal>on</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-enable-nestloop" xreflabel="enable_nestloop">
<term><varname>enable_nestloop</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>enable_nestloop</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables or disables the query planner's use of nested-loop join
plans. It is impossible to suppress nested-loop joins entirely,
but turning this variable off discourages the planner from using
one if there are other methods available. The default is
<literal>on</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-enable-parallel-append" xreflabel="enable_parallel_append">
<term><varname>enable_parallel_append</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>enable_parallel_append</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables or disables the query planner's use of parallel-aware
append plan types. The default is <literal>on</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-enable-parallel-hash" xreflabel="enable_parallel_hash">
<term><varname>enable_parallel_hash</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>enable_parallel_hash</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables or disables the query planner's use of hash-join plan
types with parallel hash. Has no effect if hash-join plans are not
also enabled. The default is <literal>on</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-enable-partition-pruning" xreflabel="enable_partition_pruning">
<term><varname>enable_partition_pruning</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>enable_partition_pruning</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables or disables the query planner's ability to eliminate a
partitioned table's partitions from query plans. This also controls
the planner's ability to generate query plans which allow the query
executor to remove (ignore) partitions during query execution. The
default is <literal>on</literal>.
See <xref linkend="ddl-partition-pruning"/> for details.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-enable-partitionwise-join" xreflabel="enable_partitionwise_join">
<term><varname>enable_partitionwise_join</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>enable_partitionwise_join</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables or disables the query planner's use of partitionwise join,
which allows a join between partitioned tables to be performed by
joining the matching partitions. Partitionwise join currently applies
only when the join conditions include all the partition keys, which
must be of the same data type and have exactly matching sets of child
partitions. Because partitionwise join planning can use significantly
more CPU time and memory during planning, the default is
<literal>off</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-enable-partitionwise-aggregate" xreflabel="enable_partitionwise_aggregate">
<term><varname>enable_partitionwise_aggregate</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>enable_partitionwise_aggregate</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables or disables the query planner's use of partitionwise grouping
or aggregation, which allows grouping or aggregation on a partitioned
tables performed separately for each partition. If the <literal>GROUP
BY</literal> clause does not include the partition keys, only partial
aggregation can be performed on a per-partition basis, and
finalization must be performed later. Because partitionwise grouping
or aggregation can use significantly more CPU time and memory during
planning, the default is <literal>off</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-enable-seqscan" xreflabel="enable_seqscan">
<term><varname>enable_seqscan</varname> (<type>boolean</type>)
<indexterm>
<primary>sequential scan</primary>
</indexterm>
<indexterm>
<primary><varname>enable_seqscan</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables or disables the query planner's use of sequential scan
plan types. It is impossible to suppress sequential scans
entirely, but turning this variable off discourages the planner
from using one if there are other methods available. The
default is <literal>on</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-enable-sort" xreflabel="enable_sort">
<term><varname>enable_sort</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>enable_sort</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables or disables the query planner's use of explicit sort
steps. It is impossible to suppress explicit sorts entirely,
but turning this variable off discourages the planner from
using one if there are other methods available. The default
is <literal>on</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-enable-tidscan" xreflabel="enable_tidscan">
<term><varname>enable_tidscan</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>enable_tidscan</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables or disables the query planner's use of <acronym>TID</acronym>
scan plan types. The default is <literal>on</literal>.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-query-constants">
<title>Planner Cost Constants</title>
<para>
The <firstterm>cost</firstterm> variables described in this section are measured
on an arbitrary scale. Only their relative values matter, hence
scaling them all up or down by the same factor will result in no change
in the planner's choices. By default, these cost variables are based on
the cost of sequential page fetches; that is,
<varname>seq_page_cost</varname> is conventionally set to <literal>1.0</literal>
and the other cost variables are set with reference to that. But
you can use a different scale if you prefer, such as actual execution
times in milliseconds on a particular machine.
</para>
<note>
<para>
Unfortunately, there is no well-defined method for determining ideal
values for the cost variables. They are best treated as averages over
the entire mix of queries that a particular installation will receive. This
means that changing them on the basis of just a few experiments is very
risky.
</para>
</note>
<variablelist>
<varlistentry id="guc-seq-page-cost" xreflabel="seq_page_cost">
<term><varname>seq_page_cost</varname> (<type>floating point</type>)
<indexterm>
<primary><varname>seq_page_cost</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the planner's estimate of the cost of a disk page fetch
that is part of a series of sequential fetches. The default is 1.0.
This value can be overridden for tables and indexes in a particular
tablespace by setting the tablespace parameter of the same name
(see <xref linkend="sql-altertablespace"/>).
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-random-page-cost" xreflabel="random_page_cost">
<term><varname>random_page_cost</varname> (<type>floating point</type>)
<indexterm>
<primary><varname>random_page_cost</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the planner's estimate of the cost of a
non-sequentially-fetched disk page. The default is 4.0.
This value can be overridden for tables and indexes in a particular
tablespace by setting the tablespace parameter of the same name
(see <xref linkend="sql-altertablespace"/>).
</para>
<para>
Reducing this value relative to <varname>seq_page_cost</varname>
will cause the system to prefer index scans; raising it will
make index scans look relatively more expensive. You can raise
or lower both values together to change the importance of disk I/O
costs relative to CPU costs, which are described by the following
parameters.
</para>
<para>
Random access to mechanical disk storage is normally much more expensive
than four times sequential access. However, a lower default is used
(4.0) because the majority of random accesses to disk, such as indexed
reads, are assumed to be in cache. The default value can be thought of
as modeling random access as 40 times slower than sequential, while
expecting 90% of random reads to be cached.
</para>
<para>
If you believe a 90% cache rate is an incorrect assumption
for your workload, you can increase random_page_cost to better
reflect the true cost of random storage reads. Correspondingly,
if your data is likely to be completely in cache, such as when
the database is smaller than the total server memory, decreasing
random_page_cost can be appropriate. Storage that has a low random
read cost relative to sequential, e.g. solid-state drives, might
also be better modeled with a lower value for random_page_cost.
</para>
<tip>
<para>
Although the system will let you set <varname>random_page_cost</varname> to
less than <varname>seq_page_cost</varname>, it is not physically sensible
to do so. However, setting them equal makes sense if the database
is entirely cached in RAM, since in that case there is no penalty
for touching pages out of sequence. Also, in a heavily-cached
database you should lower both values relative to the CPU parameters,
since the cost of fetching a page already in RAM is much smaller
than it would normally be.
</para>
</tip>
</listitem>
</varlistentry>
<varlistentry id="guc-cpu-tuple-cost" xreflabel="cpu_tuple_cost">
<term><varname>cpu_tuple_cost</varname> (<type>floating point</type>)
<indexterm>
<primary><varname>cpu_tuple_cost</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the planner's estimate of the cost of processing
each row during a query.
The default is 0.01.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-cpu-index-tuple-cost" xreflabel="cpu_index_tuple_cost">
<term><varname>cpu_index_tuple_cost</varname> (<type>floating point</type>)
<indexterm>
<primary><varname>cpu_index_tuple_cost</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the planner's estimate of the cost of processing
each index entry during an index scan.
The default is 0.005.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-cpu-operator-cost" xreflabel="cpu_operator_cost">
<term><varname>cpu_operator_cost</varname> (<type>floating point</type>)
<indexterm>
<primary><varname>cpu_operator_cost</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the planner's estimate of the cost of processing each
operator or function executed during a query.
The default is 0.0025.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-parallel-setup-cost" xreflabel="parallel_setup_cost">
<term><varname>parallel_setup_cost</varname> (<type>floating point</type>)
<indexterm>
<primary><varname>parallel_setup_cost</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the planner's estimate of the cost of launching parallel worker
processes.
The default is 1000.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-parallel-tuple-cost" xreflabel="parallel_tuple_cost">
<term><varname>parallel_tuple_cost</varname> (<type>floating point</type>)
<indexterm>
<primary><varname>parallel_tuple_cost</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the planner's estimate of the cost of transferring one tuple
from a parallel worker process to another process.
The default is 0.1.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-min-parallel-table-scan-size" xreflabel="min_parallel_table_scan_size">
<term><varname>min_parallel_table_scan_size</varname> (<type>integer</type>)
<indexterm>
<primary><varname>min_parallel_table_scan_size</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the minimum amount of table data that must be scanned in order
for a parallel scan to be considered. For a parallel sequential scan,
the amount of table data scanned is always equal to the size of the
table, but when indexes are used the amount of table data
scanned will normally be less.
If this value is specified without units, it is taken as blocks,
that is <symbol>BLCKSZ</symbol> bytes, typically 8kB.
The default is 8 megabytes (<literal>8MB</literal>).
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-min-parallel-index-scan-size" xreflabel="min_parallel_index_scan_size">
<term><varname>min_parallel_index_scan_size</varname> (<type>integer</type>)
<indexterm>
<primary><varname>min_parallel_index_scan_size</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the minimum amount of index data that must be scanned in order
for a parallel scan to be considered. Note that a parallel index scan
typically won't touch the entire index; it is the number of pages
which the planner believes will actually be touched by the scan which
is relevant. This parameter is also used to decide whether a
particular index can participate in a parallel vacuum. See
<xref linkend="sql-vacuum"/>.
If this value is specified without units, it is taken as blocks,
that is <symbol>BLCKSZ</symbol> bytes, typically 8kB.
The default is 512 kilobytes (<literal>512kB</literal>).
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-effective-cache-size" xreflabel="effective_cache_size">
<term><varname>effective_cache_size</varname> (<type>integer</type>)
<indexterm>
<primary><varname>effective_cache_size</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the planner's assumption about the effective size of the
disk cache that is available to a single query. This is
factored into estimates of the cost of using an index; a
higher value makes it more likely index scans will be used, a
lower value makes it more likely sequential scans will be
used. When setting this parameter you should consider both
<productname>PostgreSQL</productname>'s shared buffers and the
portion of the kernel's disk cache that will be used for
<productname>PostgreSQL</productname> data files, though some
data might exist in both places. Also, take
into account the expected number of concurrent queries on different
tables, since they will have to share the available
space. This parameter has no effect on the size of shared
memory allocated by <productname>PostgreSQL</productname>, nor
does it reserve kernel disk cache; it is used only for estimation
purposes. The system also does not assume data remains in
the disk cache between queries.
If this value is specified without units, it is taken as blocks,
that is <symbol>BLCKSZ</symbol> bytes, typically 8kB.
The default is 4 gigabytes (<literal>4GB</literal>).
(If <symbol>BLCKSZ</symbol> is not 8kB, the default value scales
proportionally to it.)
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-jit-above-cost" xreflabel="jit_above_cost">
<term><varname>jit_above_cost</varname> (<type>floating point</type>)
<indexterm>
<primary><varname>jit_above_cost</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the query cost above which JIT compilation is activated, if
enabled (see <xref linkend="jit"/>).
Performing <acronym>JIT</acronym> costs planning time but can
accelerate query execution.
Setting this to <literal>-1</literal> disables JIT compilation.
The default is <literal>100000</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-jit-inline-above-cost" xreflabel="jit_inline_above_cost">
<term><varname>jit_inline_above_cost</varname> (<type>floating point</type>)
<indexterm>
<primary><varname>jit_inline_above_cost</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the query cost above which JIT compilation attempts to inline
functions and operators. Inlining adds planning time, but can
improve execution speed. It is not meaningful to set this to less
than <varname>jit_above_cost</varname>.
Setting this to <literal>-1</literal> disables inlining.
The default is <literal>500000</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-jit-optimize-above-cost" xreflabel="jit_optimize_above_cost">
<term><varname>jit_optimize_above_cost</varname> (<type>floating point</type>)
<indexterm>
<primary><varname>jit_optimize_above_cost</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the query cost above which JIT compilation applies expensive
optimizations. Such optimization adds planning time, but can improve
execution speed. It is not meaningful to set this to less
than <varname>jit_above_cost</varname>, and it is unlikely to be
beneficial to set it to more
than <varname>jit_inline_above_cost</varname>.
Setting this to <literal>-1</literal> disables expensive optimizations.
The default is <literal>500000</literal>.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-query-geqo">
<title>Genetic Query Optimizer</title>
<para>
The genetic query optimizer (GEQO) is an algorithm that does query
planning using heuristic searching. This reduces planning time for
complex queries (those joining many relations), at the cost of producing
plans that are sometimes inferior to those found by the normal
exhaustive-search algorithm.
For more information see <xref linkend="geqo"/>.
</para>
<variablelist>
<varlistentry id="guc-geqo" xreflabel="geqo">
<term><varname>geqo</varname> (<type>boolean</type>)
<indexterm>
<primary>genetic query optimization</primary>
</indexterm>
<indexterm>
<primary>GEQO</primary>
<see>genetic query optimization</see>
</indexterm>
<indexterm>
<primary><varname>geqo</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables or disables genetic query optimization.
This is on by default. It is usually best not to turn it off in
production; the <varname>geqo_threshold</varname> variable provides
more granular control of GEQO.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-geqo-threshold" xreflabel="geqo_threshold">
<term><varname>geqo_threshold</varname> (<type>integer</type>)
<indexterm>
<primary><varname>geqo_threshold</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Use genetic query optimization to plan queries with at least
this many <literal>FROM</literal> items involved. (Note that a
<literal>FULL OUTER JOIN</literal> construct counts as only one <literal>FROM</literal>
item.) The default is 12. For simpler queries it is usually best
to use the regular, exhaustive-search planner, but for queries with
many tables the exhaustive search takes too long, often
longer than the penalty of executing a suboptimal plan. Thus,
a threshold on the size of the query is a convenient way to manage
use of GEQO.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-geqo-effort" xreflabel="geqo_effort">
<term><varname>geqo_effort</varname> (<type>integer</type>)
<indexterm>
<primary><varname>geqo_effort</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Controls the trade-off between planning time and query plan
quality in GEQO. This variable must be an integer in the
range from 1 to 10. The default value is five. Larger values
increase the time spent doing query planning, but also
increase the likelihood that an efficient query plan will be
chosen.
</para>
<para>
<varname>geqo_effort</varname> doesn't actually do anything
directly; it is only used to compute the default values for
the other variables that influence GEQO behavior (described
below). If you prefer, you can set the other parameters by
hand instead.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-geqo-pool-size" xreflabel="geqo_pool_size">
<term><varname>geqo_pool_size</varname> (<type>integer</type>)
<indexterm>
<primary><varname>geqo_pool_size</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Controls the pool size used by GEQO, that is the
number of individuals in the genetic population. It must be
at least two, and useful values are typically 100 to 1000. If
it is set to zero (the default setting) then a suitable
value is chosen based on <varname>geqo_effort</varname> and
the number of tables in the query.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-geqo-generations" xreflabel="geqo_generations">
<term><varname>geqo_generations</varname> (<type>integer</type>)
<indexterm>
<primary><varname>geqo_generations</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Controls the number of generations used by GEQO, that is
the number of iterations of the algorithm. It must
be at least one, and useful values are in the same range as
the pool size. If it is set to zero (the default setting)
then a suitable value is chosen based on
<varname>geqo_pool_size</varname>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-geqo-selection-bias" xreflabel="geqo_selection_bias">
<term><varname>geqo_selection_bias</varname> (<type>floating point</type>)
<indexterm>
<primary><varname>geqo_selection_bias</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Controls the selection bias used by GEQO. The selection bias
is the selective pressure within the population. Values can be
from 1.50 to 2.00; the latter is the default.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-geqo-seed" xreflabel="geqo_seed">
<term><varname>geqo_seed</varname> (<type>floating point</type>)
<indexterm>
<primary><varname>geqo_seed</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Controls the initial value of the random number generator used
by GEQO to select random paths through the join order search space.
The value can range from zero (the default) to one. Varying the
value changes the set of join paths explored, and may result in a
better or worse best path being found.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-query-other">
<title>Other Planner Options</title>
<variablelist>
<varlistentry id="guc-default-statistics-target" xreflabel="default_statistics_target">
<term><varname>default_statistics_target</varname> (<type>integer</type>)
<indexterm>
<primary><varname>default_statistics_target</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the default statistics target for table columns without
a column-specific target set via <command>ALTER TABLE
SET STATISTICS</command>. Larger values increase the time needed to
do <command>ANALYZE</command>, but might improve the quality of the
planner's estimates. The default is 100. For more information
on the use of statistics by the <productname>PostgreSQL</productname>
query planner, refer to <xref linkend="planner-stats"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-constraint-exclusion" xreflabel="constraint_exclusion">
<term><varname>constraint_exclusion</varname> (<type>enum</type>)
<indexterm>
<primary>constraint exclusion</primary>
</indexterm>
<indexterm>
<primary><varname>constraint_exclusion</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Controls the query planner's use of table constraints to
optimize queries.
The allowed values of <varname>constraint_exclusion</varname> are
<literal>on</literal> (examine constraints for all tables),
<literal>off</literal> (never examine constraints), and
<literal>partition</literal> (examine constraints only for inheritance
child tables and <literal>UNION ALL</literal> subqueries).
<literal>partition</literal> is the default setting.
It is often used with traditional inheritance trees to improve
performance.
</para>
<para>
When this parameter allows it for a particular table, the planner
compares query conditions with the table's <literal>CHECK</literal>
constraints, and omits scanning tables for which the conditions
contradict the constraints. For example:
<programlisting>
CREATE TABLE parent(key integer, ...);
CREATE TABLE child1000(check (key between 1000 and 1999)) INHERITS(parent);
CREATE TABLE child2000(check (key between 2000 and 2999)) INHERITS(parent);
...
SELECT * FROM parent WHERE key = 2400;
</programlisting>
With constraint exclusion enabled, this <command>SELECT</command>
will not scan <structname>child1000</structname> at all, improving performance.
</para>
<para>
Currently, constraint exclusion is enabled by default
only for cases that are often used to implement table partitioning via
inheritance trees. Turning it on for all tables imposes extra
planning overhead that is quite noticeable on simple queries, and most
often will yield no benefit for simple queries. If you have no
tables that are partitioned using traditional inheritance, you might
prefer to turn it off entirely. (Note that the equivalent feature for
partitioned tables is controlled by a separate parameter,
<xref linkend="guc-enable-partition-pruning"/>.)
</para>
<para>
Refer to <xref linkend="ddl-partitioning-constraint-exclusion"/> for
more information on using constraint exclusion to implement
partitioning.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-cursor-tuple-fraction" xreflabel="cursor_tuple_fraction">
<term><varname>cursor_tuple_fraction</varname> (<type>floating point</type>)
<indexterm>
<primary><varname>cursor_tuple_fraction</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the planner's estimate of the fraction of a cursor's rows that
will be retrieved. The default is 0.1. Smaller values of this
setting bias the planner towards using <quote>fast start</quote> plans
for cursors, which will retrieve the first few rows quickly while
perhaps taking a long time to fetch all rows. Larger values
put more emphasis on the total estimated time. At the maximum
setting of 1.0, cursors are planned exactly like regular queries,
considering only the total estimated time and not how soon the
first rows might be delivered.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-from-collapse-limit" xreflabel="from_collapse_limit">
<term><varname>from_collapse_limit</varname> (<type>integer</type>)
<indexterm>
<primary><varname>from_collapse_limit</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The planner will merge sub-queries into upper queries if the
resulting <literal>FROM</literal> list would have no more than
this many items. Smaller values reduce planning time but might
yield inferior query plans. The default is eight.
