685 lines
27 KiB
Plaintext
685 lines
27 KiB
Plaintext
<!-- doc/src/sgml/postgres-fdw.sgml -->
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<sect1 id="postgres-fdw" xreflabel="postgres_fdw">
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<title>postgres_fdw</title>
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<indexterm zone="postgres-fdw">
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<primary>postgres_fdw</primary>
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</indexterm>
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<para>
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The <filename>postgres_fdw</filename> module provides the foreign-data wrapper
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<literal>postgres_fdw</literal>, which can be used to access data
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stored in external <productname>PostgreSQL</productname> servers.
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</para>
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<para>
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The functionality provided by this module overlaps substantially
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with the functionality of the older <xref linkend="dblink"/> module.
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But <filename>postgres_fdw</filename> provides more transparent and
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standards-compliant syntax for accessing remote tables, and can give
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better performance in many cases.
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</para>
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<para>
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To prepare for remote access using <filename>postgres_fdw</filename>:
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<orderedlist spacing="compact">
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<listitem>
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<para>
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Install the <filename>postgres_fdw</filename> extension using <xref
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linkend="sql-createextension"/>.
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</para>
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</listitem>
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<listitem>
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<para>
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Create a foreign server object, using <xref linkend="sql-createserver"/>,
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to represent each remote database you want to connect to.
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Specify connection information, except <literal>user</literal> and
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<literal>password</literal>, as options of the server object.
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</para>
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</listitem>
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<listitem>
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<para>
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Create a user mapping, using <xref linkend="sql-createusermapping"/>, for
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each database user you want to allow to access each foreign server.
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Specify the remote user name and password to use as
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<literal>user</literal> and <literal>password</literal> options of the
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user mapping.
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</para>
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</listitem>
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<listitem>
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<para>
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Create a foreign table, using <xref linkend="sql-createforeigntable"/>
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or <xref linkend="sql-importforeignschema"/>,
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for each remote table you want to access. The columns of the foreign
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table must match the referenced remote table. You can, however, use
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table and/or column names different from the remote table's, if you
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specify the correct remote names as options of the foreign table object.
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</para>
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</listitem>
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</orderedlist>
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</para>
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<para>
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Now you need only <command>SELECT</command> from a foreign table to access
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the data stored in its underlying remote table. You can also modify
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the remote table using <command>INSERT</command>, <command>UPDATE</command>, or
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<command>DELETE</command>. (Of course, the remote user you have specified
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in your user mapping must have privileges to do these things.)
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</para>
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<para>
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Note that <filename>postgres_fdw</filename> currently lacks support for
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<command>INSERT</command> statements with an <literal>ON CONFLICT DO
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UPDATE</literal> clause. However, the <literal>ON CONFLICT DO NOTHING</literal>
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clause is supported, provided a unique index inference specification
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is omitted.
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Note also that <filename>postgres_fdw</filename> supports row movement
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invoked by <command>UPDATE</command> statements executed on partitioned
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tables, but it currently does not handle the case where a remote partition
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chosen to insert a moved row into is also an <command>UPDATE</command>
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target partition that will be updated later.
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</para>
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<para>
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It is generally recommended that the columns of a foreign table be declared
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with exactly the same data types, and collations if applicable, as the
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referenced columns of the remote table. Although <filename>postgres_fdw</filename>
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is currently rather forgiving about performing data type conversions at
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need, surprising semantic anomalies may arise when types or collations do
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not match, due to the remote server interpreting <literal>WHERE</literal> clauses
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slightly differently from the local server.
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</para>
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<para>
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Note that a foreign table can be declared with fewer columns, or with a
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different column order, than its underlying remote table has. Matching
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of columns to the remote table is by name, not position.
