288 lines
11 KiB
Plaintext
288 lines
11 KiB
Plaintext
<!--
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doc/src/sgml/ref/analyze.sgml
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PostgreSQL documentation
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-->
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<refentry id="sql-analyze">
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<indexterm zone="sql-analyze">
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<primary>ANALYZE</primary>
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</indexterm>
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<refmeta>
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<refentrytitle>ANALYZE</refentrytitle>
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<manvolnum>7</manvolnum>
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<refmiscinfo>SQL - Language Statements</refmiscinfo>
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</refmeta>
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<refnamediv>
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<refname>ANALYZE</refname>
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<refpurpose>collect statistics about a database</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<synopsis>
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ANALYZE [ ( <replaceable class="parameter">option</replaceable> [, ...] ) ] [ <replaceable class="parameter">table_and_columns</replaceable> [, ...] ]
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ANALYZE [ VERBOSE ] [ <replaceable class="parameter">table_and_columns</replaceable> [, ...] ]
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<phrase>where <replaceable class="parameter">option</replaceable> can be one of:</phrase>
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VERBOSE [ <replaceable class="parameter">boolean</replaceable> ]
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SKIP_LOCKED [ <replaceable class="parameter">boolean</replaceable> ]
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<phrase>and <replaceable class="parameter">table_and_columns</replaceable> is:</phrase>
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<replaceable class="parameter">table_name</replaceable> [ ( <replaceable class="parameter">column_name</replaceable> [, ...] ) ]
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</synopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para>
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<command>ANALYZE</command> collects statistics about the contents
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of tables in the database, and stores the results in the <link
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linkend="catalog-pg-statistic"><structname>pg_statistic</structname></link>
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system catalog. Subsequently, the query planner uses these
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statistics to help determine the most efficient execution plans for
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queries.
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</para>
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<para>
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Without a <replaceable class="parameter">table_and_columns</replaceable>
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list, <command>ANALYZE</command> processes every table and materialized view
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in the current database that the current user has permission to analyze.
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With a list, <command>ANALYZE</command> processes only those table(s).
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It is further possible to give a list of column names for a table,
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in which case only the statistics for those columns are collected.
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</para>
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<para>
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When the option list is surrounded by parentheses, the options can be
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written in any order. The parenthesized syntax was added in
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<productname>PostgreSQL</productname> 11; the unparenthesized syntax
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is deprecated.
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</para>
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</refsect1>
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<refsect1>
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<title>Parameters</title>
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<variablelist>
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<varlistentry>
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<term><literal>VERBOSE</literal></term>
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<listitem>
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<para>
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Enables display of progress messages.
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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>SKIP_LOCKED</literal></term>
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<listitem>
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<para>
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Specifies that <command>ANALYZE</command> should not wait for any
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conflicting locks to be released when beginning work on a relation:
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if a relation cannot be locked immediately without waiting, the relation
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is skipped. Note that even with this option, <command>ANALYZE</command>
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may still block when opening the relation's indexes or when acquiring
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sample rows from partitions, table inheritance children, and some
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types of foreign tables. Also, while <command>ANALYZE</command>
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ordinarily processes all partitions of specified partitioned tables,
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this option will cause <command>ANALYZE</command> to skip all
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partitions if there is a conflicting lock on the partitioned table.
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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><replaceable class="parameter">boolean</replaceable></term>
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<listitem>
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<para>
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Specifies whether the selected option should be turned on or off.
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You can write <literal>TRUE</literal>, <literal>ON</literal>, or
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<literal>1</literal> to enable the option, and <literal>FALSE</literal>,
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<literal>OFF</literal>, or <literal>0</literal> to disable it. The
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<replaceable class="parameter">boolean</replaceable> value can also
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be omitted, in which case <literal>TRUE</literal> is assumed.
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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><replaceable class="parameter">table_name</replaceable></term>
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<listitem>
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<para>
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The name (possibly schema-qualified) of a specific table to
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analyze. If omitted, all regular tables, partitioned tables, and
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materialized views in the current database are analyzed (but not
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foreign tables). If the specified table is a partitioned table, both the
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inheritance statistics of the partitioned table as a whole and
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statistics of the individual partitions are updated.
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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><replaceable class="parameter">column_name</replaceable></term>
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<listitem>
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<para>
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The name of a specific column to analyze. Defaults to all columns.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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<refsect1>
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<title>Outputs</title>
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<para>
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When <literal>VERBOSE</literal> is specified, <command>ANALYZE</command> emits
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progress messages to indicate which table is currently being
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processed. Various statistics about the tables are printed as well.
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</para>
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</refsect1>
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<refsect1>
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<title>Notes</title>
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<para>
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Foreign tables are analyzed only when explicitly selected. Not all
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foreign data wrappers support <command>ANALYZE</command>. If the table's
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wrapper does not support <command>ANALYZE</command>, the command prints a
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warning and does nothing.