For more information see <xref linkend="explicit-joins"/>.
</para>
<para>
Setting this value to <xref linkend="guc-geqo-threshold"/> or more
may trigger use of the GEQO planner, resulting in non-optimal
plans. See <xref linkend="runtime-config-query-geqo"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-jit" xreflabel="jit">
<term><varname>jit</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>jit</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Determines whether <acronym>JIT</acronym> compilation may be used by
<productname>PostgreSQL</productname>, if available (see <xref
linkend="jit"/>).
The default is <literal>on</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-join-collapse-limit" xreflabel="join_collapse_limit">
<term><varname>join_collapse_limit</varname> (<type>integer</type>)
<indexterm>
<primary><varname>join_collapse_limit</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The planner will rewrite explicit <literal>JOIN</literal>
constructs (except <literal>FULL JOIN</literal>s) into lists of
<literal>FROM</literal> items whenever a list of no more than this many items
would result. Smaller values reduce planning time but might
yield inferior query plans.
</para>
<para>
By default, this variable is set the same as
<varname>from_collapse_limit</varname>, which is appropriate
for most uses. Setting it to 1 prevents any reordering of
explicit <literal>JOIN</literal>s. Thus, the explicit join order
specified in the query will be the actual order in which the
relations are joined. Because the query planner does not always choose
the optimal join order, advanced users can elect to
temporarily set this variable to 1, and then specify the join
order they desire explicitly.
For more information see <xref linkend="explicit-joins"/>.
</para>
<para>
Setting this value to <xref linkend="guc-geqo-threshold"/> or more
may trigger use of the GEQO planner, resulting in non-optimal
plans. See <xref linkend="runtime-config-query-geqo"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-parallel-leader-participation" xreflabel="parallel_leader_participation">
<term>
<varname>parallel_leader_participation</varname> (<type>boolean</type>)
<indexterm>
<primary>
<varname>parallel_leader_participation</varname> configuration
parameter
</primary>
</indexterm>
</term>
<listitem>
<para>
Allows the leader process to execute the query plan under
<literal>Gather</literal> and <literal>Gather Merge</literal> nodes
instead of waiting for worker processes. The default is
<literal>on</literal>. Setting this value to <literal>off</literal>
reduces the likelihood that workers will become blocked because the
leader is not reading tuples fast enough, but requires the leader
process to wait for worker processes to start up before the first
tuples can be produced. The degree to which the leader can help or
hinder performance depends on the plan type, number of workers and
query duration.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-force-parallel-mode" xreflabel="force_parallel_mode">
<term><varname>force_parallel_mode</varname> (<type>enum</type>)
<indexterm>
<primary><varname>force_parallel_mode</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Allows the use of parallel queries for testing purposes even in cases
where no performance benefit is expected.
The allowed values of <varname>force_parallel_mode</varname> are
<literal>off</literal> (use parallel mode only when it is expected to improve
performance), <literal>on</literal> (force parallel query for all queries
for which it is thought to be safe), and <literal>regress</literal> (like
<literal>on</literal>, but with additional behavior changes as explained
below).
</para>
<para>
More specifically, setting this value to <literal>on</literal> will add
a <literal>Gather</literal> node to the top of any query plan for which this
appears to be safe, so that the query runs inside of a parallel worker.
Even when a parallel worker is not available or cannot be used,
operations such as starting a subtransaction that would be prohibited
in a parallel query context will be prohibited unless the planner
believes that this will cause the query to fail. If failures or
unexpected results occur when this option is set, some functions used
by the query may need to be marked <literal>PARALLEL UNSAFE</literal>
(or, possibly, <literal>PARALLEL RESTRICTED</literal>).
</para>
<para>
Setting this value to <literal>regress</literal> has all of the same effects
as setting it to <literal>on</literal> plus some additional effects that are
intended to facilitate automated regression testing. Normally,
messages from a parallel worker include a context line indicating that,
but a setting of <literal>regress</literal> suppresses this line so that the
output is the same as in non-parallel execution. Also,
the <literal>Gather</literal> nodes added to plans by this setting are hidden
in <literal>EXPLAIN</literal> output so that the output matches what
would be obtained if this setting were turned <literal>off</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-plan-cache_mode" xreflabel="plan_cache_mode">
<term><varname>plan_cache_mode</varname> (<type>enum</type>)
<indexterm>
<primary><varname>plan_cache_mode</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Prepared statements (either explicitly prepared or implicitly
generated, for example by PL/pgSQL) can be executed using custom or
generic plans. Custom plans are made afresh for each execution
using its specific set of parameter values, while generic plans do
not rely on the parameter values and can be re-used across
executions. Thus, use of a generic plan saves planning time, but if
the ideal plan depends strongly on the parameter values then a
generic plan may be inefficient. The choice between these options
is normally made automatically, but it can be overridden
with <varname>plan_cache_mode</varname>.
The allowed values are <literal>auto</literal> (the default),
<literal>force_custom_plan</literal> and
<literal>force_generic_plan</literal>.
This setting is considered when a cached plan is to be executed,
not when it is prepared.
For more information see <xref linkend="sql-prepare"/>.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
</sect1>
<sect1 id="runtime-config-logging">
<title>Error Reporting and Logging</title>
<indexterm zone="runtime-config-logging">
<primary>server log</primary>
</indexterm>
<sect2 id="runtime-config-logging-where">
<title>Where to Log</title>
<indexterm zone="runtime-config-logging-where">
<primary>where to log</primary>
</indexterm>
<indexterm>
<primary>current_logfiles</primary>
<secondary>and the log_destination configuration parameter</secondary>
</indexterm>
<variablelist>
<varlistentry id="guc-log-destination" xreflabel="log_destination">
<term><varname>log_destination</varname> (<type>string</type>)
<indexterm>
<primary><varname>log_destination</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
<productname>PostgreSQL</productname> supports several methods
for logging server messages, including
<systemitem>stderr</systemitem>, <systemitem>csvlog</systemitem> and
<systemitem>syslog</systemitem>. On Windows,
<systemitem>eventlog</systemitem> is also supported. Set this
parameter to a list of desired log destinations separated by
commas. The default is to log to <systemitem>stderr</systemitem>
only.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
<para>
If <systemitem>csvlog</systemitem> is included in <varname>log_destination</varname>,
log entries are output in <quote>comma separated
value</quote> (<acronym>CSV</acronym>) format, which is convenient for
loading logs into programs.
See <xref linkend="runtime-config-logging-csvlog"/> for details.
<xref linkend="guc-logging-collector"/> must be enabled to generate
CSV-format log output.
</para>
<para>
When either <systemitem>stderr</systemitem> or
<systemitem>csvlog</systemitem> are included, the file
<filename>current_logfiles</filename> is created to record the location
of the log file(s) currently in use by the logging collector and the
associated logging destination. This provides a convenient way to
find the logs currently in use by the instance. Here is an example of
this file's content:
<programlisting>
stderr log/postgresql.log
csvlog log/postgresql.csv
</programlisting>
<filename>current_logfiles</filename> is recreated when a new log file
is created as an effect of rotation, and
when <varname>log_destination</varname> is reloaded. It is removed when
neither <systemitem>stderr</systemitem>
nor <systemitem>csvlog</systemitem> are included
in <varname>log_destination</varname>, and when the logging collector is
disabled.
</para>
<note>
<para>
On most Unix systems, you will need to alter the configuration of
your system's <application>syslog</application> daemon in order
to make use of the <systemitem>syslog</systemitem> option for
<varname>log_destination</varname>. <productname>PostgreSQL</productname>
can log to <application>syslog</application> facilities
<literal>LOCAL0</literal> through <literal>LOCAL7</literal> (see <xref
linkend="guc-syslog-facility"/>), but the default
<application>syslog</application> configuration on most platforms
will discard all such messages. You will need to add something like:
<programlisting>
local0.* /var/log/postgresql
</programlisting>
to the <application>syslog</application> daemon's configuration file
to make it work.
</para>
<para>
On Windows, when you use the <literal>eventlog</literal>
option for <varname>log_destination</varname>, you should
register an event source and its library with the operating
system so that the Windows Event Viewer can display event
log messages cleanly.
See <xref linkend="event-log-registration"/> for details.
</para>
</note>
</listitem>
</varlistentry>
<varlistentry id="guc-logging-collector" xreflabel="logging_collector">
<term><varname>logging_collector</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>logging_collector</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This parameter enables the <firstterm>logging collector</firstterm>, which
is a background process that captures log messages
sent to <systemitem>stderr</systemitem> and redirects them into log files.
This approach is often more useful than
logging to <application>syslog</application>, since some types of messages
might not appear in <application>syslog</application> output. (One common
example is dynamic-linker failure messages; another is error messages
produced by scripts such as <varname>archive_command</varname>.)
This parameter can only be set at server start.
</para>
<note>
<para>
It is possible to log to <systemitem>stderr</systemitem> without using the
logging collector; the log messages will just go to wherever the
server's <systemitem>stderr</systemitem> is directed. However, that method is
only suitable for low log volumes, since it provides no convenient
way to rotate log files. Also, on some platforms not using the
logging collector can result in lost or garbled log output, because
multiple processes writing concurrently to the same log file can
overwrite each other's output.
</para>
</note>
<note>
<para>
The logging collector is designed to never lose messages. This means
that in case of extremely high load, server processes could be
blocked while trying to send additional log messages when the
collector has fallen behind. In contrast, <application>syslog</application>
prefers to drop messages if it cannot write them, which means it
may fail to log some messages in such cases but it will not block
the rest of the system.
</para>
</note>
</listitem>
</varlistentry>
<varlistentry id="guc-log-directory" xreflabel="log_directory">
<term><varname>log_directory</varname> (<type>string</type>)
<indexterm>
<primary><varname>log_directory</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When <varname>logging_collector</varname> is enabled,
this parameter determines the directory in which log files will be created.
It can be specified as an absolute path, or relative to the
cluster data directory.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
The default is <literal>log</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-log-filename" xreflabel="log_filename">
<term><varname>log_filename</varname> (<type>string</type>)
<indexterm>
<primary><varname>log_filename</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When <varname>logging_collector</varname> is enabled,
this parameter sets the file names of the created log files. The value
is treated as a <function>strftime</function> pattern,
so <literal>%</literal>-escapes can be used to specify time-varying
file names. (Note that if there are
any time-zone-dependent <literal>%</literal>-escapes, the computation
is done in the zone specified
by <xref linkend="guc-log-timezone"/>.)
The supported <literal>%</literal>-escapes are similar to those
listed in the Open Group's <ulink
url="http://pubs.opengroup.org/onlinepubs/009695399/functions/strftime.html">strftime
</ulink> specification.
Note that the system's <function>strftime</function> is not used
directly, so platform-specific (nonstandard) extensions do not work.
The default is <literal>postgresql-%Y-%m-%d_%H%M%S.log</literal>.
</para>
<para>
If you specify a file name without escapes, you should plan to
use a log rotation utility to avoid eventually filling the
entire disk. In releases prior to 8.4, if
no <literal>%</literal> escapes were
present, <productname>PostgreSQL</productname> would append
the epoch of the new log file's creation time, but this is no
longer the case.
</para>
<para>
If CSV-format output is enabled in <varname>log_destination</varname>,
<literal>.csv</literal> will be appended to the timestamped
log file name to create the file name for CSV-format output.
(If <varname>log_filename</varname> ends in <literal>.log</literal>, the suffix is
replaced instead.)
</para>
<para>
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-log-file-mode" xreflabel="log_file_mode">
<term><varname>log_file_mode</varname> (<type>integer</type>)
<indexterm>
<primary><varname>log_file_mode</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
On Unix systems this parameter sets the permissions for log files
when <varname>logging_collector</varname> is enabled. (On Microsoft
Windows this parameter is ignored.)
The parameter value is expected to be a numeric mode
specified in the format accepted by the
<function>chmod</function> and <function>umask</function>
system calls. (To use the customary octal format the number
must start with a <literal>0</literal> (zero).)
</para>
<para>
The default permissions are <literal>0600</literal>, meaning only the
server owner can read or write the log files. The other commonly
useful setting is <literal>0640</literal>, allowing members of the owner's
group to read the files. Note however that to make use of such a
setting, you'll need to alter <xref linkend="guc-log-directory"/> to
store the files somewhere outside the cluster data directory. In
any case, it's unwise to make the log files world-readable, since
they might contain sensitive data.
</para>
<para>
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-log-rotation-age" xreflabel="log_rotation_age">
<term><varname>log_rotation_age</varname> (<type>integer</type>)
<indexterm>
<primary><varname>log_rotation_age</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When <varname>logging_collector</varname> is enabled,
this parameter determines the maximum amount of time to use an
individual log file, after which a new log file will be created.
If this value is specified without units, it is taken as minutes.
The default is 24 hours.
Set to zero to disable time-based creation of new log files.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-log-rotation-size" xreflabel="log_rotation_size">
<term><varname>log_rotation_size</varname> (<type>integer</type>)
<indexterm>
<primary><varname>log_rotation_size</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When <varname>logging_collector</varname> is enabled,
this parameter determines the maximum size of an individual log file.
After this amount of data has been emitted into a log file,
a new log file will be created.
If this value is specified without units, it is taken as kilobytes.
The default is 10 megabytes.
Set to zero to disable size-based creation of new log files.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-log-truncate-on-rotation" xreflabel="log_truncate_on_rotation">
<term><varname>log_truncate_on_rotation</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>log_truncate_on_rotation</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When <varname>logging_collector</varname> is enabled,
this parameter will cause <productname>PostgreSQL</productname> to truncate (overwrite),
rather than append to, any existing log file of the same name.
However, truncation will occur only when a new file is being opened
due to time-based rotation, not during server startup or size-based
rotation. When off, pre-existing files will be appended to in
all cases. For example, using this setting in combination with
a <varname>log_filename</varname> like <literal>postgresql-%H.log</literal>
would result in generating twenty-four hourly log files and then
cyclically overwriting them.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
<para>
Example: To keep 7 days of logs, one log file per day named
<literal>server_log.Mon</literal>, <literal>server_log.Tue</literal>,
etc, and automatically overwrite last week's log with this week's log,
set <varname>log_filename</varname> to <literal>server_log.%a</literal>,
<varname>log_truncate_on_rotation</varname> to <literal>on</literal>, and
<varname>log_rotation_age</varname> to <literal>1440</literal>.
</para>
<para>
Example: To keep 24 hours of logs, one log file per hour, but
also rotate sooner if the log file size exceeds 1GB, set
<varname>log_filename</varname> to <literal>server_log.%H%M</literal>,
<varname>log_truncate_on_rotation</varname> to <literal>on</literal>,
<varname>log_rotation_age</varname> to <literal>60</literal>, and
<varname>log_rotation_size</varname> to <literal>1000000</literal>.
Including <literal>%M</literal> in <varname>log_filename</varname> allows
any size-driven rotations that might occur to select a file name
different from the hour's initial file name.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-syslog-facility" xreflabel="syslog_facility">
<term><varname>syslog_facility</varname> (<type>enum</type>)
<indexterm>
<primary><varname>syslog_facility</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When logging to <application>syslog</application> is enabled, this parameter
determines the <application>syslog</application>
<quote>facility</quote> to be used. You can choose
from <literal>LOCAL0</literal>, <literal>LOCAL1</literal>,
<literal>LOCAL2</literal>, <literal>LOCAL3</literal>, <literal>LOCAL4</literal>,
<literal>LOCAL5</literal>, <literal>LOCAL6</literal>, <literal>LOCAL7</literal>;
the default is <literal>LOCAL0</literal>. See also the
documentation of your system's
<application>syslog</application> daemon.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-syslog-ident" xreflabel="syslog_ident">
<term><varname>syslog_ident</varname> (<type>string</type>)
<indexterm>
<primary><varname>syslog_ident</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When logging to <application>syslog</application> is enabled, this parameter
determines the program name used to identify
<productname>PostgreSQL</productname> messages in
<application>syslog</application> logs. The default is
<literal>postgres</literal>.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-syslog-sequence-numbers" xreflabel="syslog_sequence_numbers">
<term><varname>syslog_sequence_numbers</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>syslog_sequence_numbers</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When logging to <application>syslog</application> and this is on (the
default), then each message will be prefixed by an increasing
sequence number (such as <literal>[2]</literal>). This circumvents
the <quote>--- last message repeated N times ---</quote> suppression
that many syslog implementations perform by default. In more modern
syslog implementations, repeated message suppression can be configured
(for example, <literal>$RepeatedMsgReduction</literal>
in <productname>rsyslog</productname>), so this might not be
necessary. Also, you could turn this off if you actually want to
suppress repeated messages.
</para>
<para>
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-syslog-split-messages" xreflabel="syslog_split_messages">
<term><varname>syslog_split_messages</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>syslog_split_messages</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When logging to <application>syslog</application> is enabled, this parameter
determines how messages are delivered to syslog. When on (the
default), messages are split by lines, and long lines are split so
that they will fit into 1024 bytes, which is a typical size limit for
traditional syslog implementations. When off, PostgreSQL server log
messages are delivered to the syslog service as is, and it is up to
the syslog service to cope with the potentially bulky messages.
</para>
<para>
If syslog is ultimately logging to a text file, then the effect will
be the same either way, and it is best to leave the setting on, since
most syslog implementations either cannot handle large messages or
would need to be specially configured to handle them. But if syslog
is ultimately writing into some other medium, it might be necessary or
more useful to keep messages logically together.
</para>
<para>
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-event-source" xreflabel="event_source">
<term><varname>event_source</varname> (<type>string</type>)
<indexterm>
<primary><varname>event_source</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When logging to <application>event log</application> is enabled, this parameter
determines the program name used to identify
<productname>PostgreSQL</productname> messages in
the log. The default is <literal>PostgreSQL</literal>.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-logging-when">
<title>When to Log</title>
<variablelist>
<varlistentry id="guc-log-min-messages" xreflabel="log_min_messages">
<term><varname>log_min_messages</varname> (<type>enum</type>)
<indexterm>
<primary><varname>log_min_messages</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Controls which <link linkend="runtime-config-severity-levels">message
levels</link> are written to the server log.
Valid values are <literal>DEBUG5</literal>, <literal>DEBUG4</literal>,
<literal>DEBUG3</literal>, <literal>DEBUG2</literal>, <literal>DEBUG1</literal>,
<literal>INFO</literal>, <literal>NOTICE</literal>, <literal>WARNING</literal>,
<literal>ERROR</literal>, <literal>LOG</literal>, <literal>FATAL</literal>, and
<literal>PANIC</literal>. Each level includes all the levels that
follow it. The later the level, the fewer messages are sent
to the log. The default is <literal>WARNING</literal>. Note that
<literal>LOG</literal> has a different rank here than in
<xref linkend="guc-client-min-messages"/>.
Only superusers can change this setting.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-log-min-error-statement" xreflabel="log_min_error_statement">
<term><varname>log_min_error_statement</varname> (<type>enum</type>)
<indexterm>
<primary><varname>log_min_error_statement</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Controls which SQL statements that cause an error
condition are recorded in the server log. The current
SQL statement is included in the log entry for any message of
the specified
<link linkend="runtime-config-severity-levels">severity</link>
or higher.
Valid values are <literal>DEBUG5</literal>,
<literal>DEBUG4</literal>, <literal>DEBUG3</literal>,
<literal>DEBUG2</literal>, <literal>DEBUG1</literal>,
<literal>INFO</literal>, <literal>NOTICE</literal>,
<literal>WARNING</literal>, <literal>ERROR</literal>,
<literal>LOG</literal>,
<literal>FATAL</literal>, and <literal>PANIC</literal>.