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</para>
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<sect2>
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<title>FDW Options of postgres_fdw</title>
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<sect3>
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<title>Connection Options</title>
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<para>
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A foreign server using the <filename>postgres_fdw</filename> foreign data wrapper
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can have the same options that <application>libpq</application> accepts in
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connection strings, as described in <xref linkend="libpq-paramkeywords"/>,
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except that these options are not allowed:
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<itemizedlist spacing="compact">
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<listitem>
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<para>
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<literal>user</literal>, <literal>password</literal> and <literal>sslpassword</literal> (specify these
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in a user mapping, instead)
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</para>
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</listitem>
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<listitem>
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<para>
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<literal>client_encoding</literal> (this is automatically set from the local
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server encoding)
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</para>
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</listitem>
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<listitem>
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<para>
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<literal>fallback_application_name</literal> (always set to
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<literal>postgres_fdw</literal>)
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</para>
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</listitem>
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</itemizedlist>
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</para>
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<para>
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Only superusers may connect to foreign servers without password
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authentication, so always specify the <literal>password</literal> option
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for user mappings belonging to non-superusers.
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</para>
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</sect3>
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<sect3>
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<title>Object Name Options</title>
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<para>
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These options can be used to control the names used in SQL statements
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sent to the remote <productname>PostgreSQL</productname> server. These
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options are needed when a foreign table is created with names different
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from the underlying remote table's names.
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</para>
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<variablelist>
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<varlistentry>
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<term><literal>schema_name</literal></term>
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<listitem>
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<para>
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This option, which can be specified for a foreign table, gives the
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schema name to use for the foreign table on the remote server. If this
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option is omitted, the name of the foreign table's schema is used.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>table_name</literal></term>
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<listitem>
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<para>
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This option, which can be specified for a foreign table, gives the
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table name to use for the foreign table on the remote server. If this
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option is omitted, the foreign table's name is used.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>column_name</literal></term>
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<listitem>
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<para>
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This option, which can be specified for a column of a foreign table,
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gives the column name to use for the column on the remote server.
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If this option is omitted, the column's name is used.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</sect3>
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<sect3>
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<title>Cost Estimation Options</title>
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<para>
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<filename>postgres_fdw</filename> retrieves remote data by executing queries
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against remote servers, so ideally the estimated cost of scanning a
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foreign table should be whatever it costs to be done on the remote
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server, plus some overhead for communication. The most reliable way to
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get such an estimate is to ask the remote server and then add something
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for overhead — but for simple queries, it may not be worth the cost
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of an additional remote query to get a cost estimate.
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So <filename>postgres_fdw</filename> provides the following options to control
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how cost estimation is done:
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</para>
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<variablelist>
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<varlistentry>
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<term><literal>use_remote_estimate</literal></term>
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<listitem>
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<para>
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This option, which can be specified for a foreign table or a foreign
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server, controls whether <filename>postgres_fdw</filename> issues remote
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<command>EXPLAIN</command> commands to obtain cost estimates.
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A setting for a foreign table overrides any setting for its server,
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but only for that table.
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The default is <literal>false</literal>.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>fdw_startup_cost</literal></term>
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<listitem>
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<para>
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This option, which can be specified for a foreign server, is a numeric
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value that is added to the estimated startup cost of any foreign-table
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scan on that server. This represents the additional overhead of
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establishing a connection, parsing and planning the query on the
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remote side, etc.
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The default value is <literal>100</literal>.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>fdw_tuple_cost</literal></term>
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<listitem>
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<para>
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This option, which can be specified for a foreign server, is a numeric
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value that is used as extra cost per-tuple for foreign-table
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scans on that server. This represents the additional overhead of
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data transfer between servers. You might increase or decrease this
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number to reflect higher or lower network delay to the remote server.
