path: root/doc/src/sgml/ref/vacuum.sgml

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

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

 <refnamediv>
  <refname>VACUUM</refname>
  <refpurpose>garbage-collect and optionally analyze a database</refpurpose>
 </refnamediv>

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

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

    FULL [ <replaceable class="parameter">boolean</replaceable> ]
    FREEZE [ <replaceable class="parameter">boolean</replaceable> ]
    VERBOSE [ <replaceable class="parameter">boolean</replaceable> ]
    ANALYZE [ <replaceable class="parameter">boolean</replaceable> ]
    DISABLE_PAGE_SKIPPING [ <replaceable class="parameter">boolean</replaceable> ]
    SKIP_LOCKED [ <replaceable class="parameter">boolean</replaceable> ]
    INDEX_CLEANUP { AUTO | ON | OFF }
    PROCESS_MAIN [ <replaceable class="parameter">boolean</replaceable> ]
    PROCESS_TOAST [ <replaceable class="parameter">boolean</replaceable> ]
    TRUNCATE [ <replaceable class="parameter">boolean</replaceable> ]
    PARALLEL <replaceable class="parameter">integer</replaceable>
    SKIP_DATABASE_STATS [ <replaceable class="parameter">boolean</replaceable> ]
    ONLY_DATABASE_STATS [ <replaceable class="parameter">boolean</replaceable> ]
    BUFFER_USAGE_LIMIT <replaceable class="parameter">size</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>VACUUM</command> reclaims storage occupied by dead tuples.
   In normal <productname>PostgreSQL</productname> operation, tuples that
   are deleted or obsoleted by an update are not physically removed from
   their table; they remain present until a <command>VACUUM</command> is
   done.  Therefore it's necessary to do <command>VACUUM</command>
   periodically, especially on frequently-updated tables.
  </para>

  <para>
   Without a <replaceable class="parameter">table_and_columns</replaceable>
   list, <command>VACUUM</command> processes every table and materialized view
   in the current database that the current user has permission to vacuum.
   With a list, <command>VACUUM</command> processes only those table(s).
  </para>

  <para>
   <command>VACUUM ANALYZE</command> performs a <command>VACUUM</command>
   and then an <command>ANALYZE</command> for each selected table.  This
   is a handy combination form for routine maintenance scripts.  See
   <xref linkend="sql-analyze"/>
   for more details about its processing.
  </para>

  <para>
   Plain <command>VACUUM</command> (without <literal>FULL</literal>) simply reclaims
   space and makes it
   available for re-use.  This form of the command can operate in parallel
   with normal reading and writing of the table, as an exclusive lock
   is not obtained.  However, extra space is not returned to the operating
   system (in most cases); it's just kept available for re-use within the
   same table.  It also allows us to leverage multiple CPUs in order to process
   indexes.  This feature is known as <firstterm>parallel vacuum</firstterm>.
   To disable this feature, one can use <literal>PARALLEL</literal> option and
   specify parallel workers as zero.  <command>VACUUM FULL</command> rewrites
   the entire contents of the table into a new disk file with no extra space,
   allowing unused space to be returned to the operating system.  This form is
   much slower and requires an <literal>ACCESS EXCLUSIVE</literal> lock on
   each table while it is being processed.
  </para>

  <para>
   When the option list is surrounded by parentheses, the options can be
   written in any order.  Without parentheses, options must be specified
   in exactly the order shown above.
   The parenthesized syntax was added in
   <productname>PostgreSQL</productname> 9.0;  the unparenthesized
   syntax is deprecated.
  </para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