The default is <literal>ERROR</literal>, which means statements
causing errors, log messages, fatal errors, or panics will be logged.
To effectively turn off logging of failing statements,
set this parameter to <literal>PANIC</literal>.
Only superusers can change this setting.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-log-min-duration-statement" xreflabel="log_min_duration_statement">
<term><varname>log_min_duration_statement</varname> (<type>integer</type>)
<indexterm>
<primary><varname>log_min_duration_statement</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Causes the duration of each completed statement to be logged
if the statement ran for at least the specified amount of time.
If this value is specified without units, it is taken as milliseconds.
Setting this to zero prints all statement durations.
Minus-one (the default) disables logging statement durations.
For example, if you set it to <literal>250ms</literal>
then all SQL statements that run 250ms or longer will be
logged. Enabling this parameter can be helpful in tracking down
unoptimized queries in your applications.
Only superusers can change this setting.
</para>
<para>
This overrides <xref linkend="guc-log-min-duration-sample"/>,
meaning that queries with duration exceeding this setting are not
subject to sampling and are always logged.
</para>
<para>
For clients using extended query protocol, durations of the Parse,
Bind, and Execute steps are logged independently.
</para>
<note>
<para>
When using this option together with
<xref linkend="guc-log-statement"/>,
the text of statements that are logged because of
<varname>log_statement</varname> will not be repeated in the
duration log message.
If you are not using <application>syslog</application>, it is recommended
that you log the PID or session ID using
<xref linkend="guc-log-line-prefix"/>
so that you can link the statement message to the later
duration message using the process ID or session ID.
</para>
</note>
</listitem>
</varlistentry>
<varlistentry id="guc-log-min-duration-sample" xreflabel="log_min_duration_sample">
<term><varname>log_min_duration_sample</varname> (<type>integer</type>)
<indexterm>
<primary><varname>log_min_duration_sample</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Allows to sample the logging of the duration of each completed
statement if the statement ran for at least the specified amount of
time. If this value is specified without units, it is taken as milliseconds.
Setting this to zero samples all statement durations.
Minus-one (the default) disables sampling statement durations.
For example, if you set it to <literal>250ms</literal>
then all SQL statements that run 250ms or longer will be considered
for sampling, with sample rate is controlled by <xref linkend="guc-log-statement-sample-rate"/>.
Enabling this parameter can be helpful when the traffic too high to
sample all queries.
Only superusers can change this setting.
</para>
<para>
This option has lower priority than <xref linkend="guc-log-min-duration-statement"/>,
meaning that statements with durations exceeding <xref linkend="guc-log-min-duration-statement"/>
are not subject to sampling and are always logged.
</para>
<para>
For clients using extended query protocol, durations of the Parse,
Bind, and Execute steps are logged independently.
</para>
<note>
<para>
When using this option together with
<xref linkend="guc-log-statement"/>,
the text of statements that are logged because of
<varname>log_statement</varname> will not be repeated in the
duration log message.
If you are not using <application>syslog</application>, it is recommended
that you log the PID or session ID using
<xref linkend="guc-log-line-prefix"/>
so that you can link the statement message to the later
duration message using the process ID or session ID.
</para>
</note>
</listitem>
</varlistentry>
<varlistentry id="guc-log-statement-sample-rate" xreflabel="log_statement_sample_rate">
<term><varname>log_statement_sample_rate</varname> (<type>real</type>)
<indexterm>
<primary><varname>log_statement_sample_rate</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Determines the fraction of statements with duration exceeding
<xref linkend="guc-log-min-duration-sample"/> to be logged.
This is a statistical parameter, for example <literal>0.5</literal>
means there is statistically one in two chances to log the statement.
The default is <literal>1.0</literal>, meaning log all such
statements.
Setting this to zero disables sampling logging, same as setting
<varname>log_min_duration_sample</varname> to
<literal>-1</literal>.
<varname>log_statement_sample_rate</varname> is helpful when the
traffic is too high to log all queries.
Only superusers can change this setting.
</para>
<note>
<para>
Like all statement-logging options, this option can add significant
overhead.
</para>
</note>
</listitem>
</varlistentry>
<varlistentry id="guc-log-transaction-sample-rate" xreflabel="log_transaction_sample_rate">
<term><varname>log_transaction_sample_rate</varname> (<type>real</type>)
<indexterm>
<primary><varname>log_transaction_sample_rate</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Set the fraction of transactions whose statements are all logged,
in addition to statements logged for other reasons. It applies to
each new transaction regardless of its statements' durations.
The default is <literal>0</literal>, meaning not to log statements
from any additional transaction. Setting this to <literal>1</literal>
logs all statements for all transactions.
<varname>log_transaction_sample_rate</varname> is helpful to track a
sample of transaction.
Only superusers can change this setting.
</para>
<note>
<para>
Like all statement-logging options, this option can add significant
overhead.
</para>
</note>
</listitem>
</varlistentry>
</variablelist>
<para>
<xref linkend="runtime-config-severity-levels"/> explains the message
severity levels used by <productname>PostgreSQL</productname>. If logging output
is sent to <systemitem>syslog</systemitem> or Windows'
<systemitem>eventlog</systemitem>, the severity levels are translated
as shown in the table.
</para>
<table id="runtime-config-severity-levels">
<title>Message Severity Levels</title>
<tgroup cols="4">
<thead>
<row>
<entry>Severity</entry>
<entry>Usage</entry>
<entry><systemitem>syslog</systemitem></entry>
<entry><systemitem>eventlog</systemitem></entry>
</row>
</thead>
<tbody>
<row>
<entry><literal>DEBUG1..DEBUG5</literal></entry>
<entry>Provides successively-more-detailed information for use by
developers.</entry>
<entry><literal>DEBUG</literal></entry>
<entry><literal>INFORMATION</literal></entry>
</row>
<row>
<entry><literal>INFO</literal></entry>
<entry>Provides information implicitly requested by the user,
e.g., output from <command>VACUUM VERBOSE</command>.</entry>
<entry><literal>INFO</literal></entry>
<entry><literal>INFORMATION</literal></entry>
</row>
<row>
<entry><literal>NOTICE</literal></entry>
<entry>Provides information that might be helpful to users, e.g.,
notice of truncation of long identifiers.</entry>
<entry><literal>NOTICE</literal></entry>
<entry><literal>INFORMATION</literal></entry>
</row>
<row>
<entry><literal>WARNING</literal></entry>
<entry>Provides warnings of likely problems, e.g., <command>COMMIT</command>
outside a transaction block.</entry>
<entry><literal>NOTICE</literal></entry>
<entry><literal>WARNING</literal></entry>
</row>
<row>
<entry><literal>ERROR</literal></entry>
<entry>Reports an error that caused the current command to
abort.</entry>
<entry><literal>WARNING</literal></entry>
<entry><literal>ERROR</literal></entry>
</row>
<row>
<entry><literal>LOG</literal></entry>
<entry>Reports information of interest to administrators, e.g.,
checkpoint activity.</entry>
<entry><literal>INFO</literal></entry>
<entry><literal>INFORMATION</literal></entry>
</row>
<row>
<entry><literal>FATAL</literal></entry>
<entry>Reports an error that caused the current session to
abort.</entry>
<entry><literal>ERR</literal></entry>
<entry><literal>ERROR</literal></entry>
</row>
<row>
<entry><literal>PANIC</literal></entry>
<entry>Reports an error that caused all database sessions to abort.</entry>
<entry><literal>CRIT</literal></entry>
<entry><literal>ERROR</literal></entry>
</row>
</tbody>
</tgroup>
</table>
</sect2>
<sect2 id="runtime-config-logging-what">
<title>What to Log</title>
<variablelist>
<varlistentry id="guc-application-name" xreflabel="application_name">
<term><varname>application_name</varname> (<type>string</type>)
<indexterm>
<primary><varname>application_name</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The <varname>application_name</varname> can be any string of less than
<symbol>NAMEDATALEN</symbol> characters (64 characters in a standard build).
It is typically set by an application upon connection to the server.
The name will be displayed in the <structname>pg_stat_activity</structname> view
and included in CSV log entries. It can also be included in regular
log entries via the <xref linkend="guc-log-line-prefix"/> parameter.
Only printable ASCII characters may be used in the
<varname>application_name</varname> value. Other characters will be
replaced with question marks (<literal>?</literal>).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>debug_print_parse</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>debug_print_parse</varname> configuration parameter</primary>
</indexterm>
</term>
<term><varname>debug_print_rewritten</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>debug_print_rewritten</varname> configuration parameter</primary>
</indexterm>
</term>
<term><varname>debug_print_plan</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>debug_print_plan</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
These parameters enable various debugging output to be emitted.
When set, they print the resulting parse tree, the query rewriter
output, or the execution plan for each executed query.
These messages are emitted at <literal>LOG</literal> message level, so by
default they will appear in the server log but will not be sent to the
client. You can change that by adjusting
<xref linkend="guc-client-min-messages"/> and/or
<xref linkend="guc-log-min-messages"/>.
These parameters are off by default.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>debug_pretty_print</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>debug_pretty_print</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When set, <varname>debug_pretty_print</varname> indents the messages
produced by <varname>debug_print_parse</varname>,
<varname>debug_print_rewritten</varname>, or
<varname>debug_print_plan</varname>. This results in more readable
but much longer output than the <quote>compact</quote> format used when
it is off. It is on by default.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-log-checkpoints" xreflabel="log_checkpoints">
<term><varname>log_checkpoints</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>log_checkpoints</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Causes checkpoints and restartpoints to be logged in the server log.
Some statistics are included in the log messages, including the number
of buffers written and the time spent writing them.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line. The default is off.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-log-connections" xreflabel="log_connections">
<term><varname>log_connections</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>log_connections</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Causes each attempted connection to the server to be logged,
as well as successful completion of client authentication.
Only superusers can change this parameter at session start,
and it cannot be changed at all within a session.
The default is <literal>off</literal>.
</para>
<note>
<para>
Some client programs, like <application>psql</application>, attempt
to connect twice while determining if a password is required, so
duplicate <quote>connection received</quote> messages do not
necessarily indicate a problem.
</para>
</note>
</listitem>
</varlistentry>
<varlistentry id="guc-log-disconnections" xreflabel="log_disconnections">
<term><varname>log_disconnections</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>log_disconnections</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Causes session terminations to be logged. The log output
provides information similar to <varname>log_connections</varname>,
plus the duration of the session.
Only superusers can change this parameter at session start,
and it cannot be changed at all within a session.
The default is <literal>off</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-log-duration" xreflabel="log_duration">
<term><varname>log_duration</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>log_duration</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Causes the duration of every completed statement to be logged.
The default is <literal>off</literal>.
Only superusers can change this setting.
</para>
<para>
For clients using extended query protocol, durations of the Parse,
Bind, and Execute steps are logged independently.
</para>
<note>
<para>
The difference between enabling <varname>log_duration</varname> and setting
<xref linkend="guc-log-min-duration-statement"/> to zero is that
exceeding <varname>log_min_duration_statement</varname> forces the text of
the query to be logged, but this option doesn't. Thus, if
<varname>log_duration</varname> is <literal>on</literal> and
<varname>log_min_duration_statement</varname> has a positive value, all
durations are logged but the query text is included only for
statements exceeding the threshold. This behavior can be useful for
gathering statistics in high-load installations.
</para>
</note>
</listitem>
</varlistentry>
<varlistentry id="guc-log-error-verbosity" xreflabel="log_error_verbosity">
<term><varname>log_error_verbosity</varname> (<type>enum</type>)
<indexterm>
<primary><varname>log_error_verbosity</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Controls the amount of detail written in the server log for each
message that is logged. Valid values are <literal>TERSE</literal>,
<literal>DEFAULT</literal>, and <literal>VERBOSE</literal>, each adding more
fields to displayed messages. <literal>TERSE</literal> excludes
the logging of <literal>DETAIL</literal>, <literal>HINT</literal>,
<literal>QUERY</literal>, and <literal>CONTEXT</literal> error information.
<literal>VERBOSE</literal> output includes the <symbol>SQLSTATE</symbol> error
code (see also <xref linkend="errcodes-appendix"/>) and the source code file name, function name,
and line number that generated the error.
Only superusers can change this setting.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-log-hostname" xreflabel="log_hostname">
<term><varname>log_hostname</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>log_hostname</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
By default, connection log messages only show the IP address of the
connecting host. Turning this parameter on causes logging of the
host name as well. Note that depending on your host name resolution
setup this might impose a non-negligible performance penalty.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-log-line-prefix" xreflabel="log_line_prefix">
<term><varname>log_line_prefix</varname> (<type>string</type>)
<indexterm>
<primary><varname>log_line_prefix</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This is a <function>printf</function>-style string that is output at the
beginning of each log line.
<literal>%</literal> characters begin <quote>escape sequences</quote>
that are replaced with status information as outlined below.
Unrecognized escapes are ignored. Other
characters are copied straight to the log line. Some escapes are
only recognized by session processes, and will be treated as empty by
background processes such as the main server process. Status
information may be aligned either left or right by specifying a
numeric literal after the % and before the option. A negative
value will cause the status information to be padded on the
right with spaces to give it a minimum width, whereas a positive
value will pad on the left. Padding can be useful to aid human
readability in log files.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line. The default is
<literal>'%m [%p] '</literal> which logs a time stamp and the process ID.
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Escape</entry>
<entry>Effect</entry>
<entry>Session only</entry>
</row>
</thead>
<tbody>
<row>
<entry><literal>%a</literal></entry>
<entry>Application name</entry>
<entry>yes</entry>
</row>
<row>
<entry><literal>%u</literal></entry>
<entry>User name</entry>
<entry>yes</entry>
</row>
<row>
<entry><literal>%d</literal></entry>
<entry>Database name</entry>
<entry>yes</entry>
</row>
<row>
<entry><literal>%r</literal></entry>
<entry>Remote host name or IP address, and remote port</entry>
<entry>yes</entry>
</row>
<row>
<entry><literal>%h</literal></entry>
<entry>Remote host name or IP address</entry>
<entry>yes</entry>
</row>
<row>
<entry><literal>%p</literal></entry>
<entry>Process ID</entry>
<entry>no</entry>
</row>
<row>
<entry><literal>%t</literal></entry>
<entry>Time stamp without milliseconds</entry>
<entry>no</entry>
</row>
<row>
<entry><literal>%m</literal></entry>
<entry>Time stamp with milliseconds</entry>
<entry>no</entry>
</row>
<row>
<entry><literal>%n</literal></entry>
<entry>Time stamp with milliseconds (as a Unix epoch)</entry>
<entry>no</entry>
</row>
<row>
<entry><literal>%i</literal></entry>
<entry>Command tag: type of session's current command</entry>
<entry>yes</entry>
</row>
<row>
<entry><literal>%e</literal></entry>
<entry>SQLSTATE error code</entry>
<entry>no</entry>
</row>
<row>
<entry><literal>%c</literal></entry>
<entry>Session ID: see below</entry>
<entry>no</entry>
</row>
<row>
<entry><literal>%l</literal></entry>
<entry>Number of the log line for each session or process, starting at 1</entry>
<entry>no</entry>
</row>
<row>
<entry><literal>%s</literal></entry>
<entry>Process start time stamp</entry>
<entry>no</entry>
</row>
<row>
<entry><literal>%v</literal></entry>
<entry>Virtual transaction ID (backendID/localXID)</entry>
<entry>no</entry>
</row>
<row>
<entry><literal>%x</literal></entry>
<entry>Transaction ID (0 if none is assigned)</entry>
<entry>no</entry>
</row>
<row>
<entry><literal>%q</literal></entry>
<entry>Produces no output, but tells non-session
processes to stop at this point in the string; ignored by
session processes</entry>
<entry>no</entry>
</row>
<row>
<entry><literal>%%</literal></entry>
<entry>Literal <literal>%</literal></entry>
<entry>no</entry>
</row>
</tbody>
</tgroup>
</informaltable>
The <literal>%c</literal> escape prints a quasi-unique session identifier,
consisting of two 4-byte hexadecimal numbers (without leading zeros)
separated by a dot. The numbers are the process start time and the
process ID, so <literal>%c</literal> can also be used as a space saving way
of printing those items. For example, to generate the session
identifier from <literal>pg_stat_activity</literal>, use this query:
<programlisting>
SELECT to_hex(trunc(EXTRACT(EPOCH FROM backend_start))::integer) || '.' ||
to_hex(pid)
FROM pg_stat_activity;
</programlisting>
</para>
<tip>
<para>
If you set a nonempty value for <varname>log_line_prefix</varname>,
you should usually make its last character be a space, to provide
visual separation from the rest of the log line. A punctuation
character can be used too.
</para>
</tip>
<tip>
<para>
<application>Syslog</application> produces its own
time stamp and process ID information, so you probably do not want to
include those escapes if you are logging to <application>syslog</application>.
</para>
</tip>
<tip>
<para>
The <literal>%q</literal> escape is useful when including information that is
only available in session (backend) context like user or database
name. For example:
<programlisting>
log_line_prefix = '%m [%p] %q%u@%d/%a '
</programlisting>
</para>
</tip>
</listitem>
</varlistentry>
<varlistentry id="guc-log-lock-waits" xreflabel="log_lock_waits">
<term><varname>log_lock_waits</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>log_lock_waits</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Controls whether a log message is produced when a session waits
longer than <xref linkend="guc-deadlock-timeout"/> to acquire a
lock. This is useful in determining if lock waits are causing
poor performance. The default is <literal>off</literal>.
Only superusers can change this setting.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-log-parameters-on-error" xreflabel="log_parameters_on_error">
<term><varname>log_parameters_on_error</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>log_parameters_on_error</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Controls whether bind parameters are logged when a statement is logged
as a result of <xref linkend="guc-log-min-error-statement"/>.
It adds some overhead, as postgres will compute and store textual
representations of parameter values in memory for all statements,
even if they eventually do not get logged.
This setting has no effect on statements logged due to
<xref linkend="guc-log-min-duration-statement"/> or
<xref linkend="guc-log-statement"/> settings, as they are always logged
with parameters.
The default is <literal>off</literal>.
Only superusers can change this setting.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-log-statement" xreflabel="log_statement">
<term><varname>log_statement</varname> (<type>enum</type>)
<indexterm>
<primary><varname>log_statement</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Controls which SQL statements are logged. Valid values are
<literal>none</literal> (off), <literal>ddl</literal>, <literal>mod</literal>, and
<literal>all</literal> (all statements). <literal>ddl</literal> logs all data definition
statements, such as <command>CREATE</command>, <command>ALTER</command>, and
<command>DROP</command> statements. <literal>mod</literal> logs all
<literal>ddl</literal> statements, plus data-modifying statements
such as <command>INSERT</command>,
<command>UPDATE</command>, <command>DELETE</command>, <command>TRUNCATE</command>,
and <command>COPY FROM</command>.
<command>PREPARE</command>, <command>EXECUTE</command>, and
<command>EXPLAIN ANALYZE</command> statements are also logged if their
contained command is of an appropriate type. For clients using
extended query protocol, logging occurs when an Execute message
is received, and values of the Bind parameters are included
(with any embedded single-quote marks doubled).
</para>
<para>
The default is <literal>none</literal>. Only superusers can change this
setting.