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The default value is <literal>0.01</literal>.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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<para>
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When <literal>use_remote_estimate</literal> is true,
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<filename>postgres_fdw</filename> obtains row count and cost estimates from the
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remote server and then adds <literal>fdw_startup_cost</literal> and
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<literal>fdw_tuple_cost</literal> to the cost estimates. When
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<literal>use_remote_estimate</literal> is false,
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<filename>postgres_fdw</filename> performs local row count and cost estimation
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and then adds <literal>fdw_startup_cost</literal> and
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<literal>fdw_tuple_cost</literal> to the cost estimates. This local
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estimation is unlikely to be very accurate unless local copies of the
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remote table's statistics are available. Running
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<xref linkend="sql-analyze"/> on the foreign table is the way to update
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the local statistics; this will perform a scan of the remote table and
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then calculate and store statistics just as though the table were local.
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Keeping local statistics can be a useful way to reduce per-query planning
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overhead for a remote table — but if the remote table is
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frequently updated, the local statistics will soon be obsolete.
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</para>
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</sect3>
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<sect3>
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<title>Remote Execution Options</title>
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<para>
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By default, only <literal>WHERE</literal> clauses using built-in operators and
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functions will be considered for execution on the remote server. Clauses
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involving non-built-in functions are checked locally after rows are
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fetched. If such functions are available on the remote server and can be
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relied on to produce the same results as they do locally, performance can
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be improved by sending such <literal>WHERE</literal> clauses for remote
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execution. This behavior can be controlled using the following option:
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</para>
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<variablelist>
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<varlistentry>
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<term><literal>extensions</literal></term>
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<listitem>
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<para>
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This option is a comma-separated list of names
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of <productname>PostgreSQL</productname> extensions that are installed, in
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compatible versions, on both the local and remote servers. Functions
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and operators that are immutable and belong to a listed extension will
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be considered shippable to the remote server.
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This option can only be specified for foreign servers, not per-table.
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</para>
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<para>
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When using the <literal>extensions</literal> option, <emphasis>it is the
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user's responsibility</emphasis> that the listed extensions exist and behave
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identically on both the local and remote servers. Otherwise, remote
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queries may fail or behave unexpectedly.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>fetch_size</literal></term>
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<listitem>
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<para>
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This option specifies the number of rows <filename>postgres_fdw</filename>
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should get in each fetch operation. It can be specified for a foreign
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table or a foreign server. The option specified on a table overrides
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an option specified for the server.
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The default is <literal>100</literal>.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</sect3>
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<sect3>
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<title>Updatability Options</title>
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<para>
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By default all foreign tables using <filename>postgres_fdw</filename> are assumed
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to be updatable. This may be overridden using the following option:
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</para>
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<variablelist>
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<varlistentry>
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<term><literal>updatable</literal></term>
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<listitem>
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<para>
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This option controls whether <filename>postgres_fdw</filename> allows foreign
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tables to be modified using <command>INSERT</command>, <command>UPDATE</command> and
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<command>DELETE</command> commands. It can be specified for a foreign table
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or a foreign server. A table-level option overrides a server-level
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option.
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The default is <literal>true</literal>.
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</para>
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<para>
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Of course, if the remote table is not in fact updatable, an error
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would occur anyway. Use of this option primarily allows the error to
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be thrown locally without querying the remote server. Note however
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that the <literal>information_schema</literal> views will report a
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<filename>postgres_fdw</filename> foreign table to be updatable (or not)
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according to the setting of this option, without any check of the
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remote server.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</sect3>
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<sect3>
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<title>Importing Options</title>
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<para>
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<filename>postgres_fdw</filename> is able to import foreign table definitions
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using <xref linkend="sql-importforeignschema"/>. This command creates
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foreign table definitions on the local server that match tables or
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views present on the remote server. If the remote tables to be imported
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have columns of user-defined data types, the local server must have
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compatible types of the same names.
