<!--
$Header: /cvsroot/pgsql/doc/src/sgml/ref/vacuum.sgml,v 1.6 1999/11/28 02:17:04 tgl Exp $
Postgres documentation
-->

<refentry id="SQL-VACUUM">
 <refmeta>
  <refentrytitle id="sql-vacuum-title">
   VACUUM
  </refentrytitle>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>
 <refnamediv>
  <refname>
   VACUUM
  </refname>
  <refpurpose>
   Clean and analyze a <productname>Postgres</productname> database
  </refpurpose>
 </refnamediv>
 <refsynopsisdiv>
  <refsynopsisdivinfo>
   <date>1999-07-20</date>
  </refsynopsisdivinfo>
  <synopsis>
VACUUM [ VERBOSE ] [ ANALYZE ] [ <replaceable class="PARAMETER">table</replaceable> ]
VACUUM [ VERBOSE ] ANALYZE [ <replaceable class="PARAMETER">ER">t</replaceable>BLE> [ (<replaceable class="PARAMETER">column</replaceable> [, ...] ) ] ]
  </synopsis>

  <refsect2 id="R2-SQL-VACUUM-1">
   <refsect2info>
    <date>1998-10-04</date>
   </refsect2info>
   <title>
    Inputs
   </title>

   <para>
    <variablelist>
     <varlistentry>
      <term>VERBOSE</term>
      <listitem>
       <para>
	Prints a detailed vacuum activity report for each table.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term>ANALYZE</term>
      <listitem>
       <para>
	Updates column statistics used by the optimizer to
	determine the most efficient way to execute a query.
	The statistics represent the disbursion of the data in each column.
	This information is valuable when several execution paths are possible.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><replaceable class="PARAMETER">table</replaceable></term>
      <listitem>
       <para>
	The name of a specific table to vacuum. 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-VACUUM-2">
   <refsect2info>
    <date>1998-10-04</date>
   </refsect2info>
   <title>
    Outputs
   </title>
   <para>

    <variablelist>
     <varlistentry>
      <term><computeroutput>
<returnvalue>VACUUM</returnvalue>
       </computeroutput></term>
      <listitem>
       <para>
	The command has been accepted and the database is being cleaned.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><computeroutput>
NOTICE:  --Relation <replaceable class="PARAMETER">table</replaceable>--
       </computeroutput></term>
      <listitem>
       <para>
	The report header for <replaceable class="PARAMETER">table</replaceable>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><computeroutput>
NOTICE:  Pages 98: Changed 25, Reapped 74, Empty 0, New 0;
         Tup 1000: Vac 3000, Crash 0, UnUsed 0, MinLen 188, MaxLen 188;
         Re-using: Free/Avail. Space 586952/586952; EndEmpty/Avail. Pages 0/74.
         Elapsed 0/0 sec.
       </computeroutput></term>
      <listitem>
       <para>
	The analysis for <replaceable class="PARAMETER">table</replaceable> itself.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><computeroutput>
NOTICE:  Index <replaceable class="PARAMETER">index</replaceable>: Pages 28;
         Tuples 1000: Deleted 3000. Elapsed 0/0 sec.
       </computeroutput></term>
      <listitem>
       <para>
	The analysis for an index on the target table.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </refsect2>
 </refsynopsisdiv>

 <refsect1 id="R1-SQL-VACUUM-1">
  <refsect1info>
   <date>1998-10-04</date>
  </refsect1info>
  <title>
   Description
  </title>
  <para>
   <command>VACUUM</command> serves two purposes in 
   <productname>Postgres</productname> as both a means to reclaim storage and
   also a means to collect information for the optimizer.
  </para>

  <para>
   <command>VACUUM</command> opens every class in the database,
   cleans out records from rolled back transactions, and updates statistics in the
   system catalogs.  The statistics maintained include the number of
   tuples and number of pages stored in all classes.
  </para>

  <para>
   Running <command>VACUUM</command>
   periodically will increase the speed of the database in processing user queries.
  </para>

  <refsect2 id="R2-SQL-VACUUM-3">
   <refsect2info>
    <date>1998-10-04</date>
   </refsect2info>
   <title>
    Notes
   </title>
   <para>
    The open database is the target for <command>VACUUM</command>.
   </para>
   <para>
    We recommend that active production databases be cleaned nightly, in order
    to keep statistics relatively current.  The <command>VACUUM</command>
    query may be executed at any time, however.  In particular, after
    copying a large class into <productname>Postgres</productname> 
    or after deleting a large number of
    records, it may be a good idea to issue a <command>VACUUM</command>
    query.  This will update the system catalogs with the results of all
    recent changes, and allow the <productname>Postgres</productname> 
    query optimizer to make better choices in planning user queries.
   </para>
  </refsect2>
 </refsect1>

 <refsect1 id="R1-SQL-VACUUM-2">
  <title>
   Usage
  </title>
  <para>
   The following is an example from running <command>VACUUM</command> on a table
   in the regression database:

   <programlisting>
regression=> vacuum verbose analyze onek;
NOTICE:  --Relation onek--
NOTICE:  Pages 98: Changed 25, Reapped 74, Empty 0, New 0;
         Tup 1000: Vac 3000, Crash 0, UnUsed 0, MinLen 188, MaxLen 188;
         Re-using: Free/Avail. Space 586952/586952; EndEmpty/Avail. Pages 0/74.
         Elapsed 0/0 sec.
NOTICE:  Index onek_stringu1: Pages 28; Tuples 1000: Deleted 3000. Elapsed 0/0 sec.
NOTICE:  Index onek_hundred: Pages 12; Tuples 1000: Deleted 3000. Elapsed 0/0 sec.
NOTICE:  Index onek_unique2: Pages 19; Tuples 1000: Deleted 3000. Elapsed 0/0 sec.
NOTICE:  Index onek_unique1: Pages 17; Tuples 1000: Deleted 3000. Elapsed 0/0 sec.
NOTICE:  Rel onek: Pages: 98 --> 25; Tuple(s) moved: 1000. Elapsed 0/1 sec.
NOTICE:  Index onek_stringu1: Pages 28; Tuples 1000: Deleted 1000. Elapsed 0/0 sec.
NOTICE:  Index onek_hundred: Pages 12; Tuples 1000: Deleted 1000. Elapsed 0/0 sec.
NOTICE:  Index onek_unique2: Pages 19; Tuples 1000: Deleted 1000. Elapsed 0/0 sec.
NOTICE:  Index onek_unique1: Pages 17; Tuples 1000: Deleted 1000. Elapsed 0/0 sec.
VACUUM
   </programlisting>
  </para>
 </refsect1>

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

  <refsect2 id="R2-SQL-VACUUM-4">
   <refsect2info>
    <date>1998-10-04</date>
   </refsect2info>
   <title>
    SQL92
   </title>
   <para>
    There is no <command>VACUUM</command> statement in <acronym>SQL92</acronym>.
   </para>
  </refsect2>
 </refsect1>
</refentry>

<!-- Keep this comment at the end of the file
Local variables:
mode: sgml
sgml-omittag:nil
sgml-shorttag:t
sgml-minimize-attributes:nil
sgml-always-quote-attributes:t
sgml-indent-step:1
sgml-indent-data:t
sgml-parent-document:nil
sgml-default-dtd-file:"../reference.ced"
sgml-exposed-tags:nil
sgml-local-catalogs:"/usr/lib/sgml/catalog"
sgml-local-ecat-files:nil
End:
-->