</para>
<note>
<para>
Statements that contain simple syntax errors are not logged
even by the <varname>log_statement</varname> = <literal>all</literal> setting,
because the log message is emitted only after basic parsing has
been done to determine the statement type. In the case of extended
query protocol, this setting likewise does not log statements that
fail before the Execute phase (i.e., during parse analysis or
planning). Set <varname>log_min_error_statement</varname> to
<literal>ERROR</literal> (or lower) to log such statements.
</para>
</note>
</listitem>
</varlistentry>
<varlistentry id="guc-log-replication-commands" xreflabel="log_replication_commands">
<term><varname>log_replication_commands</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>log_replication_commands</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Causes each replication command to be logged in the server log.
See <xref linkend="protocol-replication"/> for more information about
replication command. The default value is <literal>off</literal>.
Only superusers can change this setting.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-log-temp-files" xreflabel="log_temp_files">
<term><varname>log_temp_files</varname> (<type>integer</type>)
<indexterm>
<primary><varname>log_temp_files</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Controls logging of temporary file names and sizes.
Temporary files can be
created for sorts, hashes, and temporary query results.
If enabled by this setting, a log entry is emitted for each
temporary file when it is deleted.
A value of zero logs all temporary file information, while positive
values log only files whose size is greater than or equal to
the specified amount of data.
If this value is specified without units, it is taken as kilobytes.
The default setting is -1, which disables such logging.
Only superusers can change this setting.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-log-timezone" xreflabel="log_timezone">
<term><varname>log_timezone</varname> (<type>string</type>)
<indexterm>
<primary><varname>log_timezone</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the time zone used for timestamps written in the server log.
Unlike <xref linkend="guc-timezone"/>, this value is cluster-wide,
so that all sessions will report timestamps consistently.
The built-in default is <literal>GMT</literal>, but that is typically
overridden in <filename>postgresql.conf</filename>; <application>initdb</application>
will install a setting there corresponding to its system environment.
See <xref linkend="datatype-timezones"/> for more information.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-logging-csvlog">
<title>Using CSV-Format Log Output</title>
<para>
Including <literal>csvlog</literal> in the <varname>log_destination</varname> list
provides a convenient way to import log files into a database table.
This option emits log lines in comma-separated-values
(<acronym>CSV</acronym>) format,
with these columns:
time stamp with milliseconds,
user name,
database name,
process ID,
client host:port number,
session ID,
per-session line number,
command tag,
session start time,
virtual transaction ID,
regular transaction ID,
error severity,
SQLSTATE code,
error message,
error message detail,
hint,
internal query that led to the error (if any),
character count of the error position therein,
error context,
user query that led to the error (if any and enabled by
<varname>log_min_error_statement</varname>),
character count of the error position therein,
location of the error in the PostgreSQL source code
(if <varname>log_error_verbosity</varname> is set to <literal>verbose</literal>),
and application name.
Here is a sample table definition for storing CSV-format log output:
<programlisting>
CREATE TABLE postgres_log
(
log_time timestamp(3) with time zone,
user_name text,
database_name text,
process_id integer,
connection_from text,
session_id text,
session_line_num bigint,
command_tag text,
session_start_time timestamp with time zone,
virtual_transaction_id text,
transaction_id bigint,
error_severity text,
sql_state_code text,
message text,
detail text,
hint text,
internal_query text,
internal_query_pos integer,
context text,
query text,
query_pos integer,
location text,
application_name text,
PRIMARY KEY (session_id, session_line_num)
);
</programlisting>
</para>
<para>
To import a log file into this table, use the <command>COPY FROM</command>
command:
<programlisting>
COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
</programlisting>
</para>
<para>
There are a few things you need to do to simplify importing CSV log
files:
<orderedlist>
<listitem>
<para>
Set <varname>log_filename</varname> and
<varname>log_rotation_age</varname> to provide a consistent,
predictable naming scheme for your log files. This lets you
predict what the file name will be and know when an individual log
file is complete and therefore ready to be imported.
</para>
</listitem>
<listitem>
<para>
Set <varname>log_rotation_size</varname> to 0 to disable
size-based log rotation, as it makes the log file name difficult
to predict.
</para>
</listitem>
<listitem>
<para>
Set <varname>log_truncate_on_rotation</varname> to <literal>on</literal> so
that old log data isn't mixed with the new in the same file.
</para>
</listitem>
<listitem>
<para>
The table definition above includes a primary key specification.
This is useful to protect against accidentally importing the same
information twice. The <command>COPY</command> command commits all of the
data it imports at one time, so any error will cause the entire
import to fail. If you import a partial log file and later import
the file again when it is complete, the primary key violation will
cause the import to fail. Wait until the log is complete and
closed before importing. This procedure will also protect against
accidentally importing a partial line that hasn't been completely
written, which would also cause <command>COPY</command> to fail.
</para>
</listitem>
</orderedlist>
</para>
</sect2>
<sect2>
<title>Process Title</title>
<para>
These settings control how process titles of server processes are
modified. Process titles are typically viewed using programs like
<application>ps</application> or, on Windows, <application>Process Explorer</application>.
See <xref linkend="monitoring-ps"/> for details.
</para>
<variablelist>
<varlistentry id="guc-cluster-name" xreflabel="cluster_name">
<term><varname>cluster_name</varname> (<type>string</type>)
<indexterm>
<primary><varname>cluster_name</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets a name that identifies this database cluster (instance) for
various purposes. The cluster name appears in the process title for
all server processes in this cluster. Moreover, it is the default
application name for a standby connection (see <xref
linkend="guc-synchronous-standby-names"/>.)
</para>
<para>
The name can be any string of less
than <symbol>NAMEDATALEN</symbol> characters (64 characters in a standard
build). Only printable ASCII characters may be used in the
<varname>cluster_name</varname> value. Other characters will be
replaced with question marks (<literal>?</literal>). No name is shown
if this parameter is set to the empty string <literal>''</literal> (which is
the default). This parameter can only be set at server start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-update-process-title" xreflabel="update_process_title">
<term><varname>update_process_title</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>update_process_title</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables updating of the process title every time a new SQL command
is received by the server.
This setting defaults to <literal>on</literal> on most platforms, but it
defaults to <literal>off</literal> on Windows due to that platform's larger
overhead for updating the process title.
Only superusers can change this setting.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
</sect1>
<sect1 id="runtime-config-statistics">
<title>Run-time Statistics</title>
<sect2 id="runtime-config-statistics-collector">
<title>Query and Index Statistics Collector</title>
<para>
These parameters control server-wide statistics collection features.
When statistics collection is enabled, the data that is produced can be
accessed via the <structname>pg_stat</structname> and
<structname>pg_statio</structname> family of system views.
Refer to <xref linkend="monitoring"/> for more information.
</para>
<variablelist>
<varlistentry id="guc-track-activities" xreflabel="track_activities">
<term><varname>track_activities</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>track_activities</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables the collection of information on the currently
executing command of each session, along with the time when
that command began execution. This parameter is on by
default. Note that even when enabled, this information is not
visible to all users, only to superusers and the user owning
the session being reported on, so it should not represent a
security risk.
Only superusers can change this setting.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-track-activity-query-size" xreflabel="track_activity_query_size">
<term><varname>track_activity_query_size</varname> (<type>integer</type>)
<indexterm>
<primary><varname>track_activity_query_size</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the amount of memory reserved to store the text of the
currently executing command for each active session, for the
<structname>pg_stat_activity</structname>.<structfield>query</structfield> field.
If this value is specified without units, it is taken as bytes.
The default value is 1024 bytes.
This parameter can only be set at server start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-track-counts" xreflabel="track_counts">
<term><varname>track_counts</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>track_counts</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables collection of statistics on database activity.
This parameter is on by default, because the autovacuum
daemon needs the collected information.
Only superusers can change this setting.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-track-io-timing" xreflabel="track_io_timing">
<term><varname>track_io_timing</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>track_io_timing</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables timing of database I/O calls. This parameter is off by
default, because it will repeatedly query the operating system for
the current time, which may cause significant overhead on some
platforms. You can use the <xref linkend="pgtesttiming"/> tool to
measure the overhead of timing on your system.
I/O timing information is
displayed in <xref linkend="pg-stat-database-view"/>, in the output of
<xref linkend="sql-explain"/> when the <literal>BUFFERS</literal> option is
used, and by <xref linkend="pgstatstatements"/>. Only superusers can
change this setting.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-track-functions" xreflabel="track_functions">
<term><varname>track_functions</varname> (<type>enum</type>)
<indexterm>
<primary><varname>track_functions</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables tracking of function call counts and time used. Specify
<literal>pl</literal> to track only procedural-language functions,
<literal>all</literal> to also track SQL and C language functions.
The default is <literal>none</literal>, which disables function
statistics tracking. Only superusers can change this setting.
</para>
<note>
<para>
SQL-language functions that are simple enough to be <quote>inlined</quote>
into the calling query will not be tracked, regardless of this
setting.
</para>
</note>
</listitem>
</varlistentry>
<varlistentry id="guc-stats-temp-directory" xreflabel="stats_temp_directory">
<term><varname>stats_temp_directory</varname> (<type>string</type>)
<indexterm>
<primary><varname>stats_temp_directory</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the directory to store temporary statistics data in. This can be
a path relative to the data directory or an absolute path. The default
is <filename>pg_stat_tmp</filename>. Pointing this at a RAM-based
file system will decrease physical I/O requirements and can lead to
improved performance.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-statistics-monitor">
<title>Statistics Monitoring</title>
<variablelist>
<varlistentry>
<term><varname>log_statement_stats</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>log_statement_stats</varname> configuration parameter</primary>
</indexterm>
</term>
<term><varname>log_parser_stats</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>log_parser_stats</varname> configuration parameter</primary>
</indexterm>
</term>
<term><varname>log_planner_stats</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>log_planner_stats</varname> configuration parameter</primary>
</indexterm>
</term>
<term><varname>log_executor_stats</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>log_executor_stats</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
For each query, output performance statistics of the respective
module to the server log. This is a crude profiling
instrument, similar to the Unix <function>getrusage()</function> operating
system facility. <varname>log_statement_stats</varname> reports total
statement statistics, while the others report per-module statistics.
<varname>log_statement_stats</varname> cannot be enabled together with
any of the per-module options. All of these options are disabled by
default. Only superusers can change these settings.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
</sect1>
<sect1 id="runtime-config-autovacuum">
<title>Automatic Vacuuming</title>
<indexterm>
<primary>autovacuum</primary>
<secondary>configuration parameters</secondary>
</indexterm>
<para>
These settings control the behavior of the <firstterm>autovacuum</firstterm>
feature. Refer to <xref linkend="autovacuum"/> for more information.
Note that many of these settings can be overridden on a per-table
basis; see <xref linkend="sql-createtable-storage-parameters"
endterm="sql-createtable-storage-parameters-title"/>.
</para>
<variablelist>
<varlistentry id="guc-autovacuum" xreflabel="autovacuum">
<term><varname>autovacuum</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>autovacuum</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Controls whether the server should run the
autovacuum launcher daemon. This is on by default; however,
<xref linkend="guc-track-counts"/> must also be enabled for
autovacuum to work.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line; however, autovacuuming can be
disabled for individual tables by changing table storage parameters.
</para>
<para>
Note that even when this parameter is disabled, the system
will launch autovacuum processes if necessary to
prevent transaction ID wraparound. See <xref
linkend="vacuum-for-wraparound"/> for more information.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-log-autovacuum-min-duration" xreflabel="log_autovacuum_min_duration">
<term><varname>log_autovacuum_min_duration</varname> (<type>integer</type>)
<indexterm>
<primary><varname>log_autovacuum_min_duration</varname></primary>
<secondary>configuration parameter</secondary>
</indexterm>
</term>
<listitem>
<para>
Causes each action executed by autovacuum to be logged if it ran for at
least the specified amount of time. Setting this to zero logs
all autovacuum actions. <literal>-1</literal> (the default) disables
logging autovacuum actions.
If this value is specified without units, it is taken as milliseconds.
For example, if you set this to
<literal>250ms</literal> then all automatic vacuums and analyzes that run
250ms or longer will be logged. In addition, when this parameter is
set to any value other than <literal>-1</literal>, a message will be
logged if an autovacuum action is skipped due to a conflicting lock or a
concurrently dropped relation. Enabling this parameter can be helpful
in tracking autovacuum activity. This parameter can only be set in
the <filename>postgresql.conf</filename> file or on the server command line;
but the setting can be overridden for individual tables by
changing table storage parameters.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-autovacuum-max-workers" xreflabel="autovacuum_max_workers">
<term><varname>autovacuum_max_workers</varname> (<type>integer</type>)
<indexterm>
<primary><varname>autovacuum_max_workers</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the maximum number of autovacuum processes (other than the
autovacuum launcher) that may be running at any one time. The default
is three. This parameter can only be set at server start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-autovacuum-naptime" xreflabel="autovacuum_naptime">
<term><varname>autovacuum_naptime</varname> (<type>integer</type>)
<indexterm>
<primary><varname>autovacuum_naptime</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the minimum delay between autovacuum runs on any given
database. In each round the daemon examines the
database and issues <command>VACUUM</command> and <command>ANALYZE</command> commands
as needed for tables in that database.
If this value is specified without units, it is taken as seconds.
The default is one minute (<literal>1min</literal>).
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-autovacuum-vacuum-threshold" xreflabel="autovacuum_vacuum_threshold">
<term><varname>autovacuum_vacuum_threshold</varname> (<type>integer</type>)
<indexterm>
<primary><varname>autovacuum_vacuum_threshold</varname></primary>
<secondary>configuration parameter</secondary>
</indexterm>
</term>
<listitem>
<para>
Specifies the minimum number of updated or deleted tuples needed
to trigger a <command>VACUUM</command> in any one table.
The default is 50 tuples.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line;
but the setting can be overridden for individual tables by
changing table storage parameters.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-autovacuum-analyze-threshold" xreflabel="autovacuum_analyze_threshold">
<term><varname>autovacuum_analyze_threshold</varname> (<type>integer</type>)
<indexterm>
<primary><varname>autovacuum_analyze_threshold</varname></primary>
<secondary>configuration parameter</secondary>
</indexterm>
</term>
<listitem>
<para>
Specifies the minimum number of inserted, updated or deleted tuples
needed to trigger an <command>ANALYZE</command> in any one table.
The default is 50 tuples.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line;
but the setting can be overridden for individual tables by
changing table storage parameters.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-autovacuum-vacuum-scale-factor" xreflabel="autovacuum_vacuum_scale_factor">
<term><varname>autovacuum_vacuum_scale_factor</varname> (<type>floating point</type>)
<indexterm>
<primary><varname>autovacuum_vacuum_scale_factor</varname></primary>
<secondary>configuration parameter</secondary>
</indexterm>
</term>
<listitem>
<para>
Specifies a fraction of the table size to add to
<varname>autovacuum_vacuum_threshold</varname>
when deciding whether to trigger a <command>VACUUM</command>.
The default is 0.2 (20% of table size).
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line;
but the setting can be overridden for individual tables by
changing table storage parameters.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-autovacuum-analyze-scale-factor" xreflabel="autovacuum_analyze_scale_factor">
<term><varname>autovacuum_analyze_scale_factor</varname> (<type>floating point</type>)
<indexterm>
<primary><varname>autovacuum_analyze_scale_factor</varname></primary>
<secondary>configuration parameter</secondary>
</indexterm>
</term>
<listitem>
<para>
Specifies a fraction of the table size to add to
<varname>autovacuum_analyze_threshold</varname>
when deciding whether to trigger an <command>ANALYZE</command>.
The default is 0.1 (10% of table size).
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line;
but the setting can be overridden for individual tables by
changing table storage parameters.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-autovacuum-freeze-max-age" xreflabel="autovacuum_freeze_max_age">
<term><varname>autovacuum_freeze_max_age</varname> (<type>integer</type>)
<indexterm>
<primary><varname>autovacuum_freeze_max_age</varname></primary>
<secondary>configuration parameter</secondary>
</indexterm>
</term>
<listitem>
<para>
Specifies the maximum age (in transactions) that a table's
<structname>pg_class</structname>.<structfield>relfrozenxid</structfield> field can
attain before a <command>VACUUM</command> operation is forced
to prevent transaction ID wraparound within the table.
Note that the system will launch autovacuum processes to
prevent wraparound even when autovacuum is otherwise disabled.
</para>
<para>
Vacuum also allows removal of old files from the
<filename>pg_xact</filename> subdirectory, which is why the default
is a relatively low 200 million transactions.
This parameter can only be set at server start, but the setting
can be reduced for individual tables by
changing table storage parameters.
For more information see <xref linkend="vacuum-for-wraparound"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-autovacuum-multixact-freeze-max-age" xreflabel="autovacuum_multixact_freeze_max_age">
<term><varname>autovacuum_multixact_freeze_max_age</varname> (<type>integer</type>)
<indexterm>
<primary><varname>autovacuum_multixact_freeze_max_age</varname></primary>
<secondary>configuration parameter</secondary>
</indexterm>
</term>
<listitem>
<para>
Specifies the maximum age (in multixacts) that a table's
<structname>pg_class</structname>.<structfield>relminmxid</structfield> field can
attain before a <command>VACUUM</command> operation is forced to
prevent multixact ID wraparound within the table.
Note that the system will launch autovacuum processes to
prevent wraparound even when autovacuum is otherwise disabled.
</para>
<para>
Vacuuming multixacts also allows removal of old files from the
<filename>pg_multixact/members</filename> and <filename>pg_multixact/offsets</filename>
subdirectories, which is why the default is a relatively low
400 million multixacts.
This parameter can only be set at server start, but the setting can
be reduced for individual tables by changing table storage parameters.
For more information see <xref linkend="vacuum-for-multixact-wraparound"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-autovacuum-vacuum-cost-delay" xreflabel="autovacuum_vacuum_cost_delay">
<term><varname>autovacuum_vacuum_cost_delay</varname> (<type>floating point</type>)
<indexterm>
<primary><varname>autovacuum_vacuum_cost_delay</varname></primary>
<secondary>configuration parameter</secondary>
</indexterm>
</term>
<listitem>
<para>
Specifies the cost delay value that will be used in automatic
<command>VACUUM</command> operations. If -1 is specified, the regular
<xref linkend="guc-vacuum-cost-delay"/> value will be used.
If this value is specified without units, it is taken as milliseconds.
The default value is 2 milliseconds.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line;
but the setting can be overridden for individual tables by
changing table storage parameters.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-autovacuum-vacuum-cost-limit" xreflabel="autovacuum_vacuum_cost_limit">
<term><varname>autovacuum_vacuum_cost_limit</varname> (<type>integer</type>)
<indexterm>
<primary><varname>autovacuum_vacuum_cost_limit</varname></primary>
<secondary>configuration parameter</secondary>
</indexterm>
</term>
<listitem>
<para>
Specifies the cost limit value that will be used in automatic
<command>VACUUM</command> operations. If -1 is specified (which is the
default), the regular
<xref linkend="guc-vacuum-cost-limit"/> value will be used. Note that
the value is distributed proportionally among the running autovacuum
workers, if there is more than one, so that the sum of the limits for
each worker does not exceed the value of this variable.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line;
but the setting can be overridden for individual tables by
changing table storage parameters.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect1>
<sect1 id="runtime-config-client">
<title>Client Connection Defaults</title>
<sect2 id="runtime-config-client-statement">
<title>Statement Behavior</title>
<variablelist>
<varlistentry id="guc-client-min-messages" xreflabel="client_min_messages">
<term><varname>client_min_messages</varname> (<type>enum</type>)
<indexterm>
<primary><varname>client_min_messages</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Controls which
<link linkend="runtime-config-severity-levels">message levels</link>
are sent to the client.