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</para>
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<para>
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Importing behavior can be customized with the following options
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(given in the <command>IMPORT FOREIGN SCHEMA</command> command):
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</para>
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<variablelist>
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<varlistentry>
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<term><literal>import_collate</literal></term>
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<listitem>
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<para>
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This option controls whether column <literal>COLLATE</literal> options
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are included in the definitions of foreign tables imported
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from a foreign server. The default is <literal>true</literal>. You might
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need to turn this off if the remote server has a different set of
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collation names than the local server does, which is likely to be the
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case if it's running on a different operating system.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>import_default</literal></term>
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<listitem>
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<para>
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This option controls whether column <literal>DEFAULT</literal> expressions
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are included in the definitions of foreign tables imported
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from a foreign server. The default is <literal>false</literal>. If you
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enable this option, be wary of defaults that might get computed
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differently on the local server than they would be on the remote
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server; <function>nextval()</function> is a common source of problems.
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The <command>IMPORT</command> will fail altogether if an imported default
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expression uses a function or operator that does not exist locally.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>import_not_null</literal></term>
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<listitem>
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<para>
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This option controls whether column <literal>NOT NULL</literal>
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constraints are included in the definitions of foreign tables imported
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from a foreign server. The default is <literal>true</literal>.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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<para>
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Note that constraints other than <literal>NOT NULL</literal> will never be
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imported from the remote tables. Although <productname>PostgreSQL</productname>
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does support <literal>CHECK</literal> constraints on foreign tables, there is no
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provision for importing them automatically, because of the risk that a
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constraint expression could evaluate differently on the local and remote
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servers. Any such inconsistency in the behavior of a <literal>CHECK</literal>
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constraint could lead to hard-to-detect errors in query optimization.
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So if you wish to import <literal>CHECK</literal> constraints, you must do so
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manually, and you should verify the semantics of each one carefully.
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For more detail about the treatment of <literal>CHECK</literal> constraints on
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foreign tables, see <xref linkend="sql-createforeigntable"/>.
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</para>
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<para>
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Tables or foreign tables which are partitions of some other table are
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automatically excluded. Partitioned tables are imported, unless they
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are a partition of some other table. Since all data can be accessed
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through the partitioned table which is the root of the partitioning
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hierarchy, this approach should allow access to all the data without
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creating extra objects.
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</para>
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</sect3>
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</sect2>
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<sect2>
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<title>Connection Management</title>
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<para>
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<filename>postgres_fdw</filename> establishes a connection to a
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foreign server during the first query that uses a foreign table
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associated with the foreign server. This connection is kept and
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re-used for subsequent queries in the same session. However, if
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multiple user identities (user mappings) are used to access the foreign
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server, a connection is established for each user mapping.
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</para>
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</sect2>
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<sect2>
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<title>Transaction Management</title>
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<para>
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During a query that references any remote tables on a foreign server,
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<filename>postgres_fdw</filename> opens a transaction on the
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remote server if one is not already open corresponding to the current
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local transaction. The remote transaction is committed or aborted when
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the local transaction commits or aborts. Savepoints are similarly
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managed by creating corresponding remote savepoints.
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</para>
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<para>
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The remote transaction uses <literal>SERIALIZABLE</literal>
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isolation level when the local transaction has <literal>SERIALIZABLE</literal>
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isolation level; otherwise it uses <literal>REPEATABLE READ</literal>
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isolation level. This choice ensures that if a query performs multiple
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table scans on the remote server, it will get snapshot-consistent results
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for all the scans. A consequence is that successive queries within a
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single transaction will see the same data from the remote server, even if
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concurrent updates are occurring on the remote server due to other
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activities. That behavior would be expected anyway if the local
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transaction uses <literal>SERIALIZABLE</literal> or <literal>REPEATABLE READ</literal>
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isolation level, but it might be surprising for a <literal>READ
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COMMITTED</literal> local transaction. A future
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<productname>PostgreSQL</productname> release might modify these rules.
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</para>
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<para>
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Note that it is currently not supported by
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<filename>postgres_fdw</filename> to prepare the remote transaction for
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two-phase commit.