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</para>
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<para>
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In the default <productname>PostgreSQL</productname> configuration,
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the autovacuum daemon (see <xref linkend="autovacuum"/>)
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takes care of automatic analyzing of tables when they are first loaded
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with data, and as they change throughout regular operation.
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When autovacuum is disabled,
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it is a good idea to run <command>ANALYZE</command> periodically, or
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just after making major changes in the contents of a table. Accurate
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statistics will help the planner to choose the most appropriate query
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plan, and thereby improve the speed of query processing. A common
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strategy for read-mostly databases is to run <xref linkend="sql-vacuum"/>
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and <command>ANALYZE</command> once a day during a low-usage time of day.
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(This will not be sufficient if there is heavy update activity.)
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</para>
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<para>
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<command>ANALYZE</command>
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requires only a read lock on the target table, so it can run in
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parallel with other activity on the table.
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</para>
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<para>
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The statistics collected by <command>ANALYZE</command> usually
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include a list of some of the most common values in each column and
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a histogram showing the approximate data distribution in each
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column. One or both of these can be omitted if
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<command>ANALYZE</command> deems them uninteresting (for example,
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in a unique-key column, there are no common values) or if the
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column data type does not support the appropriate operators. There
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is more information about the statistics in <xref
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linkend="maintenance"/>.
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</para>
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<para>
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For large tables, <command>ANALYZE</command> takes a random sample
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of the table contents, rather than examining every row. This
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allows even very large tables to be analyzed in a small amount of
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time. Note, however, that the statistics are only approximate, and
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will change slightly each time <command>ANALYZE</command> is run,
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even if the actual table contents did not change. This might result
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in small changes in the planner's estimated costs shown by
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<xref linkend="sql-explain"/>.
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In rare situations, this non-determinism will cause the planner's
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choices of query plans to change after <command>ANALYZE</command> is run.
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To avoid this, raise the amount of statistics collected by
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<command>ANALYZE</command>, as described below.
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</para>
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<para>
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The extent of analysis can be controlled by adjusting the
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<xref linkend="guc-default-statistics-target"/> configuration variable, or
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on a column-by-column basis by setting the per-column statistics
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target with <command>ALTER TABLE ... ALTER COLUMN ... SET
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STATISTICS</command> (see <xref linkend="sql-altertable"/>).
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The target value sets the
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maximum number of entries in the most-common-value list and the
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maximum number of bins in the histogram. The default target value
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is 100, but this can be adjusted up or down to trade off accuracy of
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planner estimates against the time taken for
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<command>ANALYZE</command> and the amount of space occupied in
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<literal>pg_statistic</literal>. In particular, setting the
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statistics target to zero disables collection of statistics for
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that column. It might be useful to do that for columns that are
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never used as part of the <literal>WHERE</literal>, <literal>GROUP BY</literal>,
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or <literal>ORDER BY</literal> clauses of queries, since the planner will
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have no use for statistics on such columns.
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</para>
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<para>
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The largest statistics target among the columns being analyzed determines
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the number of table rows sampled to prepare the statistics. Increasing
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the target causes a proportional increase in the time and space needed
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to do <command>ANALYZE</command>.
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</para>
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<para>
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One of the values estimated by <command>ANALYZE</command> is the number of
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distinct values that appear in each column. Because only a subset of the
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rows are examined, this estimate can sometimes be quite inaccurate, even
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with the largest possible statistics target. If this inaccuracy leads to
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bad query plans, a more accurate value can be determined manually and then
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installed with
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<command>ALTER TABLE ... ALTER COLUMN ... SET (n_distinct = ...)</command>
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(see <xref linkend="sql-altertable"/>).
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</para>
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<para>
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If the table being analyzed has one or more children,
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<command>ANALYZE</command> will gather statistics twice: once on the
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rows of the parent table only, and a second time on the rows of the
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parent table with all of its children. This second set of statistics
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is needed when planning queries that traverse the entire inheritance
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tree. The autovacuum daemon, however, will only consider inserts or
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updates on the parent table itself when deciding whether to trigger an
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automatic analyze for that table. If that table is rarely inserted into
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or updated, the inheritance statistics will not be up to date unless you
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run <command>ANALYZE</command> manually.
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</para>
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<para>
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If any of the child tables are foreign tables whose foreign data wrappers
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do not support <command>ANALYZE</command>, those child tables are ignored while
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gathering inheritance statistics.
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</para>
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<para>
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If the table being analyzed is completely empty, <command>ANALYZE</command>
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will not record new statistics for that table. Any existing statistics
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will be retained.
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</para>
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</refsect1>
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<refsect1>
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<title>Compatibility</title>
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<para>
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There is no <command>ANALYZE</command> statement in the SQL standard.
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</para>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<simplelist type="inline">
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<member><xref linkend="sql-vacuum"/></member>
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<member><xref linkend="app-vacuumdb"/></member>
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<member><xref linkend="runtime-config-resource-vacuum-cost"/></member>
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<member><xref linkend="autovacuum"/></member>
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</simplelist>
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</refsect1>
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</refentry>
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