Valid values are <literal>DEBUG5</literal>,
<literal>DEBUG4</literal>, <literal>DEBUG3</literal>, <literal>DEBUG2</literal>,
<literal>DEBUG1</literal>, <literal>LOG</literal>, <literal>NOTICE</literal>,
<literal>WARNING</literal>, and <literal>ERROR</literal>.
Each level includes all the levels that follow it. The later the level,
the fewer messages are sent. The default is
<literal>NOTICE</literal>. Note that <literal>LOG</literal> has a different
rank here than in <xref linkend="guc-log-min-messages"/>.
</para>
<para>
<literal>INFO</literal> level messages are always sent to the client.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-search-path" xreflabel="search_path">
<term><varname>search_path</varname> (<type>string</type>)
<indexterm>
<primary><varname>search_path</varname> configuration parameter</primary>
</indexterm>
<indexterm><primary>path</primary><secondary>for schemas</secondary></indexterm>
</term>
<listitem>
<para>
This variable specifies the order in which schemas are searched
when an object (table, data type, function, etc.) is referenced by a
simple name with no schema specified. When there are objects of
identical names in different schemas, the one found first
in the search path is used. An object that is not in any of the
schemas in the search path can only be referenced by specifying
its containing schema with a qualified (dotted) name.
</para>
<para>
The value for <varname>search_path</varname> must be a comma-separated
list of schema names. Any name that is not an existing schema, or is
a schema for which the user does not have <literal>USAGE</literal>
permission, is silently ignored.
</para>
<para>
If one of the list items is the special name
<literal>$user</literal>, then the schema having the name returned by
<function>CURRENT_USER</function> is substituted, if there is such a schema
and the user has <literal>USAGE</literal> permission for it.
(If not, <literal>$user</literal> is ignored.)
</para>
<para>
The system catalog schema, <literal>pg_catalog</literal>, is always
searched, whether it is mentioned in the path or not. If it is
mentioned in the path then it will be searched in the specified
order. If <literal>pg_catalog</literal> is not in the path then it will
be searched <emphasis>before</emphasis> searching any of the path items.
</para>
<!-- To further split hairs, funcname('foo') does not use the temporary
schema, even when it considers typname='funcname'. This paragraph
refers to function names in a loose sense, "pg_proc.proname or
func_name grammar production". -->
<para>
Likewise, the current session's temporary-table schema,
<literal>pg_temp_<replaceable>nnn</replaceable></literal>, is always searched if it
exists. It can be explicitly listed in the path by using the
alias <literal>pg_temp</literal><indexterm><primary>pg_temp</primary></indexterm>. If it is not listed in the path then
it is searched first (even before <literal>pg_catalog</literal>). However,
the temporary schema is only searched for relation (table, view,
sequence, etc) and data type names. It is never searched for
function or operator names.
</para>
<para>
When objects are created without specifying a particular target
schema, they will be placed in the first valid schema named in
<varname>search_path</varname>. An error is reported if the search
path is empty.
</para>
<para>
The default value for this parameter is
<literal>"$user", public</literal>.
This setting supports shared use of a database (where no users
have private schemas, and all share use of <literal>public</literal>),
private per-user schemas, and combinations of these. Other
effects can be obtained by altering the default search path
setting, either globally or per-user.
</para>
<para>
For more information on schema handling, see
<xref linkend="ddl-schemas"/>. In particular, the default
configuration is suitable only when the database has a single user or
a few mutually-trusting users.
</para>
<para>
The current effective value of the search path can be examined
via the <acronym>SQL</acronym> function
<function>current_schemas</function>
(see <xref linkend="functions-info"/>).
This is not quite the same as
examining the value of <varname>search_path</varname>, since
<function>current_schemas</function> shows how the items
appearing in <varname>search_path</varname> were resolved.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-row-security" xreflabel="row_security">
<term><varname>row_security</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>row_security</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This variable controls whether to raise an error in lieu of applying a
row security policy. When set to <literal>on</literal>, policies apply
normally. When set to <literal>off</literal>, queries fail which would
otherwise apply at least one policy. The default is <literal>on</literal>.
Change to <literal>off</literal> where limited row visibility could cause
incorrect results; for example, <application>pg_dump</application> makes that
change by default. This variable has no effect on roles which bypass
every row security policy, to wit, superusers and roles with
the <literal>BYPASSRLS</literal> attribute.
</para>
<para>
For more information on row security policies,
see <xref linkend="sql-createpolicy"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-default-table-access-method" xreflabel="default_table_access_method">
<term><varname>default_table_access_method</varname> (<type>string</type>)
<indexterm>
<primary><varname>default_table_access_method</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This parameter specifies the default table access method to use when
creating tables or materialized views if the <command>CREATE</command>
command does not explicitly specify an access method, or when
<command>SELECT ... INTO</command> is used, which does not allow to
specify a table access method. The default is <literal>heap</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-default-tablespace" xreflabel="default_tablespace">
<term><varname>default_tablespace</varname> (<type>string</type>)
<indexterm>
<primary><varname>default_tablespace</varname> configuration parameter</primary>
</indexterm>
<indexterm><primary>tablespace</primary><secondary>default</secondary></indexterm>
</term>
<listitem>
<para>
This variable specifies the default tablespace in which to create
objects (tables and indexes) when a <command>CREATE</command> command does
not explicitly specify a tablespace. It also determines the tablespace
that a partitioned relation will direct future partitions to.
</para>
<para>
The value is either the name of a tablespace, or an empty string
to specify using the default tablespace of the current database.
If the value does not match the name of any existing tablespace,
<productname>PostgreSQL</productname> will automatically use the default
tablespace of the current database. If a nondefault tablespace
is specified, the user must have <literal>CREATE</literal> privilege
for it, or creation attempts will fail.
</para>
<para>
This variable is not used for temporary tables; for them,
<xref linkend="guc-temp-tablespaces"/> is consulted instead.
</para>
<para>
This variable is also not used when creating databases.
By default, a new database inherits its tablespace setting from
the template database it is copied from.
</para>
<para>
For more information on tablespaces,
see <xref linkend="manage-ag-tablespaces"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-temp-tablespaces" xreflabel="temp_tablespaces">
<term><varname>temp_tablespaces</varname> (<type>string</type>)
<indexterm>
<primary><varname>temp_tablespaces</varname> configuration parameter</primary>
</indexterm>
<indexterm><primary>tablespace</primary><secondary>temporary</secondary></indexterm>
</term>
<listitem>
<para>
This variable specifies tablespaces in which to create temporary
objects (temp tables and indexes on temp tables) when a
<command>CREATE</command> command does not explicitly specify a tablespace.
Temporary files for purposes such as sorting large data sets
are also created in these tablespaces.
</para>
<para>
The value is a list of names of tablespaces. When there is more than
one name in the list, <productname>PostgreSQL</productname> chooses a random
member of the list each time a temporary object is to be created;
except that within a transaction, successively created temporary
objects are placed in successive tablespaces from the list.
If the selected element of the list is an empty string,
<productname>PostgreSQL</productname> will automatically use the default
tablespace of the current database instead.
</para>
<para>
When <varname>temp_tablespaces</varname> is set interactively, specifying a
nonexistent tablespace is an error, as is specifying a tablespace for
which the user does not have <literal>CREATE</literal> privilege. However,
when using a previously set value, nonexistent tablespaces are
ignored, as are tablespaces for which the user lacks
<literal>CREATE</literal> privilege. In particular, this rule applies when
using a value set in <filename>postgresql.conf</filename>.
</para>
<para>
The default value is an empty string, which results in all temporary
objects being created in the default tablespace of the current
database.
</para>
<para>
See also <xref linkend="guc-default-tablespace"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-check-function-bodies" xreflabel="check_function_bodies">
<term><varname>check_function_bodies</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>check_function_bodies</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This parameter is normally on. When set to <literal>off</literal>, it
disables validation of the function body string during <xref
linkend="sql-createfunction"/>. Disabling validation avoids side
effects of the validation process and avoids false positives due
to problems such as forward references. Set this parameter
to <literal>off</literal> before loading functions on behalf of other
users; <application>pg_dump</application> does so automatically.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-default-transaction-isolation" xreflabel="default_transaction_isolation">
<term><varname>default_transaction_isolation</varname> (<type>enum</type>)
<indexterm>
<primary>transaction isolation level</primary>
<secondary>setting default</secondary>
</indexterm>
<indexterm>
<primary><varname>default_transaction_isolation</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Each SQL transaction has an isolation level, which can be
either <quote>read uncommitted</quote>, <quote>read
committed</quote>, <quote>repeatable read</quote>, or
<quote>serializable</quote>. This parameter controls the
default isolation level of each new transaction. The default
is <quote>read committed</quote>.
</para>
<para>
Consult <xref linkend="mvcc"/> and <xref
linkend="sql-set-transaction"/> for more information.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-default-transaction-read-only" xreflabel="default_transaction_read_only">
<term><varname>default_transaction_read_only</varname> (<type>boolean</type>)
<indexterm>
<primary>read-only transaction</primary>
<secondary>setting default</secondary>
</indexterm>
<indexterm>
<primary><varname>default_transaction_read_only</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
A read-only SQL transaction cannot alter non-temporary tables.
This parameter controls the default read-only status of each new
transaction. The default is <literal>off</literal> (read/write).
</para>
<para>
Consult <xref linkend="sql-set-transaction"/> for more information.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-default-transaction-deferrable" xreflabel="default_transaction_deferrable">
<term><varname>default_transaction_deferrable</varname> (<type>boolean</type>)
<indexterm>
<primary>deferrable transaction</primary>
<secondary>setting default</secondary>
</indexterm>
<indexterm>
<primary><varname>default_transaction_deferrable</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When running at the <literal>serializable</literal> isolation level,
a deferrable read-only SQL transaction may be delayed before
it is allowed to proceed. However, once it begins executing
it does not incur any of the overhead required to ensure
serializability; so serialization code will have no reason to
force it to abort because of concurrent updates, making this
option suitable for long-running read-only transactions.
</para>
<para>
This parameter controls the default deferrable status of each
new transaction. It currently has no effect on read-write
transactions or those operating at isolation levels lower
than <literal>serializable</literal>. The default is <literal>off</literal>.
</para>
<para>
Consult <xref linkend="sql-set-transaction"/> for more information.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-session-replication-role" xreflabel="session_replication_role">
<term><varname>session_replication_role</varname> (<type>enum</type>)
<indexterm>
<primary><varname>session_replication_role</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Controls firing of replication-related triggers and rules for the
current session. Setting this variable requires
superuser privilege and results in discarding any previously cached
query plans. Possible values are <literal>origin</literal> (the default),
<literal>replica</literal> and <literal>local</literal>.
</para>
<para>
The intended use of this setting is that logical replication systems
set it to <literal>replica</literal> when they are applying replicated
changes. The effect of that will be that triggers and rules (that
have not been altered from their default configuration) will not fire
on the replica. See the <xref linkend="sql-altertable"/> clauses
<literal>ENABLE TRIGGER</literal> and <literal>ENABLE RULE</literal>
for more information.
</para>
<para>
PostgreSQL treats the settings <literal>origin</literal> and
<literal>local</literal> the same internally. Third-party replication
systems may use these two values for their internal purposes, for
example using <literal>local</literal> to designate a session whose
changes should not be replicated.
</para>
<para>
Since foreign keys are implemented as triggers, setting this parameter
to <literal>replica</literal> also disables all foreign key checks,
which can leave data in an inconsistent state if improperly used.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-statement-timeout" xreflabel="statement_timeout">
<term><varname>statement_timeout</varname> (<type>integer</type>)
<indexterm>
<primary><varname>statement_timeout</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Abort any statement that takes more than the specified amount of time.
If <varname>log_min_error_statement</varname> is set
to <literal>ERROR</literal> or lower, the statement that timed out
will also be logged.
If this value is specified without units, it is taken as milliseconds.
A value of zero (the default) disables the timeout.
</para>
<para>
The timeout is measured from the time a command arrives at the
server until it is completed by the server. If multiple SQL
statements appear in a single simple-Query message, the timeout
is applied to each statement separately.
(<productname>PostgreSQL</productname> versions before 13 usually
treated the timeout as applying to the whole query string.)
In extended query protocol, the timeout starts running when any
query-related message (Parse, Bind, Execute, Describe) arrives, and
it is cancelled by completion of an Execute or Sync message.
</para>
<para>
Setting <varname>statement_timeout</varname> in
<filename>postgresql.conf</filename> is not recommended because it would
affect all sessions.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-lock-timeout" xreflabel="lock_timeout">
<term><varname>lock_timeout</varname> (<type>integer</type>)
<indexterm>
<primary><varname>lock_timeout</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Abort any statement that waits longer than the specified amount of
time while attempting to acquire a lock on a table, index,
row, or other database object. The time limit applies separately to
each lock acquisition attempt. The limit applies both to explicit
locking requests (such as <command>LOCK TABLE</command>, or <command>SELECT
FOR UPDATE</command> without <literal>NOWAIT</literal>) and to implicitly-acquired
locks.
If this value is specified without units, it is taken as milliseconds.
A value of zero (the default) disables the timeout.
</para>
<para>
Unlike <varname>statement_timeout</varname>, this timeout can only occur
while waiting for locks. Note that if <varname>statement_timeout</varname>
is nonzero, it is rather pointless to set <varname>lock_timeout</varname> to
the same or larger value, since the statement timeout would always
trigger first. If <varname>log_min_error_statement</varname> is set to
<literal>ERROR</literal> or lower, the statement that timed out will be
logged.
</para>
<para>
Setting <varname>lock_timeout</varname> in
<filename>postgresql.conf</filename> is not recommended because it would
affect all sessions.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-idle-in-transaction-session-timeout" xreflabel="idle_in_transaction_session_timeout">
<term><varname>idle_in_transaction_session_timeout</varname> (<type>integer</type>)
<indexterm>
<primary><varname>idle_in_transaction_session_timeout</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Terminate any session with an open transaction that has been idle for
longer than the specified amount of time. This allows any
locks held by that session to be released and the connection slot to be reused;
it also allows tuples visible only to this transaction to be vacuumed. See
<xref linkend="routine-vacuuming"/> for more details about this.
</para>
<para>
If this value is specified without units, it is taken as milliseconds.
A value of zero (the default) disables the timeout.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-freeze-table-age" xreflabel="vacuum_freeze_table_age">
<term><varname>vacuum_freeze_table_age</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_freeze_table_age</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
<command>VACUUM</command> performs an aggressive scan if the table's
<structname>pg_class</structname>.<structfield>relfrozenxid</structfield> field has reached
the age specified by this setting. An aggressive scan differs from
a regular <command>VACUUM</command> in that it visits every page that might
contain unfrozen XIDs or MXIDs, not just those that might contain dead
tuples. The default is 150 million transactions. Although users can
set this value anywhere from zero to two billions, <command>VACUUM</command>
will silently limit the effective value to 95% of
<xref linkend="guc-autovacuum-freeze-max-age"/>, so that a
periodical manual <command>VACUUM</command> has a chance to run before an
anti-wraparound autovacuum is launched for the table. For more
information see
<xref linkend="vacuum-for-wraparound"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-freeze-min-age" xreflabel="vacuum_freeze_min_age">
<term><varname>vacuum_freeze_min_age</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_freeze_min_age</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the cutoff age (in transactions) that <command>VACUUM</command>
should use to decide whether to freeze row versions
while scanning a table.
The default is 50 million transactions. Although
users can set this value anywhere from zero to one billion,
<command>VACUUM</command> will silently limit the effective value to half
the value of <xref linkend="guc-autovacuum-freeze-max-age"/>, so
that there is not an unreasonably short time between forced
autovacuums. For more information see <xref
linkend="vacuum-for-wraparound"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-multixact-freeze-table-age" xreflabel="vacuum_multixact_freeze_table_age">
<term><varname>vacuum_multixact_freeze_table_age</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_multixact_freeze_table_age</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
<command>VACUUM</command> performs an aggressive scan if the table's
<structname>pg_class</structname>.<structfield>relminmxid</structfield> field has reached
the age specified by this setting. An aggressive scan differs from
a regular <command>VACUUM</command> in that it visits every page that might
contain unfrozen XIDs or MXIDs, not just those that might contain dead
tuples. The default is 150 million multixacts.
Although users can set this value anywhere from zero to two billions,
<command>VACUUM</command> will silently limit the effective value to 95% of
<xref linkend="guc-autovacuum-multixact-freeze-max-age"/>, so that a
periodical manual <command>VACUUM</command> has a chance to run before an
anti-wraparound is launched for the table.
For more information see <xref linkend="vacuum-for-multixact-wraparound"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-multixact-freeze-min-age" xreflabel="vacuum_multixact_freeze_min_age">
<term><varname>vacuum_multixact_freeze_min_age</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_multixact_freeze_min_age</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the cutoff age (in multixacts) that <command>VACUUM</command>
should use to decide whether to replace multixact IDs with a newer
transaction ID or multixact ID while scanning a table. The default
is 5 million multixacts.
Although users can set this value anywhere from zero to one billion,
<command>VACUUM</command> will silently limit the effective value to half
the value of <xref linkend="guc-autovacuum-multixact-freeze-max-age"/>,
so that there is not an unreasonably short time between forced
autovacuums.
For more information see <xref linkend="vacuum-for-multixact-wraparound"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-cleanup-index-scale-factor" xreflabel="vacuum_cleanup_index_scale_factor">
<term><varname>vacuum_cleanup_index_scale_factor</varname> (<type>floating point</type>)
<indexterm>
<primary><varname>vacuum_cleanup_index_scale_factor</varname></primary>
<secondary>configuration parameter</secondary>
</indexterm>
</term>
<listitem>
<para>
Specifies the fraction of the total number of heap tuples counted in
the previous statistics collection that can be inserted without
incurring an index scan at the <command>VACUUM</command> cleanup stage.
This setting currently applies to B-tree indexes only.
</para>
<para>
If no tuples were deleted from the heap, B-tree indexes are still
scanned at the <command>VACUUM</command> cleanup stage when at least one
of the following conditions is met: the index statistics are stale, or
the index contains deleted pages that can be recycled during cleanup.
Index statistics are considered to be stale if the number of newly
inserted tuples exceeds the <varname>vacuum_cleanup_index_scale_factor</varname>
fraction of the total number of heap tuples detected by the previous
statistics collection. The total number of heap tuples is stored in
the index meta-page. Note that the meta-page does not include this data
until <command>VACUUM</command> finds no dead tuples, so B-tree index
scan at the cleanup stage can only be skipped if the second and
subsequent <command>VACUUM</command> cycles detect no dead tuples.
</para>
<para>
The value can range from <literal>0</literal> to
<literal>10000000000</literal>.
When <varname>vacuum_cleanup_index_scale_factor</varname> is set to
<literal>0</literal>, index scans are never skipped during
<command>VACUUM</command> cleanup. The default value is <literal>0.1</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-bytea-output" xreflabel="bytea_output">
<term><varname>bytea_output</varname> (<type>enum</type>)
<indexterm>
<primary><varname>bytea_output</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the output format for values of type <type>bytea</type>.
