doc/src/sgml/ref/analyze.sgml

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doc/src/sgml/ref/analyze.sgml
PostgreSQL documentation
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<refentry id="sql-analyze">
 <indexterm zone="sql-analyze">
  <primary>ANALYZE</primary>
 </indexterm>

 <refmeta>
  <refentrytitle>ANALYZE</refentrytitle>
  <manvolnum>7</manvolnum>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>ANALYZE</refname>
  <refpurpose>collect statistics about a database</refpurpose>
 </refnamediv>

 <refsynopsisdiv>
<synopsis>
ANALYZE [ ( <replaceable class="parameter">option</replaceable> [, ...] ) ] [ <replaceable class="parameter">table_and_columns</replaceable> [, ...] ]
ANALYZE [ VERBOSE ] [ <replaceable class="parameter">table_and_columns</replaceable> [, ...] ]

<phrase>where <replaceable class="parameter">option</replaceable> can be one of:</phrase>

    VERBOSE [ <replaceable class="parameter">boolean</replaceable> ]
    SKIP_LOCKED [ <replaceable class="parameter">boolean</replaceable> ]

<phrase>and <replaceable class="parameter">table_and_columns</replaceable> is:</phrase>

    <replaceable class="parameter">table_name</replaceable> [ ( <replaceable class="parameter">column_name</replaceable> [, ...] ) ]
</synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   <command>ANALYZE</command> collects statistics about the contents
   of tables in the database, and stores the results in the <link
   linkend="catalog-pg-statistic"><structname>pg_statistic</structname></link>
   system catalog.  Subsequently, the query planner uses these
   statistics to help determine the most efficient execution plans for
   queries.
  </para>

  <para>
   Without a <replaceable class="parameter">table_and_columns</replaceable>
   list, <command>ANALYZE</command> processes every table and materialized view
   in the current database that the current user has permission to analyze.
   With a list, <command>ANALYZE</command> processes only those table(s).
   It is further possible to give a list of column names for a table,
   in which case only the statistics for those columns are collected.
  </para>

  <para>
   When the option list is surrounded by parentheses, the options can be
   written in any order.  The parenthesized syntax was added in
   <productname>PostgreSQL</productname> 11;  the unparenthesized syntax
   is deprecated.
  </para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

  <variablelist>
   <varlistentry>
    <term><literal>VERBOSE</literal></term>
    <listitem>
     <para>
      Enables display of progress messages.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>SKIP_LOCKED</literal></term>
    <listitem>
     <para>
      Specifies that <command>ANALYZE</command> should not wait for any
      conflicting locks to be released when beginning work on a relation:
      if a relation cannot be locked immediately without waiting, the relation
      is skipped.  Note that even with this option, <command>ANALYZE</command>
      may still block when opening the relation's indexes or when acquiring
      sample rows from partitions, table inheritance children, and some
      types of foreign tables.  Also, while <command>ANALYZE</command>
      ordinarily processes all partitions of specified partitioned tables,
      this option will cause <command>ANALYZE</command> to skip all
      partitions if there is a conflicting lock on the partitioned table.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">boolean</replaceable></term>
    <listitem>
     <para>
      Specifies whether the selected option should be turned on or off.
      You can write <literal>TRUE</literal>, <literal>ON</literal>, or
      <literal>1</literal> to enable the option, and <literal>FALSE</literal>,
      <literal>OFF</literal>, or <literal>0</literal> to disable it.  The
      <replaceable class="parameter">boolean</replaceable> value can also
      be omitted, in which case <literal>TRUE</literal> is assumed.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">table_name</replaceable></term>
    <listitem>
     <para>
      The name (possibly schema-qualified) of a specific table to
      analyze.  If omitted, all regular tables, partitioned tables, and
      materialized views in the current database are analyzed (but not
      foreign tables).  If the specified table is a partitioned table, both the
      inheritance statistics of the partitioned table as a whole and
      statistics of the individual partitions are updated.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">column_name</replaceable></term>
    <listitem>
     <para>
      The name of a specific column to analyze. Defaults to all columns.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </refsect1>

 <refsect1>
  <title>Outputs</title>

   <para>
    When <literal>VERBOSE</literal> is specified, <command>ANALYZE</command> emits
    progress messages to indicate which table is currently being
    processed.  Various statistics about the tables are printed as well.
   </para>
 </refsect1>

 <refsect1>
  <title>Notes</title>

  <para>
   To analyze a table, one must ordinarily be the table's owner or a
   superuser.  However, database owners are allowed to
   analyze all tables in their databases, except shared catalogs.
   (The restriction for shared catalogs means that a true database-wide
   <command>ANALYZE</command> can only be performed by a superuser.)
   <command>ANALYZE</command> will skip over any tables that the calling user
   does not have permission to analyze.
  </para>

  <para>
   Foreign tables are analyzed only when explicitly selected.  Not all
   foreign data wrappers support <command>ANALYZE</command>.  If the table's
   wrapper does not support <command>ANALYZE</command>, the command prints a
   warning and does nothing.
  </para>

  <para>
   In the default <productname>PostgreSQL</productname> configuration,
   the autovacuum daemon (see <xref linkend="autovacuum"/>)
   takes care of automatic analyzing of tables when they are first loaded
   with data, and as they change throughout regular operation.
   When autovacuum is disabled,
   it is a good idea to run <command>ANALYZE</command> periodically, or
   just after making major changes in the contents of a table.  Accurate
   statistics will help the planner to choose the most appropriate query
   plan, and thereby improve the speed of query processing.  A common
   strategy for read-mostly databases is to run <link linkend="sql-vacuum"><command>VACUUM</command></link>
   and <command>ANALYZE</command> once a day during a low-usage time of day.
   (This will not be sufficient if there is heavy update activity.)
  </para>

