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$Header: /cvsroot/pgsql/doc/src/sgml/ref/analyze.sgml,v 1.5 2001/12/08 03:24:34 thomas Exp $
PostgreSQL documentation
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<refentry id="SQL-ANALYZE">
 <refmeta>
  <refentrytitle id="sql-analyze-title">
   ANALYZE
  </refentrytitle>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>
 <refnamediv>
  <refname>
   ANALYZE
  </refname>
  <refpurpose>
   collect statistics about a database
  </refpurpose>
 </refnamediv>
 <refsynopsisdiv>
  <refsynopsisdivinfo>
   <date>2001-05-04</date>
  </refsynopsisdivinfo>
  <synopsis>
ANALYZE [ VERBOSE ] [ <replaceable class="PARAMETER">table</replaceable> [ (<replaceable class="PARAMETER">column</replaceable> [, ...] ) ] ]
  </synopsis>

  <refsect2 id="R2-SQL-ANALYZE-1">
   <refsect2info>
    <date>2001-05-04</date>
   </refsect2info>
   <title>
    Inputs
   </title>

   <para>
    <variablelist>
     <varlistentry>
      <term>VERBOSE</term>
      <listitem>
       <para>
	Enables display of progress messages.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><replaceable class="PARAMETER">table</replaceable></term>
      <listitem>
       <para>
	The name of a specific table to analyze. Defaults to all tables.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><replaceable class="PARAMETER">column</replaceable></term>
      <listitem>
       <para>
	The name of a specific column to analyze. Defaults to all columns.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </refsect2>

  <refsect2 id="R2-SQL-ANALYZE-2">
   <refsect2info>
    <date>2001-05-04</date>
   </refsect2info>
   <title>
    Outputs
   </title>
   <para>

    <variablelist>
     <varlistentry>
      <term><computeroutput>
<returnvalue>ANALYZE</returnvalue>
       </computeroutput></term>
      <listitem>
       <para>
	The command is complete.
       </para>
      </listitem>
     </varlistentry>

    </variablelist>
   </para>
  </refsect2>
 </refsynopsisdiv>

 <refsect1 id="R1-SQL-ANALYZE-1">
  <refsect1info>
   <date>2001-05-04</date>
  </refsect1info>
  <title>
   Description
  </title>
  <para>
   <command>ANALYZE</command> collects statistics about the contents of
   <productname>PostgreSQL</productname> tables, and stores the results in
   the system table <literal>pg_statistic</literal>.  Subsequently,
   the query planner uses the statistics to help determine the most efficient
   execution plans for queries.
  </para>

  <para>
   With no parameter, <command>ANALYZE</command> examines every table in the
   current database.  With a parameter, <command>ANALYZE</command> examines
   only that table.  It is further possible to give a list of column names,
   in which case only the statistics for those columns are updated.
  </para>

  <refsect2 id="R2-SQL-ANALYZE-3">
   <refsect2info>
    <date>2001-05-04</date>
   </refsect2info>
   <title>
    Notes
   </title>

  <para>
   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 is to run <xref linkend="sql-vacuum" endterm="sql-vacuum-title">
   and <command>ANALYZE</command> once a day during a low-usage time of day.
  </para>

  <para>
   Unlike <command>VACUUM FULL</command>,
   <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>
   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 may result in small changes in the planner's
   estimated costs shown by <command>EXPLAIN</command>.
  </para>

  <para>
   The collected statistics 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 may 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
   datatype does not support the appropriate operators.  There is more
   information about the statistics in the <citetitle>User's
   Guide</citetitle>.
  </para>

  <para>
   The extent of analysis can be controlled by adjusting the per-column
   statistics target with <command>ALTER TABLE ALTER COLUMN SET
   STATISTICS</command> (see
   <xref linkend="sql-altertable" endterm="sql-altertable-title">).  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 10, 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 may be useful to do that for columns that
   are never used as part of the WHERE, GROUP BY, or ORDER BY 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>

  </refsect2>
 </refsect1>

 <refsect1 id="R1-SQL-ANALYZE-3">
  <title>
   Compatibility
  </title>

  <refsect2 id="R2-SQL-ANALYZE-4">
   <refsect2info>
    <date>2001-05-04</date>
   </refsect2info>
   <title>
    SQL92
   </title>
   <para>
    There is no <command>ANALYZE</command> statement in <acronym>SQL92</acronym>.
   </para>
  </refsect2>
 </refsect1>
</refentry>

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