  <variablelist>
   <varlistentry>
    <term><literal>FULL</literal></term>
    <listitem>
     <para>
      Selects <quote>full</quote> vacuum, which can reclaim more
      space, but takes much longer and exclusively locks the table.
      This method also requires extra disk space, since it writes a
      new copy of the table and doesn't release the old copy until
      the operation is complete.  Usually this should only be used when a
      significant amount of space needs to be reclaimed from within the table.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>FREEZE</literal></term>
    <listitem>
     <para>
      Selects aggressive <quote>freezing</quote> of tuples.
      Specifying <literal>FREEZE</literal> is equivalent to performing
      <command>VACUUM</command> with the
      <xref linkend="guc-vacuum-freeze-min-age"/> and
      <xref linkend="guc-vacuum-freeze-table-age"/> parameters
      set to zero.  Aggressive freezing is always performed when the
      table is rewritten, so this option is redundant when <literal>FULL</literal>
      is specified.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>VERBOSE</literal></term>
    <listitem>
     <para>
      Prints a detailed vacuum activity report for each table.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>ANALYZE</literal></term>
    <listitem>
     <para>
      Updates statistics used by the planner to determine the most
      efficient way to execute a query.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>DISABLE_PAGE_SKIPPING</literal></term>
    <listitem>
     <para>
      Normally, <command>VACUUM</command> will skip pages based on the <link
      linkend="vacuum-for-visibility-map">visibility map</link>.  Pages where
      all tuples are known to be frozen can always be skipped, and those
      where all tuples are known to be visible to all transactions may be
      skipped except when performing an aggressive vacuum.  Furthermore,
      except when performing an aggressive vacuum, some pages may be skipped
      in order to avoid waiting for other sessions to finish using them.
      This option disables all page-skipping behavior, and is intended to
      be used only when the contents of the visibility map are
      suspect, which should happen only if there is a hardware or software
      issue causing database corruption.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>SKIP_LOCKED</literal></term>
    <listitem>
     <para>
      Specifies that <command>VACUUM</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>VACUUM</command> may still block when opening the relation's
      indexes.  Additionally, <command>VACUUM ANALYZE</command> may still
      block when acquiring sample rows from partitions, table inheritance
      children, and some types of foreign tables.  Also, while
      <command>VACUUM</command> ordinarily processes all partitions of
      specified partitioned tables, this option will cause
      <command>VACUUM</command> to skip all partitions if there is a
      conflicting lock on the partitioned table.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>INDEX_CLEANUP</literal></term>
    <listitem>
     <para>
      Normally, <command>VACUUM</command> will skip index vacuuming
      when there are very few dead tuples in the table.  The cost of
      processing all of the table's indexes is expected to greatly
      exceed the benefit of removing dead index tuples when this
      happens.  This option can be used to force
      <command>VACUUM</command> to process indexes when there are more
      than zero dead tuples.  The default is <literal>AUTO</literal>,
      which allows <command>VACUUM</command> to skip index vacuuming
      when appropriate.  If <literal>INDEX_CLEANUP</literal> is set to
      <literal>ON</literal>, <command>VACUUM</command> will
      conservatively remove all dead tuples from indexes.  This may be
      useful for backwards compatibility with earlier releases of
      <productname>PostgreSQL</productname> where this was the
      standard behavior.
     </para>
     <para>
      <literal>INDEX_CLEANUP</literal> can also be set to
      <literal>OFF</literal> to force <command>VACUUM</command> to
      <emphasis>always</emphasis> skip index vacuuming, even when
      there are many dead tuples in the table.  This may be useful
      when it is necessary to make <command>VACUUM</command> run as
      quickly as possible to avoid imminent transaction ID wraparound
      (see <xref linkend="vacuum-for-wraparound"/>).  However, the
      wraparound failsafe mechanism controlled by <xref
       linkend="guc-vacuum-failsafe-age"/>  will generally trigger
      automatically to avoid transaction ID wraparound failure, and
      should be preferred.  If index cleanup is not performed
      regularly, performance may suffer, because as the table is
      modified indexes will accumulate dead tuples and the table
      itself will accumulate dead line pointers that cannot be removed
      until index cleanup is completed.
     </para>
     <para>
      This option has no effect for tables that have no index and is
      ignored if the <literal>FULL</literal> option is used.  It also
      has no effect on the transaction ID wraparound failsafe
      mechanism.  When triggered it will skip index vacuuming, even