  <para>
   <command>ANALYZE</command>
   requires only a read lock on the target table, so it can run in
   parallel with other activity on the table.
  </para>

  <para>
   The statistics collected by <command>ANALYZE</command> usually
   include a list of some of the most common values in each column and
   a histogram showing the approximate data distribution in each
   column.  One or both of these can be omitted if
   <command>ANALYZE</command> deems them uninteresting (for example,
   in a unique-key column, there are no common values) or if the
   column data type does not support the appropriate operators.  There
   is more information about the statistics in <xref
   linkend="maintenance"/>.
  </para>

  <para>
   For large tables, <command>ANALYZE</command> takes a random sample
   of the table contents, rather than examining every row.  This
   allows even very large tables to be analyzed in a small amount of
   time.  Note, however, that the statistics are only approximate, and
   will change slightly each time <command>ANALYZE</command> is run,
   even if the actual table contents did not change.  This might result
   in small changes in the planner's estimated costs shown by
   <link linkend="sql-explain"><command>EXPLAIN</command></link>.
   In rare situations, this non-determinism will cause the planner's
   choices of query plans to change after <command>ANALYZE</command> is run.
   To avoid this, raise the amount of statistics collected by
   <command>ANALYZE</command>, as described below.
  </para>

  <para>
   The extent of analysis can be controlled by adjusting the
   <xref linkend="guc-default-statistics-target"/> configuration variable, or
   on a column-by-column basis by setting the per-column statistics
   target with <link linkend="sql-altertable"><command>ALTER TABLE ... ALTER COLUMN ... SET
   STATISTICS</command></link>.
   The target value sets the
   maximum number of entries in the most-common-value list and the
   maximum number of bins in the histogram.  The default target value
   is 100, but this can be adjusted up or down to trade off accuracy of
   planner estimates against the time taken for
   <command>ANALYZE</command> and the amount of space occupied in
   <literal>pg_statistic</literal>.  In particular, setting the
   statistics target to zero disables collection of statistics for
   that column.  It might be useful to do that for columns that are
   never used as part of the <literal>WHERE</literal>, <literal>GROUP BY</literal>,
   or <literal>ORDER BY</literal> clauses of queries, since the planner will
   have no use for statistics on such columns.
  </para>

  <para>
   The largest statistics target among the columns being analyzed determines
   the number of table rows sampled to prepare the statistics.  Increasing
   the target causes a proportional increase in the time and space needed
   to do <command>ANALYZE</command>.
  </para>

  <para>
   One of the values estimated by <command>ANALYZE</command> is the number of
   distinct values that appear in each column.  Because only a subset of the
   rows are examined, this estimate can sometimes be quite inaccurate, even
   with the largest possible statistics target.  If this inaccuracy leads to
   bad query plans, a more accurate value can be determined manually and then
   installed with
   <link linkend="sql-altertable"><command>ALTER TABLE ... ALTER COLUMN ... SET (n_distinct = ...)</command></link>.
  </para>

  <para>
    If the table being analyzed has one or more children,
    <command>ANALYZE</command> will gather statistics twice: once on the
    rows of the parent table only, and a second time on the rows of the
    parent table with all of its children.  This second set of statistics
    is needed when planning queries that traverse the entire inheritance
    tree.  The autovacuum daemon, however, will only consider inserts or
    updates on the parent table itself when deciding whether to trigger an
    automatic analyze for that table.  If that table is rarely inserted into
    or updated, the inheritance statistics will not be up to date unless you
    run <command>ANALYZE</command> manually.
  </para>

  <para>
    For partitioned tables, <command>ANALYZE</command> gathers statistics by
    sampling rows from all partitions; in addition, it will recurse into each
    partition and update its statistics.  Each leaf partition is analyzed only
    once, even with multi-level partitioning.  No statistics are collected for
    only the parent table (without data from its partitions), because with
    partitioning it's guaranteed to be empty.
  </para>

  <para>
    By contrast, if the table being analyzed has inheritance children,
    <command>ANALYZE</command> gathers two sets of statistics: one on the rows
    of the parent table only, and a second including rows of both the parent
    table and all of its children.  This second set of statistics is needed when
    planning queries that process the inheritance tree as a whole.  The child
    tables themselves are not individually analyzed in this case.
  </para>

  <para>
    The autovacuum daemon does not process partitioned tables, nor does it
    process inheritance parents if only the children are ever modified.
    It is usually necessary to periodically run a manual
    <command>ANALYZE</command> to keep the statistics of the table hierarchy
    up to date.
  </para>

  <para>
    If any child tables or partitions are foreign tables whose foreign
    data wrappers do not support <command>ANALYZE</command>, those tables are
    ignored while gathering inheritance statistics.
  </para>

  <para>
    If the table being analyzed is completely empty, <command>ANALYZE</command>
    will not record new statistics for that table.  Any existing statistics
    will be retained.
  </para>

  <para>
    Each backend running <command>ANALYZE</command> will report its progress
    in the <structname>pg_stat_progress_analyze</structname> view. See
    <xref linkend="analyze-progress-reporting"/> for details.
  </para>
 </refsect1>

 <refsect1>
  <title>Compatibility</title>

  <para>
   There is no <command>ANALYZE</command> statement in the SQL standard.
  </para>
 </refsect1>

 <refsect1>
  <title>See Also</title>

  <simplelist type="inline">
   <member><xref linkend="sql-vacuum"/></member>
   <member><xref linkend="app-vacuumdb"/></member>
   <member><xref linkend="runtime-config-resource-vacuum-cost"/></member>
   <member><xref linkend="autovacuum"/></member>
   <member><xref linkend="analyze-progress-reporting"/></member>
  </simplelist>
 </refsect1>
</refentry>