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</para>
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</sect2>
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<sect2>
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<title>Remote Query Optimization</title>
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<para>
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<filename>postgres_fdw</filename> attempts to optimize remote queries to reduce
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the amount of data transferred from foreign servers. This is done by
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sending query <literal>WHERE</literal> clauses to the remote server for
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execution, and by not retrieving table columns that are not needed for
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the current query. To reduce the risk of misexecution of queries,
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<literal>WHERE</literal> clauses are not sent to the remote server unless they use
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only data types, operators, and functions that are built-in or belong to an
|
|
extension that's listed in the foreign server's <literal>extensions</literal>
|
|
option. Operators and functions in such clauses must
|
|
be <literal>IMMUTABLE</literal> as well.
|
|
For an <command>UPDATE</command> or <command>DELETE</command> query,
|
|
<filename>postgres_fdw</filename> attempts to optimize the query execution by
|
|
sending the whole query to the remote server if there are no query
|
|
<literal>WHERE</literal> clauses that cannot be sent to the remote server,
|
|
no local joins for the query, no row-level local <literal>BEFORE</literal> or
|
|
<literal>AFTER</literal> triggers or stored generated columns on the target
|
|
table, and no <literal>CHECK OPTION</literal> constraints from parent
|
|
views. In <command>UPDATE</command>,
|
|
expressions to assign to target columns must use only built-in data types,
|
|
<literal>IMMUTABLE</literal> operators, or <literal>IMMUTABLE</literal> functions,
|
|
to reduce the risk of misexecution of the query.
|
|
</para>
|
|
|
|
<para>
|
|
When <filename>postgres_fdw</filename> encounters a join between foreign tables on
|
|
the same foreign server, it sends the entire join to the foreign server,
|
|
unless for some reason it believes that it will be more efficient to fetch
|
|
rows from each table individually, or unless the table references involved
|
|
are subject to different user mappings. While sending the <literal>JOIN</literal>
|
|
clauses, it takes the same precautions as mentioned above for the
|
|
<literal>WHERE</literal> clauses.
|
|
</para>
|
|
|
|
<para>
|
|
The query that is actually sent to the remote server for execution can
|
|
be examined using <command>EXPLAIN VERBOSE</command>.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2>
|
|
<title>Remote Query Execution Environment</title>
|
|
|
|
<para>
|
|
In the remote sessions opened by <filename>postgres_fdw</filename>,
|
|
the <xref linkend="guc-search-path"/> parameter is set to
|
|
just <literal>pg_catalog</literal>, so that only built-in objects are visible
|
|
without schema qualification. This is not an issue for queries
|
|
generated by <filename>postgres_fdw</filename> itself, because it always
|
|
supplies such qualification. However, this can pose a hazard for
|
|
functions that are executed on the remote server via triggers or rules
|
|
on remote tables. For example, if a remote table is actually a view,
|
|
any functions used in that view will be executed with the restricted
|
|
search path. It is recommended to schema-qualify all names in such
|
|
functions, or else attach <literal>SET search_path</literal> options
|
|
(see <xref linkend="sql-createfunction"/>) to such functions
|
|
to establish their expected search path environment.
|
|
</para>
|
|
|
|
<para>
|
|
<filename>postgres_fdw</filename> likewise establishes remote session settings
|
|
for various parameters:
|
|
<itemizedlist spacing="compact">
|
|
<listitem>
|
|
<para>
|
|
<xref linkend="guc-timezone"/> is set to <literal>UTC</literal>
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
<xref linkend="guc-datestyle"/> is set to <literal>ISO</literal>
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
<xref linkend="guc-intervalstyle"/> is set to <literal>postgres</literal>
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
<xref linkend="guc-extra-float-digits"/> is set to <literal>3</literal> for remote
|
|
servers 9.0 and newer and is set to <literal>2</literal> for older versions
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
These are less likely to be problematic than <varname>search_path</varname>, but
|
|
can be handled with function <literal>SET</literal> options if the need arises.