Valid values are <literal>hex</literal> (the default)
and <literal>escape</literal> (the traditional PostgreSQL
format). See <xref linkend="datatype-binary"/> for more
information. The <type>bytea</type> type always
accepts both formats on input, regardless of this setting.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-xmlbinary" xreflabel="xmlbinary">
<term><varname>xmlbinary</varname> (<type>enum</type>)
<indexterm>
<primary><varname>xmlbinary</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets how binary values are to be encoded in XML. This applies
for example when <type>bytea</type> values are converted to
XML by the functions <function>xmlelement</function> or
<function>xmlforest</function>. Possible values are
<literal>base64</literal> and <literal>hex</literal>, which
are both defined in the XML Schema standard. The default is
<literal>base64</literal>. For further information about
XML-related functions, see <xref linkend="functions-xml"/>.
</para>
<para>
The actual choice here is mostly a matter of taste,
constrained only by possible restrictions in client
applications. Both methods support all possible values,
although the hex encoding will be somewhat larger than the
base64 encoding.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-xmloption" xreflabel="xmloption">
<term><varname>xmloption</varname> (<type>enum</type>)
<indexterm>
<primary><varname>xmloption</varname> configuration parameter</primary>
</indexterm>
<indexterm>
<primary><varname>SET XML OPTION</varname></primary>
</indexterm>
<indexterm>
<primary>XML option</primary>
</indexterm>
</term>
<listitem>
<para>
Sets whether <literal>DOCUMENT</literal> or
<literal>CONTENT</literal> is implicit when converting between
XML and character string values. See <xref
linkend="datatype-xml"/> for a description of this. Valid
values are <literal>DOCUMENT</literal> and
<literal>CONTENT</literal>. The default is
<literal>CONTENT</literal>.
</para>
<para>
According to the SQL standard, the command to set this option is
<synopsis>
SET XML OPTION { DOCUMENT | CONTENT };
</synopsis>
This syntax is also available in PostgreSQL.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-gin-pending-list-limit" xreflabel="gin_pending_list_limit">
<term><varname>gin_pending_list_limit</varname> (<type>integer</type>)
<indexterm>
<primary><varname>gin_pending_list_limit</varname></primary>
<secondary>configuration parameter</secondary>
</indexterm>
</term>
<listitem>
<para>
Sets the maximum size of a GIN index's pending list, which is used
when <literal>fastupdate</literal> is enabled. If the list grows
larger than this maximum size, it is cleaned up by moving
the entries in it to the index's main GIN data structure in bulk.
If this value is specified without units, it is taken as kilobytes.
The default is four megabytes (<literal>4MB</literal>). This setting
can be overridden for individual GIN indexes by changing
index storage parameters.
See <xref linkend="gin-fast-update"/> and <xref linkend="gin-tips"/>
for more information.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-client-format">
<title>Locale and Formatting</title>
<variablelist>
<varlistentry id="guc-datestyle" xreflabel="DateStyle">
<term><varname>DateStyle</varname> (<type>string</type>)
<indexterm>
<primary><varname>DateStyle</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the display format for date and time values, as well as the
rules for interpreting ambiguous date input values. For
historical reasons, this variable contains two independent
components: the output format specification (<literal>ISO</literal>,
<literal>Postgres</literal>, <literal>SQL</literal>, or <literal>German</literal>)
and the input/output specification for year/month/day ordering
(<literal>DMY</literal>, <literal>MDY</literal>, or <literal>YMD</literal>). These
can be set separately or together. The keywords <literal>Euro</literal>
and <literal>European</literal> are synonyms for <literal>DMY</literal>; the
keywords <literal>US</literal>, <literal>NonEuro</literal>, and
<literal>NonEuropean</literal> are synonyms for <literal>MDY</literal>. See
<xref linkend="datatype-datetime"/> for more information. The
built-in default is <literal>ISO, MDY</literal>, but
<application>initdb</application> will initialize the
configuration file with a setting that corresponds to the
behavior of the chosen <varname>lc_time</varname> locale.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-intervalstyle" xreflabel="IntervalStyle">
<term><varname>IntervalStyle</varname> (<type>enum</type>)
<indexterm>
<primary><varname>IntervalStyle</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the display format for interval values.
The value <literal>sql_standard</literal> will produce
output matching <acronym>SQL</acronym> standard interval literals.
The value <literal>postgres</literal> (which is the default) will produce
output matching <productname>PostgreSQL</productname> releases prior to 8.4
when the <xref linkend="guc-datestyle"/>
parameter was set to <literal>ISO</literal>.
The value <literal>postgres_verbose</literal> will produce output
matching <productname>PostgreSQL</productname> releases prior to 8.4
when the <varname>DateStyle</varname>
parameter was set to non-<literal>ISO</literal> output.
The value <literal>iso_8601</literal> will produce output matching the time
interval <quote>format with designators</quote> defined in section
4.4.3.2 of ISO 8601.
</para>
<para>
The <varname>IntervalStyle</varname> parameter also affects the
interpretation of ambiguous interval input. See
<xref linkend="datatype-interval-input"/> for more information.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-timezone" xreflabel="TimeZone">
<term><varname>TimeZone</varname> (<type>string</type>)
<indexterm>
<primary><varname>TimeZone</varname> configuration parameter</primary>
</indexterm>
<indexterm><primary>time zone</primary></indexterm>
</term>
<listitem>
<para>
Sets the time zone for displaying and interpreting time stamps.
The built-in default is <literal>GMT</literal>, but that is typically
overridden in <filename>postgresql.conf</filename>; <application>initdb</application>
will install a setting there corresponding to its system environment.
See <xref linkend="datatype-timezones"/> for more information.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-timezone-abbreviations" xreflabel="timezone_abbreviations">
<term><varname>timezone_abbreviations</varname> (<type>string</type>)
<indexterm>
<primary><varname>timezone_abbreviations</varname> configuration parameter</primary>
</indexterm>
<indexterm><primary>time zone names</primary></indexterm>
</term>
<listitem>
<para>
Sets the collection of time zone abbreviations that will be accepted
by the server for datetime input. The default is <literal>'Default'</literal>,
which is a collection that works in most of the world; there are
also <literal>'Australia'</literal> and <literal>'India'</literal>,
and other collections can be defined for a particular installation.
See <xref linkend="datetime-config-files"/> for more information.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-extra-float-digits" xreflabel="extra_float_digits">
<term><varname>extra_float_digits</varname> (<type>integer</type>)
<indexterm>
<primary>significant digits</primary>
</indexterm>
<indexterm>
<primary>floating-point</primary>
<secondary>display</secondary>
</indexterm>
<indexterm>
<primary><varname>extra_float_digits</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This parameter adjusts the number of digits used for textual output of
floating-point values, including <type>float4</type>, <type>float8</type>,
and geometric data types.
</para>
<para>
If the value is 1 (the default) or above, float values are output in
shortest-precise format; see <xref linkend="datatype-float"/>. The
actual number of digits generated depends only on the value being
output, not on the value of this parameter. At most 17 digits are
required for <type>float8</type> values, and 9 for <type>float4</type>
values. This format is both fast and precise, preserving the original
binary float value exactly when correctly read. For historical
compatibility, values up to 3 are permitted.
</para>
<para>
If the value is zero or negative, then the output is rounded to a
given decimal precision. The precision used is the standard number of
digits for the type (<literal>FLT_DIG</literal>
or <literal>DBL_DIG</literal> as appropriate) reduced according to the
value of this parameter. (For example, specifying -1 will cause
<type>float4</type> values to be output rounded to 5 significant
digits, and <type>float8</type> values
rounded to 14 digits.) This format is slower and does not preserve all
the bits of the binary float value, but may be more human-readable.
</para>
<note>
<para>
The meaning of this parameter, and its default value, changed
in <productname>PostgreSQL</productname> 12;
see <xref linkend="datatype-float"/> for further discussion.
</para>
</note>
</listitem>
</varlistentry>
<varlistentry id="guc-client-encoding" xreflabel="client_encoding">
<term><varname>client_encoding</varname> (<type>string</type>)
<indexterm>
<primary><varname>client_encoding</varname> configuration parameter</primary>
</indexterm>
<indexterm><primary>character set</primary></indexterm>
</term>
<listitem>
<para>
Sets the client-side encoding (character set).
The default is to use the database encoding.
The character sets supported by the <productname>PostgreSQL</productname>
server are described in <xref linkend="multibyte-charset-supported"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-lc-messages" xreflabel="lc_messages">
<term><varname>lc_messages</varname> (<type>string</type>)
<indexterm>
<primary><varname>lc_messages</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the language in which messages are displayed. Acceptable
values are system-dependent; see <xref linkend="locale"/> for
more information. If this variable is set to the empty string
(which is the default) then the value is inherited from the
execution environment of the server in a system-dependent way.
</para>
<para>
On some systems, this locale category does not exist. Setting
this variable will still work, but there will be no effect.
Also, there is a chance that no translated messages for the
desired language exist. In that case you will continue to see
the English messages.
</para>
<para>
Only superusers can change this setting, because it affects the
messages sent to the server log as well as to the client, and
an improper value might obscure the readability of the server
logs.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-lc-monetary" xreflabel="lc_monetary">
<term><varname>lc_monetary</varname> (<type>string</type>)
<indexterm>
<primary><varname>lc_monetary</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the locale to use for formatting monetary amounts, for
example with the <function>to_char</function> family of
functions. Acceptable values are system-dependent; see <xref
linkend="locale"/> for more information. If this variable is
set to the empty string (which is the default) then the value
is inherited from the execution environment of the server in a
system-dependent way.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-lc-numeric" xreflabel="lc_numeric">
<term><varname>lc_numeric</varname> (<type>string</type>)
<indexterm>
<primary><varname>lc_numeric</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the locale to use for formatting numbers, for example
with the <function>to_char</function> family of
functions. Acceptable values are system-dependent; see <xref
linkend="locale"/> for more information. If this variable is
set to the empty string (which is the default) then the value
is inherited from the execution environment of the server in a
system-dependent way.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-lc-time" xreflabel="lc_time">
<term><varname>lc_time</varname> (<type>string</type>)
<indexterm>
<primary><varname>lc_time</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the locale to use for formatting dates and times, for example
with the <function>to_char</function> family of
functions. Acceptable values are system-dependent; see <xref
linkend="locale"/> for more information. If this variable is
set to the empty string (which is the default) then the value
is inherited from the execution environment of the server in a
system-dependent way.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-default-text-search-config" xreflabel="default_text_search_config">
<term><varname>default_text_search_config</varname> (<type>string</type>)
<indexterm>
<primary><varname>default_text_search_config</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Selects the text search configuration that is used by those variants
of the text search functions that do not have an explicit argument
specifying the configuration.
See <xref linkend="textsearch"/> for further information.
The built-in default is <literal>pg_catalog.simple</literal>, but
<application>initdb</application> will initialize the
configuration file with a setting that corresponds to the
chosen <varname>lc_ctype</varname> locale, if a configuration
matching that locale can be identified.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-client-preload">
<title>Shared Library Preloading</title>
<para>
Several settings are available for preloading shared libraries into the
server, in order to load additional functionality or achieve performance
benefits. For example, a setting of
<literal>'$libdir/mylib'</literal> would cause
<literal>mylib.so</literal> (or on some platforms,
<literal>mylib.sl</literal>) to be preloaded from the installation's standard
library directory. The differences between the settings are when they
take effect and what privileges are required to change them.
</para>
<para>
<productname>PostgreSQL</productname> procedural language libraries can
be preloaded in this way, typically by using the
syntax <literal>'$libdir/plXXX'</literal> where
<literal>XXX</literal> is <literal>pgsql</literal>, <literal>perl</literal>,
<literal>tcl</literal>, or <literal>python</literal>.
</para>
<para>
Only shared libraries specifically intended to be used with PostgreSQL
can be loaded this way. Every PostgreSQL-supported library has
a <quote>magic block</quote> that is checked to guarantee compatibility. For
this reason, non-PostgreSQL libraries cannot be loaded in this way. You
might be able to use operating-system facilities such
as <envar>LD_PRELOAD</envar> for that.
</para>
<para>
In general, refer to the documentation of a specific module for the
recommended way to load that module.
</para>
<variablelist>
<varlistentry id="guc-local-preload-libraries" xreflabel="local_preload_libraries">
<term><varname>local_preload_libraries</varname> (<type>string</type>)
<indexterm>
<primary><varname>local_preload_libraries</varname> configuration parameter</primary>
</indexterm>
<indexterm>
<primary><filename>$libdir/plugins</filename></primary>
</indexterm>
</term>
<listitem>
<para>
This variable specifies one or more shared libraries that are to be
preloaded at connection start.
It contains a comma-separated list of library names, where each name
is interpreted as for the <xref linkend="sql-load"/> command.
Whitespace between entries is ignored; surround a library name with
double quotes if you need to include whitespace or commas in the name.
The parameter value only takes effect at the start of the connection.
Subsequent changes have no effect. If a specified library is not
found, the connection attempt will fail.
</para>
<para>
This option can be set by any user. Because of that, the libraries
that can be loaded are restricted to those appearing in the
<filename>plugins</filename> subdirectory of the installation's
standard library directory. (It is the database administrator's
responsibility to ensure that only <quote>safe</quote> libraries
are installed there.) Entries in <varname>local_preload_libraries</varname>
can specify this directory explicitly, for example
<literal>$libdir/plugins/mylib</literal>, or just specify
the library name &mdash; <literal>mylib</literal> would have
the same effect as <literal>$libdir/plugins/mylib</literal>.
</para>
<para>
The intent of this feature is to allow unprivileged users to load
debugging or performance-measurement libraries into specific sessions
without requiring an explicit <command>LOAD</command> command. To that end,
it would be typical to set this parameter using
the <envar>PGOPTIONS</envar> environment variable on the client or by
using
<command>ALTER ROLE SET</command>.
</para>
<para>
However, unless a module is specifically designed to be used in this way by
non-superusers, this is usually not the right setting to use. Look
at <xref linkend="guc-session-preload-libraries"/> instead.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-session-preload-libraries" xreflabel="session_preload_libraries">
<term><varname>session_preload_libraries</varname> (<type>string</type>)
<indexterm>
<primary><varname>session_preload_libraries</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This variable specifies one or more shared libraries that are to be
preloaded at connection start.
It contains a comma-separated list of library names, where each name
is interpreted as for the <xref linkend="sql-load"/> command.
Whitespace between entries is ignored; surround a library name with
double quotes if you need to include whitespace or commas in the name.
The parameter value only takes effect at the start of the connection.
Subsequent changes have no effect. If a specified library is not
found, the connection attempt will fail.
Only superusers can change this setting.
</para>
<para>
The intent of this feature is to allow debugging or
performance-measurement libraries to be loaded into specific sessions
without an explicit
<command>LOAD</command> command being given. For
example, <xref linkend="auto-explain"/> could be enabled for all
sessions under a given user name by setting this parameter
with <command>ALTER ROLE SET</command>. Also, this parameter can be changed
without restarting the server (but changes only take effect when a new
session is started), so it is easier to add new modules this way, even
if they should apply to all sessions.
</para>
<para>
Unlike <xref linkend="guc-shared-preload-libraries"/>, there is no large
performance advantage to loading a library at session start rather than
when it is first used. There is some advantage, however, when
connection pooling is used.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-shared-preload-libraries" xreflabel="shared_preload_libraries">
<term><varname>shared_preload_libraries</varname> (<type>string</type>)
<indexterm>
<primary><varname>shared_preload_libraries</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This variable specifies one or more shared libraries to be preloaded at
server start.
It contains a comma-separated list of library names, where each name
is interpreted as for the <xref linkend="sql-load"/> command.
Whitespace between entries is ignored; surround a library name with
double quotes if you need to include whitespace or commas in the name.
This parameter can only be set at server start. If a specified
library is not found, the server will fail to start.
</para>
<para>
Some libraries need to perform certain operations that can only take
place at postmaster start, such as allocating shared memory, reserving
light-weight locks, or starting background workers. Those libraries
must be loaded at server start through this parameter. See the
documentation of each library for details.
</para>
<para>
Other libraries can also be preloaded. By preloading a shared library,
the library startup time is avoided when the library is first used.
However, the time to start each new server process might increase
slightly, even if that process never uses the library. So this
parameter is recommended only for libraries that will be used in most
sessions. Also, changing this parameter requires a server restart, so
this is not the right setting to use for short-term debugging tasks,
say. Use <xref linkend="guc-session-preload-libraries"/> for that
instead.
</para>
<note>
<para>
On Windows hosts, preloading a library at server start will not reduce
the time required to start each new server process; each server process
will re-load all preload libraries. However, <varname>shared_preload_libraries
</varname> is still useful on Windows hosts for libraries that need to
perform operations at postmaster start time.
</para>
</note>
</listitem>
</varlistentry>
<varlistentry id="guc-jit-provider" xreflabel="jit_provider">
<term><varname>jit_provider</varname> (<type>string</type>)
<indexterm>
<primary><varname>jit_provider</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This variable is the name of the JIT provider library to be used
(see <xref linkend="jit-pluggable"/>).
The default is <literal>llvmjit</literal>.
This parameter can only be set at server start.
</para>
<para>
If set to a non-existent library, <acronym>JIT</acronym> will not be
available, but no error will be raised. This allows JIT support to be
installed separately from the main
<productname>PostgreSQL</productname> package.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-client-other">
<title>Other Defaults</title>
<variablelist>
<varlistentry id="guc-dynamic-library-path" xreflabel="dynamic_library_path">
<term><varname>dynamic_library_path</varname> (<type>string</type>)
<indexterm>
<primary><varname>dynamic_library_path</varname> configuration parameter</primary>
</indexterm>
<indexterm><primary>dynamic loading</primary></indexterm>
</term>
<listitem>
<para>
If a dynamically loadable module needs to be opened and the
file name specified in the <command>CREATE FUNCTION</command> or
<command>LOAD</command> command
does not have a directory component (i.e., the
name does not contain a slash), the system will search this
path for the required file.
</para>
<para>
The value for <varname>dynamic_library_path</varname> must be a
list of absolute directory paths separated by colons (or semi-colons
on Windows). If a list element starts
with the special string <literal>$libdir</literal>, the
compiled-in <productname>PostgreSQL</productname> package
library directory is substituted for <literal>$libdir</literal>; this
is where the modules provided by the standard
<productname>PostgreSQL</productname> distribution are installed.
(Use <literal>pg_config --pkglibdir</literal> to find out the name of
this directory.) For example:
<programlisting>
dynamic_library_path = '/usr/local/lib/postgresql:/home/my_project/lib:$libdir'
</programlisting>
or, in a Windows environment:
<programlisting>
dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
</programlisting>
</para>
<para>
The default value for this parameter is
<literal>'$libdir'</literal>. If the value is set to an empty
string, the automatic path search is turned off.
</para>
<para>
This parameter can be changed at run time by superusers, but a
setting done that way will only persist until the end of the
client connection, so this method should be reserved for
development purposes. The recommended way to set this parameter
is in the <filename>postgresql.conf</filename> configuration
file.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-gin-fuzzy-search-limit" xreflabel="gin_fuzzy_search_limit">
<term><varname>gin_fuzzy_search_limit</varname> (<type>integer</type>)
<indexterm>
<primary><varname>gin_fuzzy_search_limit</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Soft upper limit of the size of the set returned by GIN index scans. For more
information see <xref linkend="gin-tips"/>.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
</sect1>
<sect1 id="runtime-config-locks">
<title>Lock Management</title>
<variablelist>
<varlistentry id="guc-deadlock-timeout" xreflabel="deadlock_timeout">
<term><varname>deadlock_timeout</varname> (<type>integer</type>)
<indexterm>
<primary>deadlock</primary>
<secondary>timeout during</secondary>
</indexterm>
<indexterm>
<primary>timeout</primary>
<secondary>deadlock</secondary>
</indexterm>
<indexterm>
<primary><varname>deadlock_timeout</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This is the amount of time to wait on a lock
before checking to see if there is a deadlock condition. The
check for deadlock is relatively expensive, so the server doesn't run
it every time it waits for a lock. We optimistically assume
that deadlocks are not common in production applications and
just wait on the lock for a while before checking for a
deadlock. Increasing this value reduces the amount of time
wasted in needless deadlock checks, but slows down reporting of
real deadlock errors.
If this value is specified without units, it is taken as milliseconds.
The default is one second (<literal>1s</literal>),
which is probably about the smallest value you would want in
practice. On a heavily loaded server you might want to raise it.
Ideally the setting should exceed your typical transaction time,
so as to improve the odds that a lock will be released before
the waiter decides to check for deadlock. Only superusers can change
this setting.
</para>
<para>
When <xref linkend="guc-log-lock-waits"/> is set,
this parameter also determines the amount of time to wait before
a log message is issued about the lock wait. If you are trying
to investigate locking delays you might want to set a shorter than
normal <varname>deadlock_timeout</varname>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-max-locks-per-transaction" xreflabel="max_locks_per_transaction">
<term><varname>max_locks_per_transaction</varname> (<type>integer</type>)
<indexterm>
<primary><varname>max_locks_per_transaction</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The shared lock table tracks locks on
<varname>max_locks_per_transaction</varname> * (<xref
linkend="guc-max-connections"/> + <xref
linkend="guc-max-prepared-transactions"/>) objects (e.g., tables);
hence, no more than this many distinct objects can be locked at
any one time. This parameter controls the average number of object
locks allocated for each transaction; individual transactions
can lock more objects as long as the locks of all transactions
fit in the lock table. This is <emphasis>not</emphasis> the number of
rows that can be locked; that value is unlimited. The default,
64, has historically proven sufficient, but you might need to
raise this value if you have queries that touch many different
tables in a single transaction, e.g. query of a parent table with
many children. This parameter can only be set at server start.
</para>
<para>
When running a standby server, you must set this parameter to the
same or higher value than on the master server. Otherwise, queries
will not be allowed in the standby server.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-max-pred-locks-per-transaction" xreflabel="max_pred_locks_per_transaction">
<term><varname>max_pred_locks_per_transaction</varname> (<type>integer</type>)
<indexterm>
<primary><varname>max_pred_locks_per_transaction</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The shared predicate lock table tracks locks on
<varname>max_pred_locks_per_transaction</varname> * (<xref
linkend="guc-max-connections"/> + <xref
linkend="guc-max-prepared-transactions"/>) objects (e.g., tables);
hence, no more than this many distinct objects can be locked at
any one time. This parameter controls the average number of object
locks allocated for each transaction; individual transactions
can lock more objects as long as the locks of all transactions
fit in the lock table. This is <emphasis>not</emphasis> the number of
rows that can be locked; that value is unlimited. The default,
64, has generally been sufficient in testing, but you might need to
raise this value if you have clients that touch many different
tables in a single serializable transaction. This parameter can
only be set at server start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-max-pred-locks-per-relation" xreflabel="max_pred_locks_per_relation">
<term><varname>max_pred_locks_per_relation</varname> (<type>integer</type>)
<indexterm>
<primary><varname>max_pred_locks_per_relation</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This controls how many pages or tuples of a single relation can be
predicate-locked before the lock is promoted to covering the whole
relation. Values greater than or equal to zero mean an absolute
limit, while negative values
mean <xref linkend="guc-max-pred-locks-per-transaction"/> divided by
the absolute value of this setting. The default is -2, which keeps
the behavior from previous versions of <productname>PostgreSQL</productname>.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-max-pred-locks-per-page" xreflabel="max_pred_locks_per_page">
<term><varname>max_pred_locks_per_page</varname> (<type>integer</type>)
<indexterm>
<primary><varname>max_pred_locks_per_page</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This controls how many rows on a single page can be predicate-locked
before the lock is promoted to covering the whole page. The default
is 2. This parameter can only be set in
the <filename>postgresql.conf</filename> file or on the server command line.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect1>
<sect1 id="runtime-config-compatible">
<title>Version and Platform Compatibility</title>
<sect2 id="runtime-config-compatible-version">
<title>Previous PostgreSQL Versions</title>
<variablelist>
<varlistentry id="guc-array-nulls" xreflabel="array_nulls">
<term><varname>array_nulls</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>array_nulls</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This controls whether the array input parser recognizes
unquoted <literal>NULL</literal> as specifying a null array element.
By default, this is <literal>on</literal>, allowing array values containing
null values to be entered. However, <productname>PostgreSQL</productname> versions
before 8.2 did not support null values in arrays, and therefore would
treat <literal>NULL</literal> as specifying a normal array element with
the string value <quote>NULL</quote>. For backward compatibility with
applications that require the old behavior, this variable can be
turned <literal>off</literal>.
</para>
<para>
Note that it is possible to create array values containing null values
even when this variable is <literal>off</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-backslash-quote" xreflabel="backslash_quote">
<term><varname>backslash_quote</varname> (<type>enum</type>)
<indexterm><primary>strings</primary><secondary>backslash quotes</secondary></indexterm>
<indexterm>
<primary><varname>backslash_quote</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This controls whether a quote mark can be represented by
<literal>\'</literal> in a string literal. The preferred, SQL-standard way
to represent a quote mark is by doubling it (<literal>''</literal>) but
<productname>PostgreSQL</productname> has historically also accepted
<literal>\'</literal>. However, use of <literal>\'</literal> creates security risks
because in some client character set encodings, there are multibyte
characters in which the last byte is numerically equivalent to ASCII
<literal>\</literal>. If client-side code does escaping incorrectly then a
SQL-injection attack is possible. This risk can be prevented by
making the server reject queries in which a quote mark appears to be
escaped by a backslash.
The allowed values of <varname>backslash_quote</varname> are
<literal>on</literal> (allow <literal>\'</literal> always),
<literal>off</literal> (reject always), and
<literal>safe_encoding</literal> (allow only if client encoding does not
allow ASCII <literal>\</literal> within a multibyte character).
<literal>safe_encoding</literal> is the default setting.
</para>
<para>
Note that in a standard-conforming string literal, <literal>\</literal> just
means <literal>\</literal> anyway. This parameter only affects the handling of
non-standard-conforming literals, including
escape string syntax (<literal>E'...'</literal>).
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-escape-string-warning" xreflabel="escape_string_warning">
<term><varname>escape_string_warning</varname> (<type>boolean</type>)
<indexterm><primary>strings</primary><secondary>escape warning</secondary></indexterm>
<indexterm>
<primary><varname>escape_string_warning</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When on, a warning is issued if a backslash (<literal>\</literal>)
appears in an ordinary string literal (<literal>'...'</literal>
syntax) and <varname>standard_conforming_strings</varname> is off.
The default is <literal>on</literal>.
</para>
<para>
Applications that wish to use backslash as escape should be
modified to use escape string syntax (<literal>E'...'</literal>),
because the default behavior of ordinary strings is now to treat
backslash as an ordinary character, per SQL standard. This variable
can be enabled to help locate code that needs to be changed.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-lo-compat-privileges" xreflabel="lo_compat_privileges">
<term><varname>lo_compat_privileges</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>lo_compat_privileges</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
In <productname>PostgreSQL</productname> releases prior to 9.0, large objects
did not have access privileges and were, therefore, always readable
and writable by all users. Setting this variable to <literal>on</literal>
disables the new privilege checks, for compatibility with prior
releases. The default is <literal>off</literal>.
Only superusers can change this setting.
</para>
<para>
Setting this variable does not disable all security checks related to
large objects &mdash; only those for which the default behavior has
changed in <productname>PostgreSQL</productname> 9.0.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-operator-precedence-warning" xreflabel="operator_precedence_warning">
<term><varname>operator_precedence_warning</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>operator_precedence_warning</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When on, the parser will emit a warning for any construct that might
have changed meanings since <productname>PostgreSQL</productname> 9.4 as a result
of changes in operator precedence. This is useful for auditing
applications to see if precedence changes have broken anything; but it
is not meant to be kept turned on in production, since it will warn
about some perfectly valid, standard-compliant SQL code.
The default is <literal>off</literal>.
</para>
<para>
See <xref linkend="sql-precedence"/> for more information.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-quote-all-identifiers" xreflabel="quote-all-identifiers">
<term><varname>quote_all_identifiers</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>quote_all_identifiers</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When the database generates SQL, force all identifiers to be quoted,
even if they are not (currently) keywords. This will affect the
output of <command>EXPLAIN</command> as well as the results of functions
like <function>pg_get_viewdef</function>. See also the
<option>--quote-all-identifiers</option> option of
<xref linkend="app-pgdump"/> and <xref linkend="app-pg-dumpall"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-standard-conforming-strings" xreflabel="standard_conforming_strings">
<term><varname>standard_conforming_strings</varname> (<type>boolean</type>)
<indexterm><primary>strings</primary><secondary>standard conforming</secondary></indexterm>
<indexterm>
<primary><varname>standard_conforming_strings</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This controls whether ordinary string literals
(<literal>'...'</literal>) treat backslashes literally, as specified in
the SQL standard.
Beginning in <productname>PostgreSQL</productname> 9.1, the default is
<literal>on</literal> (prior releases defaulted to <literal>off</literal>).
Applications can check this
parameter to determine how string literals will be processed.
The presence of this parameter can also be taken as an indication
that the escape string syntax (<literal>E'...'</literal>) is supported.
Escape string syntax (<xref linkend="sql-syntax-strings-escape"/>)
should be used if an application desires
backslashes to be treated as escape characters.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-synchronize-seqscans" xreflabel="synchronize_seqscans">
<term><varname>synchronize_seqscans</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>synchronize_seqscans</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This allows sequential scans of large tables to synchronize with each
other, so that concurrent scans read the same block at about the
same time and hence share the I/O workload. When this is enabled,
a scan might start in the middle of the table and then <quote>wrap
around</quote> the end to cover all rows, so as to synchronize with the
activity of scans already in progress. This can result in
unpredictable changes in the row ordering returned by queries that
have no <literal>ORDER BY</literal> clause. Setting this parameter to
<literal>off</literal> ensures the pre-8.3 behavior in which a sequential
scan always starts from the beginning of the table. The default
is <literal>on</literal>.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-compatible-clients">
<title>Platform and Client Compatibility</title>
<variablelist>
<varlistentry id="guc-transform-null-equals" xreflabel="transform_null_equals">
<term><varname>transform_null_equals</varname> (<type>boolean</type>)
<indexterm><primary>IS NULL</primary></indexterm>
<indexterm>
<primary><varname>transform_null_equals</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When on, expressions of the form <literal><replaceable>expr</replaceable> =
NULL</literal> (or <literal>NULL =
<replaceable>expr</replaceable></literal>) are treated as
<literal><replaceable>expr</replaceable> IS NULL</literal>, that is, they
return true if <replaceable>expr</replaceable> evaluates to the null value,
and false otherwise. The correct SQL-spec-compliant behavior of
<literal><replaceable>expr</replaceable> = NULL</literal> is to always
return null (unknown). Therefore this parameter defaults to
<literal>off</literal>.
</para>
<para>
However, filtered forms in <productname>Microsoft
Access</productname> generate queries that appear to use
<literal><replaceable>expr</replaceable> = NULL</literal> to test for
null values, so if you use that interface to access the database you
might want to turn this option on. Since expressions of the
form <literal><replaceable>expr</replaceable> = NULL</literal> always
return the null value (using the SQL standard interpretation), they are not
very useful and do not appear often in normal applications so
this option does little harm in practice. But new users are
frequently confused about the semantics of expressions
involving null values, so this option is off by default.
</para>
<para>
Note that this option only affects the exact form <literal>= NULL</literal>,
not other comparison operators or other expressions
that are computationally equivalent to some expression
involving the equals operator (such as <literal>IN</literal>).
Thus, this option is not a general fix for bad programming.
</para>
<para>
Refer to <xref linkend="functions-comparison"/> for related information.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
</sect1>
<sect1 id="runtime-config-error-handling">
<title>Error Handling</title>
<variablelist>
<varlistentry id="guc-exit-on-error" xreflabel="exit_on_error">
<term><varname>exit_on_error</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>exit_on_error</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
If on, any error will terminate the current session. By default,
this is set to off, so that only FATAL errors will terminate the
session.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-restart-after-crash" xreflabel="restart_after_crash">
<term><varname>restart_after_crash</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>restart_after_crash</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When set to on, which is the default, <productname>PostgreSQL</productname>
will automatically reinitialize after a backend crash. Leaving this
value set to on is normally the best way to maximize the availability
of the database. However, in some circumstances, such as when
<productname>PostgreSQL</productname> is being invoked by clusterware, it may be
useful to disable the restart so that the clusterware can gain
control and take any actions it deems appropriate.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-data-sync-retry" xreflabel="data_sync_retry">
<term><varname>data_sync_retry</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>data_sync_retry</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When set to off, which is the default, <productname>PostgreSQL</productname>
will raise a PANIC-level error on failure to flush modified data files
to the file system. This causes the database server to crash. This
parameter can only be set at server start.
</para>
<para>
On some operating systems, the status of data in the kernel's page
cache is unknown after a write-back failure. In some cases it might
have been entirely forgotten, making it unsafe to retry; the second
attempt may be reported as successful, when in fact the data has been
lost. In these circumstances, the only way to avoid data loss is to
recover from the WAL after any failure is reported, preferably
after investigating the root cause of the failure and replacing any
faulty hardware.
</para>
<para>
If set to on, <productname>PostgreSQL</productname> will instead
report an error but continue to run so that the data flushing
operation can be retried in a later checkpoint. Only set it to on
after investigating the operating system's treatment of buffered data
in case of write-back failure.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect1>
<sect1 id="runtime-config-preset">
<title>Preset Options</title>
<para>
The following <quote>parameters</quote> are read-only, and are determined
when <productname>PostgreSQL</productname> is compiled or when it is
installed. As such, they have been excluded from the sample
<filename>postgresql.conf</filename> file. These options report
various aspects of <productname>PostgreSQL</productname> behavior
that might be of interest to certain applications, particularly
administrative front-ends.
</para>
<variablelist>
<varlistentry id="guc-block-size" xreflabel="block_size">
<term><varname>block_size</varname> (<type>integer</type>)
<indexterm>
<primary><varname>block_size</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Reports the size of a disk block. It is determined by the value
of <literal>BLCKSZ</literal> when building the server. The default
value is 8192 bytes. The meaning of some configuration
variables (such as <xref linkend="guc-shared-buffers"/>) is
influenced by <varname>block_size</varname>. See <xref
linkend="runtime-config-resource"/> for information.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-data-checksums" xreflabel="data_checksums">
<term><varname>data_checksums</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>data_checksums</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Reports whether data checksums are enabled for this cluster.
See <xref linkend="app-initdb-data-checksums"/> for more information.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-data-directory-mode" xreflabel="data_directory_mode">
<term><varname>data_directory_mode</varname> (<type>integer</type>)
<indexterm>
<primary><varname>data_directory_mode</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
On Unix systems this parameter reports the permissions of the data
directory defined by (<xref linkend="guc-data-directory"/>) at startup.
(On Microsoft Windows this parameter will always display
<literal>0700</literal>). See
<xref linkend="app-initdb-allow-group-access"/> for more information.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-debug-assertions" xreflabel="debug_assertions">
<term><varname>debug_assertions</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>debug_assertions</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Reports whether <productname>PostgreSQL</productname> has been built
with assertions enabled. That is the case if the
macro <symbol>USE_ASSERT_CHECKING</symbol> is defined
when <productname>PostgreSQL</productname> is built (accomplished
e.g. by the <command>configure</command> option
<option>--enable-cassert</option>). By
default <productname>PostgreSQL</productname> is built without
assertions.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-integer-datetimes" xreflabel="integer_datetimes">
<term><varname>integer_datetimes</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>integer_datetimes</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Reports whether <productname>PostgreSQL</productname> was built with support for
64-bit-integer dates and times. As of <productname>PostgreSQL</productname> 10,
this is always <literal>on</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-lc-collate" xreflabel="lc_collate">
<term><varname>lc_collate</varname> (<type>string</type>)
<indexterm>
<primary><varname>lc_collate</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Reports the locale in which sorting of textual data is done.
See <xref linkend="locale"/> for more information.
This value is determined when a database is created.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-lc-ctype" xreflabel="lc_ctype">
<term><varname>lc_ctype</varname> (<type>string</type>)
<indexterm>
<primary><varname>lc_ctype</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Reports the locale that determines character classifications.
See <xref linkend="locale"/> for more information.
This value is determined when a database is created.
Ordinarily this will be the same as <varname>lc_collate</varname>,
but for special applications it might be set differently.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-max-function-args" xreflabel="max_function_args">
<term><varname>max_function_args</varname> (<type>integer</type>)
<indexterm>
<primary><varname>max_function_args</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Reports the maximum number of function arguments. It is determined by
the value of <literal>FUNC_MAX_ARGS</literal> when building the server. The
default value is 100 arguments.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-max-identifier-length" xreflabel="max_identifier_length">
<term><varname>max_identifier_length</varname> (<type>integer</type>)
<indexterm>
<primary><varname>max_identifier_length</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Reports the maximum identifier length. It is determined as one
less than the value of <literal>NAMEDATALEN</literal> when building
the server. The default value of <literal>NAMEDATALEN</literal> is
64; therefore the default
<varname>max_identifier_length</varname> is 63 bytes, which
can be less than 63 characters when using multibyte encodings.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-max-index-keys" xreflabel="max_index_keys">
<term><varname>max_index_keys</varname> (<type>integer</type>)
<indexterm>
<primary><varname>max_index_keys</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Reports the maximum number of index keys. It is determined by
the value of <literal>INDEX_MAX_KEYS</literal> when building the server. The
default value is 32 keys.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-segment-size" xreflabel="segment_size">
<term><varname>segment_size</varname> (<type>integer</type>)
<indexterm>
<primary><varname>segment_size</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Reports the number of blocks (pages) that can be stored within a file
segment. It is determined by the value of <literal>RELSEG_SIZE</literal>
when building the server. The maximum size of a segment file in bytes
is equal to <varname>segment_size</varname> multiplied by
<varname>block_size</varname>; by default this is 1GB.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-server-encoding" xreflabel="server_encoding">
<term><varname>server_encoding</varname> (<type>string</type>)
<indexterm>
<primary><varname>server_encoding</varname> configuration parameter</primary>
</indexterm>
<indexterm><primary>character set</primary></indexterm>
</term>
<listitem>
<para>
Reports the database encoding (character set).