|
|
</para>
|
|
|
|
<para>
|
|
It is <emphasis>not</emphasis> recommended that you override this behavior by
|
|
changing the session-level settings of these parameters; that is likely
|
|
to cause <filename>postgres_fdw</filename> to malfunction.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2>
|
|
<title>Cross-Version Compatibility</title>
|
|
|
|
<para>
|
|
<filename>postgres_fdw</filename> can be used with remote servers dating back
|
|
to <productname>PostgreSQL</productname> 8.3. Read-only capability is available
|
|
back to 8.1. A limitation however is that <filename>postgres_fdw</filename>
|
|
generally assumes that immutable built-in functions and operators are
|
|
safe to send to the remote server for execution, if they appear in a
|
|
<literal>WHERE</literal> clause for a foreign table. Thus, a built-in
|
|
function that was added since the remote server's release might be sent
|
|
to it for execution, resulting in <quote>function does not exist</quote> or
|
|
a similar error. This type of failure can be worked around by
|
|
rewriting the query, for example by embedding the foreign table
|
|
reference in a sub-<literal>SELECT</literal> with <literal>OFFSET 0</literal> as an
|
|
optimization fence, and placing the problematic function or operator
|
|
outside the sub-<literal>SELECT</literal>.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2>
|
|
<title>Examples</title>
|
|
|
|
<para>
|
|
Here is an example of creating a foreign table with
|
|
<literal>postgres_fdw</literal>. First install the extension:
|
|
</para>
|
|
|
|
<programlisting>
|
|
CREATE EXTENSION postgres_fdw;
|
|
</programlisting>
|
|
|
|
<para>
|
|
Then create a foreign server using <xref linkend="sql-createserver"/>.
|
|
In this example we wish to connect to a <productname>PostgreSQL</productname> server
|
|
on host <literal>192.83.123.89</literal> listening on
|
|
port <literal>5432</literal>. The database to which the connection is made
|
|
is named <literal>foreign_db</literal> on the remote server:
|
|
|
|
<programlisting>
|
|
CREATE SERVER foreign_server
|
|
FOREIGN DATA WRAPPER postgres_fdw
|
|
OPTIONS (host '192.83.123.89', port '5432', dbname 'foreign_db');
|
|
</programlisting>
|
|
</para>
|
|
|
|
<para>
|
|
A user mapping, defined with <xref linkend="sql-createusermapping"/>, is
|
|
needed as well to identify the role that will be used on the remote
|
|
server:
|
|
|
|
<programlisting>
|
|
CREATE USER MAPPING FOR local_user
|
|
SERVER foreign_server
|
|
OPTIONS (user 'foreign_user', password 'password');
|
|
</programlisting>
|
|
</para>
|
|
|
|
<para>
|
|
Now it is possible to create a foreign table with
|
|
<xref linkend="sql-createforeigntable"/>. In this example we
|
|
wish to access the table named <structname>some_schema.some_table</structname>
|
|
on the remote server. The local name for it will
|
|
be <structname>foreign_table</structname>:
|
|
|
|
<programlisting>
|
|
CREATE FOREIGN TABLE foreign_table (
|
|
id integer NOT NULL,
|
|
data text
|
|
)
|
|
SERVER foreign_server
|
|
OPTIONS (schema_name 'some_schema', table_name 'some_table');
|
|
</programlisting>
|
|
|
|
It's essential that the data types and other properties of the columns
|
|
declared in <command>CREATE FOREIGN TABLE</command> match the actual remote table.
|
|
Column names must match as well, unless you attach <literal>column_name</literal>
|
|
options to the individual columns to show how they are named in the remote
|
|
table.
|
|
In many cases, use of <xref linkend="sql-importforeignschema"/> is
|
|
preferable to constructing foreign table definitions manually.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2>
|
|
<title>Author</title>
|
|
<para>
|
|
Shigeru Hanada <email>shigeru.hanada@gmail.com</email>
|
|
</para>
|
|
</sect2>
|
|
|
|
</sect1>
|