It is determined when the database is created. Ordinarily,
clients need only be concerned with the value of <xref
linkend="guc-client-encoding"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-server-version" xreflabel="server_version">
<term><varname>server_version</varname> (<type>string</type>)
<indexterm>
<primary><varname>server_version</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Reports the version number of the server. It is determined by the
value of <literal>PG_VERSION</literal> when building the server.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-server-version-num" xreflabel="server_version_num">
<term><varname>server_version_num</varname> (<type>integer</type>)
<indexterm>
<primary><varname>server_version_num</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Reports the version number of the server as an integer. It is determined
by the value of <literal>PG_VERSION_NUM</literal> when building the server.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-ssl-library" xreflabel="ssl_library">
<term><varname>ssl_library</varname> (<type>string</type>)
<indexterm>
<primary><varname>ssl_library</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Reports the name of the SSL library that this
<productname>PostgreSQL</productname> server was built with (even if
SSL is not currently configured or in use on this instance), for
example <literal>OpenSSL</literal>, or an empty string if none.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-wal-block-size" xreflabel="wal_block_size">
<term><varname>wal_block_size</varname> (<type>integer</type>)
<indexterm>
<primary><varname>wal_block_size</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Reports the size of a WAL disk block. It is determined by the value
of <literal>XLOG_BLCKSZ</literal> when building the server. The default value
is 8192 bytes.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-wal-segment-size" xreflabel="wal_segment_size">
<term><varname>wal_segment_size</varname> (<type>integer</type>)
<indexterm>
<primary><varname>wal_segment_size</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Reports the size of write ahead log segments. The default value is
16MB. See <xref linkend="wal-configuration"/> for more information.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect1>
<sect1 id="runtime-config-custom">
<title>Customized Options</title>
<para>
This feature was designed to allow parameters not normally known to
<productname>PostgreSQL</productname> to be added by add-on modules
(such as procedural languages). This allows extension modules to be
configured in the standard ways.
</para>
<para>
Custom options have two-part names: an extension name, then a dot, then
the parameter name proper, much like qualified names in SQL. An example
is <literal>plpgsql.variable_conflict</literal>.
</para>
<para>
Because custom options may need to be set in processes that have not
loaded the relevant extension module, <productname>PostgreSQL</productname>
will accept a setting for any two-part parameter name. Such variables
are treated as placeholders and have no function until the module that
defines them is loaded. When an extension module is loaded, it will add
its variable definitions, convert any placeholder values according to
those definitions, and issue warnings for any unrecognized placeholders
that begin with its extension name.
</para>
</sect1>
<sect1 id="runtime-config-developer">
<title>Developer Options</title>
<para>
The following parameters are intended for work on the
<productname>PostgreSQL</productname> source code, and in some cases
to assist with recovery of severely damaged databases. There
should be no reason to use them on a production database.
As such, they have been excluded from the sample
<filename>postgresql.conf</filename> file. Note that many of these
parameters require special source compilation flags to work at all.
</para>
<variablelist>
<varlistentry id="guc-allow-system-table-mods" xreflabel="allow_system_table_mods">
<term><varname>allow_system_table_mods</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>allow_system_table_mods</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Allows modification of the structure of system tables as well as
certain other risky actions on system tables. This is otherwise not
allowed even for superusers. Ill-advised use of this setting can
cause irretrievable data loss or seriously corrupt the database
system. Only superusers can change this setting.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-backtrace-functions" xreflabel="backtrace_functions">
<term><varname>backtrace_functions</varname> (<type>string</type>)
<indexterm>
<primary><varname>backtrace_functions</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This parameter contains a comma-separated list of C function names.
If an error is raised and the name of the internal C function where
the error happens matches a value in the list, then a backtrace is
written to the server log together with the error message. This can
be used to debug specific areas of the source code.
</para>
<para>
Backtrace support is not available on all platforms, and the quality
of the backtraces depends on compilation options.
</para>
<para>
This parameter can only be set by superusers.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-ignore-system-indexes" xreflabel="ignore_system_indexes">
<term><varname>ignore_system_indexes</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>ignore_system_indexes</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Ignore system indexes when reading system tables (but still
update the indexes when modifying the tables). This is useful
when recovering from damaged system indexes.
This parameter cannot be changed after session start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-post-auth-delay" xreflabel="post_auth_delay">
<term><varname>post_auth_delay</varname> (<type>integer</type>)
<indexterm>
<primary><varname>post_auth_delay</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The amount of time to delay when a new
server process is started, after it conducts the
authentication procedure. This is intended to give developers an
opportunity to attach to the server process with a debugger.
If this value is specified without units, it is taken as seconds.
A value of zero (the default) disables the delay.
This parameter cannot be changed after session start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-pre-auth-delay" xreflabel="pre_auth_delay">
<term><varname>pre_auth_delay</varname> (<type>integer</type>)
<indexterm>
<primary><varname>pre_auth_delay</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The amount of time to delay just after a
new server process is forked, before it conducts the
authentication procedure. This is intended to give developers an
opportunity to attach to the server process with a debugger to
trace down misbehavior in authentication.
If this value is specified without units, it is taken as seconds.
A value of zero (the default) disables the delay.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-trace-notify" xreflabel="trace_notify">
<term><varname>trace_notify</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>trace_notify</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Generates a great amount of debugging output for the
<command>LISTEN</command> and <command>NOTIFY</command>
commands. <xref linkend="guc-client-min-messages"/> or
<xref linkend="guc-log-min-messages"/> must be
<literal>DEBUG1</literal> or lower to send this output to the
client or server logs, respectively.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-trace-recovery-messages" xreflabel="trace_recovery_messages">
<term><varname>trace_recovery_messages</varname> (<type>enum</type>)
<indexterm>
<primary><varname>trace_recovery_messages</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables logging of recovery-related debugging output that otherwise
would not be logged. This parameter allows the user to override the
normal setting of <xref linkend="guc-log-min-messages"/>, but only for
specific messages. This is intended for use in debugging Hot Standby.
Valid values are <literal>DEBUG5</literal>, <literal>DEBUG4</literal>,
<literal>DEBUG3</literal>, <literal>DEBUG2</literal>, <literal>DEBUG1</literal>, and
<literal>LOG</literal>. The default, <literal>LOG</literal>, does not affect
logging decisions at all. The other values cause recovery-related
debug messages of that priority or higher to be logged as though they
had <literal>LOG</literal> priority; for common settings of
<varname>log_min_messages</varname> this results in unconditionally sending
them to the server log.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-trace-sort" xreflabel="trace_sort">
<term><varname>trace_sort</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>trace_sort</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
If on, emit information about resource usage during sort operations.
This parameter is only available if the <symbol>TRACE_SORT</symbol> macro
was defined when <productname>PostgreSQL</productname> was compiled.
(However, <symbol>TRACE_SORT</symbol> is currently defined by default.)
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>trace_locks</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>trace_locks</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
If on, emit information about lock usage. Information dumped
includes the type of lock operation, the type of lock and the unique
identifier of the object being locked or unlocked. Also included
are bit masks for the lock types already granted on this object as
well as for the lock types awaited on this object. For each lock
type a count of the number of granted locks and waiting locks is
also dumped as well as the totals. An example of the log file output
is shown here:
<screen>
LOG: LockAcquire: new: lock(0xb7acd844) id(24688,24696,0,0,0,1)
grantMask(0) req(0,0,0,0,0,0,0)=0 grant(0,0,0,0,0,0,0)=0
wait(0) type(AccessShareLock)
LOG: GrantLock: lock(0xb7acd844) id(24688,24696,0,0,0,1)
grantMask(2) req(1,0,0,0,0,0,0)=1 grant(1,0,0,0,0,0,0)=1
wait(0) type(AccessShareLock)
LOG: UnGrantLock: updated: lock(0xb7acd844) id(24688,24696,0,0,0,1)
grantMask(0) req(0,0,0,0,0,0,0)=0 grant(0,0,0,0,0,0,0)=0
wait(0) type(AccessShareLock)
LOG: CleanUpLock: deleting: lock(0xb7acd844) id(24688,24696,0,0,0,1)
grantMask(0) req(0,0,0,0,0,0,0)=0 grant(0,0,0,0,0,0,0)=0
wait(0) type(INVALID)
</screen>
Details of the structure being dumped may be found in
<filename>src/include/storage/lock.h</filename>.
</para>
<para>
This parameter is only available if the <symbol>LOCK_DEBUG</symbol>
macro was defined when <productname>PostgreSQL</productname> was
compiled.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>trace_lwlocks</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>trace_lwlocks</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
If on, emit information about lightweight lock usage. Lightweight
locks are intended primarily to provide mutual exclusion of access
to shared-memory data structures.
</para>
<para>
This parameter is only available if the <symbol>LOCK_DEBUG</symbol>
macro was defined when <productname>PostgreSQL</productname> was
compiled.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>trace_userlocks</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>trace_userlocks</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
If on, emit information about user lock usage. Output is the same
as for <symbol>trace_locks</symbol>, only for advisory locks.
</para>
<para>
This parameter is only available if the <symbol>LOCK_DEBUG</symbol>
macro was defined when <productname>PostgreSQL</productname> was
compiled.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>trace_lock_oidmin</varname> (<type>integer</type>)
<indexterm>
<primary><varname>trace_lock_oidmin</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
If set, do not trace locks for tables below this OID. (use to avoid
output on system tables)
</para>
<para>
This parameter is only available if the <symbol>LOCK_DEBUG</symbol>
macro was defined when <productname>PostgreSQL</productname> was
compiled.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>trace_lock_table</varname> (<type>integer</type>)
<indexterm>
<primary><varname>trace_lock_table</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Unconditionally trace locks on this table (OID).
</para>
<para>
This parameter is only available if the <symbol>LOCK_DEBUG</symbol>
macro was defined when <productname>PostgreSQL</productname> was
compiled.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>debug_deadlocks</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>debug_deadlocks</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
If set, dumps information about all current locks when a
deadlock timeout occurs.
</para>
<para>
This parameter is only available if the <symbol>LOCK_DEBUG</symbol>
macro was defined when <productname>PostgreSQL</productname> was
compiled.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>log_btree_build_stats</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>log_btree_build_stats</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
If set, logs system resource usage statistics (memory and CPU) on
various B-tree operations.
</para>
<para>
This parameter is only available if the <symbol>BTREE_BUILD_STATS</symbol>
macro was defined when <productname>PostgreSQL</productname> was
compiled.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-wal-consistency-checking" xreflabel="wal_consistency_checking">
<term><varname>wal_consistency_checking</varname> (<type>string</type>)
<indexterm>
<primary><varname>wal_consistency_checking</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This parameter is intended to be used to check for bugs in the WAL
redo routines. When enabled, full-page images of any buffers modified
in conjunction with the WAL record are added to the record.
If the record is subsequently replayed, the system will first apply
each record and then test whether the buffers modified by the record
match the stored images. In certain cases (such as hint bits), minor
variations are acceptable, and will be ignored. Any unexpected
differences will result in a fatal error, terminating recovery.
</para>
<para>
The default value of this setting is the empty string, which disables
the feature. It can be set to <literal>all</literal> to check all
records, or to a comma-separated list of resource managers to check
only records originating from those resource managers. Currently,
the supported resource managers are <literal>heap</literal>,
<literal>heap2</literal>, <literal>btree</literal>, <literal>hash</literal>,
<literal>gin</literal>, <literal>gist</literal>, <literal>sequence</literal>,
<literal>spgist</literal>, <literal>brin</literal>, and <literal>generic</literal>. Only
superusers can change this setting.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-wal-debug" xreflabel="wal_debug">
<term><varname>wal_debug</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>wal_debug</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
If on, emit WAL-related debugging output. This parameter is
only available if the <symbol>WAL_DEBUG</symbol> macro was
defined when <productname>PostgreSQL</productname> was
compiled.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-ignore-checksum-failure" xreflabel="ignore_checksum_failure">
<term><varname>ignore_checksum_failure</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>ignore_checksum_failure</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Only has effect if <xref linkend="app-initdb-data-checksums"/> are enabled.
</para>
<para>
Detection of a checksum failure during a read normally causes
<productname>PostgreSQL</productname> to report an error, aborting the current
transaction. Setting <varname>ignore_checksum_failure</varname> to on causes
the system to ignore the failure (but still report a warning), and
continue processing. This behavior may <emphasis>cause crashes, propagate
or hide corruption, or other serious problems</emphasis>. However, it may allow
you to get past the error and retrieve undamaged tuples that might still be
present in the table if the block header is still sane. If the header is
corrupt an error will be reported even if this option is enabled. The
default setting is <literal>off</literal>, and it can only be changed by a superuser.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-zero-damaged-pages" xreflabel="zero_damaged_pages">
<term><varname>zero_damaged_pages</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>zero_damaged_pages</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Detection of a damaged page header normally causes
<productname>PostgreSQL</productname> to report an error, aborting the current
transaction. Setting <varname>zero_damaged_pages</varname> to on causes
the system to instead report a warning, zero out the damaged
page in memory, and continue processing. This behavior <emphasis>will destroy data</emphasis>,
namely all the rows on the damaged page. However, it does allow you to get
past the error and retrieve rows from any undamaged pages that might
be present in the table. It is useful for recovering data if
corruption has occurred due to a hardware or software error. You should
generally not set this on until you have given up hope of recovering
data from the damaged pages of a table. Zeroed-out pages are not
forced to disk so it is recommended to recreate the table or
the index before turning this parameter off again. The
default setting is <literal>off</literal>, and it can only be changed
by a superuser.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-ignore-invalid-pages" xreflabel="ignore_invalid_pages">
<term><varname>ignore_invalid_pages</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>ignore_invalid_pages</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
If set to <literal>off</literal> (the default), detection of
WAL records having references to invalid pages during
recovery causes <productname>PostgreSQL</productname> to
raise a PANIC-level error, aborting the recovery. Setting
<varname>ignore_invalid_pages</varname> to <literal>on</literal>
causes the system to ignore invalid page references in WAL records
(but still report a warning), and continue the recovery.
This behavior may <emphasis>cause crashes, data loss,
propagate or hide corruption, or other serious problems</emphasis>.
However, it may allow you to get past the PANIC-level error,
to finish the recovery, and to cause the server to start up.
The parameter can only be set at server start. It only has effect
during recovery or in standby mode.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-jit-debugging-support" xreflabel="jit_debugging_support">
<term><varname>jit_debugging_support</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>jit_debugging_support</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
If LLVM has the required functionality, register generated functions
with <productname>GDB</productname>. This makes debugging easier.
The default setting is <literal>off</literal>.
This parameter can only be set at server start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-jit-dump-bitcode" xreflabel="jit_dump_bitcode">
<term><varname>jit_dump_bitcode</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>jit_dump_bitcode</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Writes the generated <productname>LLVM</productname> IR out to the
file system, inside <xref linkend="guc-data-directory"/>. This is only
useful for working on the internals of the JIT implementation.
The default setting is <literal>off</literal>.
This parameter can only be changed by a superuser.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-jit-expressions" xreflabel="jit_expressions">
<term><varname>jit_expressions</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>jit_expressions</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Determines whether expressions are JIT compiled, when JIT compilation
is activated (see <xref linkend="jit-decision"/>). The default is
<literal>on</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-jit-profiling-support" xreflabel="jit_profiling_support">
<term><varname>jit_profiling_support</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>jit_profiling_support</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
If LLVM has the required functionality, emit the data needed to allow
<productname>perf</productname> to profile functions generated by JIT.
This writes out files to <filename>$HOME/.debug/jit/</filename>; the
user is responsible for performing cleanup when desired.
The default setting is <literal>off</literal>.
This parameter can only be set at server start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-jit-tuple-deforming" xreflabel="jit_tuple_deforming">
<term><varname>jit_tuple_deforming</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>jit_tuple_deforming</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Determines whether tuple deforming is JIT compiled, when JIT
compilation is activated (see <xref linkend="jit-decision"/>).
The default is <literal>on</literal>.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect1>
<sect1 id="runtime-config-short">
<title>Short Options</title>
<para>
For convenience there are also single letter command-line option
switches available for some parameters. They are described in
<xref linkend="runtime-config-short-table"/>. Some of these
options exist for historical reasons, and their presence as a
single-letter option does not necessarily indicate an endorsement
to use the option heavily.
</para>
<table id="runtime-config-short-table">
<title>Short Option Key</title>
<tgroup cols="2">
<thead>
<row>
<entry>Short Option</entry>
<entry>Equivalent</entry>
</row>
</thead>
<tbody>
<row>
<entry><option>-B <replaceable>x</replaceable></option></entry>
<entry><literal>shared_buffers = <replaceable>x</replaceable></literal></entry>
</row>
<row>
<entry><option>-d <replaceable>x</replaceable></option></entry>
<entry><literal>log_min_messages = DEBUG<replaceable>x</replaceable></literal></entry>
</row>
<row>
<entry><option>-e</option></entry>
<entry><literal>datestyle = euro</literal></entry>
</row>
<row>
<entry>
<option>-fb</option>, <option>-fh</option>, <option>-fi</option>,
<option>-fm</option>, <option>-fn</option>, <option>-fo</option>,
<option>-fs</option>, <option>-ft</option>
</entry>
<entry>
<literal>enable_bitmapscan = off</literal>,
<literal>enable_hashjoin = off</literal>,
<literal>enable_indexscan = off</literal>,
<literal>enable_mergejoin = off</literal>,
<literal>enable_nestloop = off</literal>,
<literal>enable_indexonlyscan = off</literal>,
<literal>enable_seqscan = off</literal>,
<literal>enable_tidscan = off</literal>
</entry>
</row>
<row>
<entry><option>-F</option></entry>
<entry><literal>fsync = off</literal></entry>
</row>
<row>
<entry><option>-h <replaceable>x</replaceable></option></entry>
<entry><literal>listen_addresses = <replaceable>x</replaceable></literal></entry>
</row>
<row>
<entry><option>-i</option></entry>
<entry><literal>listen_addresses = '*'</literal></entry>
</row>
<row>
<entry><option>-k <replaceable>x</replaceable></option></entry>
<entry><literal>unix_socket_directories = <replaceable>x</replaceable></literal></entry>
</row>
<row>
<entry><option>-l</option></entry>
<entry><literal>ssl = on</literal></entry>
</row>
<row>
<entry><option>-N <replaceable>x</replaceable></option></entry>
<entry><literal>max_connections = <replaceable>x</replaceable></literal></entry>
</row>
<row>
<entry><option>-O</option></entry>
<entry><literal>allow_system_table_mods = on</literal></entry>
</row>
<row>
<entry><option>-p <replaceable>x</replaceable></option></entry>
<entry><literal>port = <replaceable>x</replaceable></literal></entry>
</row>
<row>
<entry><option>-P</option></entry>
<entry><literal>ignore_system_indexes = on</literal></entry>
</row>
<row>
<entry><option>-s</option></entry>
<entry><literal>log_statement_stats = on</literal></entry>
</row>
<row>
<entry><option>-S <replaceable>x</replaceable></option></entry>
<entry><literal>work_mem = <replaceable>x</replaceable></literal></entry>
</row>
<row>
<entry><option>-tpa</option>, <option>-tpl</option>, <option>-te</option></entry>
<entry><literal>log_parser_stats = on</literal>,
<literal>log_planner_stats = on</literal>,
<literal>log_executor_stats = on</literal></entry>
</row>
<row>
<entry><option>-W <replaceable>x</replaceable></option></entry>
<entry><literal>post_auth_delay = <replaceable>x</replaceable></literal></entry>
</row>
</tbody>
</tgroup>
</table>
</sect1>
</chapter>
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