      when <literal>INDEX_CLEANUP</literal> is set to
      <literal>ON</literal>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>PROCESS_MAIN</literal></term>
    <listitem>
     <para>
      Specifies that <command>VACUUM</command> should attempt to process the
      main relation. This is usually the desired behavior and is the default.
      Setting this option to false may be useful when it is only necessary to
      vacuum a relation's corresponding <literal>TOAST</literal> table.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>PROCESS_TOAST</literal></term>
    <listitem>
     <para>
      Specifies that <command>VACUUM</command> should attempt to process the
      corresponding <literal>TOAST</literal> table for each relation, if one
      exists. This is usually the desired behavior and is the default.
      Setting this option to false may be useful when it is only necessary to
      vacuum the main relation. This option is required when the
      <literal>FULL</literal> option is used.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>TRUNCATE</literal></term>
    <listitem>
     <para>
      Specifies that <command>VACUUM</command> should attempt to
      truncate off any empty pages at the end of the table and allow
      the disk space for the truncated pages to be returned to
      the operating system. This is normally the desired behavior
      and is the default unless the <literal>vacuum_truncate</literal>
      option has been set to false for the table to be vacuumed.
      Setting this option to false may be useful to avoid
      <literal>ACCESS EXCLUSIVE</literal> lock on the table that
      the truncation requires. This option is ignored if the
      <literal>FULL</literal> option is used.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>PARALLEL</literal></term>
    <listitem>
     <para>
      Perform index vacuum and index cleanup phases of <command>VACUUM</command>
      in parallel using <replaceable class="parameter">integer</replaceable>
      background workers (for the details of each vacuum phase, please
      refer to <xref linkend="vacuum-phases"/>).  The number of workers used
      to perform the operation is equal to the number of indexes on the
      relation that support parallel vacuum which is limited by the number of
      workers specified with <literal>PARALLEL</literal> option if any which is
      further limited by <xref linkend="guc-max-parallel-maintenance-workers"/>.
      An index can participate in parallel vacuum if and only if the size of the
      index is more than <xref linkend="guc-min-parallel-index-scan-size"/>.
      Please note that it is not guaranteed that the number of parallel workers
      specified in <replaceable class="parameter">integer</replaceable> will be
      used during execution.  It is possible for a vacuum to run with fewer
      workers than specified, or even with no workers at all.  Only one worker
      can be used per index.  So parallel workers are launched only when there
      are at least <literal>2</literal> indexes in the table.  Workers for
      vacuum are launched before the start of each phase and exit at the end of
      the phase.  These behaviors might change in a future release.  This
      option can't be used with the <literal>FULL</literal> option.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>SKIP_DATABASE_STATS</literal></term>
    <listitem>
     <para>
      Specifies that <command>VACUUM</command> should skip updating the
      database-wide statistics about oldest unfrozen XIDs.  Normally
      <command>VACUUM</command> will update these statistics once at the
      end of the command.  However, this can take awhile in a database
      with a very large number of tables, and it will accomplish nothing
      unless the table that had contained the oldest unfrozen XID was
      among those vacuumed.  Moreover, if multiple <command>VACUUM</command>
      commands are issued in parallel, only one of them can update the
      database-wide statistics at a time.  Therefore, if an application
      intends to issue a series of many <command>VACUUM</command>
      commands, it can be helpful to set this option in all but the last
      such command; or set it in all the commands and separately
      issue <literal>VACUUM (ONLY_DATABASE_STATS)</literal> afterwards.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>ONLY_DATABASE_STATS</literal></term>
    <listitem>
     <para>
      Specifies that <command>VACUUM</command> should do nothing except
      update the database-wide statistics about oldest unfrozen XIDs.
      When this option is specified,
      the <replaceable class="parameter">table_and_columns</replaceable>
      list must be empty, and no other option may be enabled
      except <literal>VERBOSE</literal>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>BUFFER_USAGE_LIMIT</literal></term>
    <listitem>
     <para>
      Specifies the
      <glossterm linkend="glossary-buffer-access-strategy">Buffer Access Strategy</glossterm>
      ring buffer size for <command>VACUUM</command>.  This size is used to
      calculate the number of shared buffers which will be reused as part of
      this strategy.  <literal>0</literal> disables use of a
      <literal>Buffer Access Strategy</literal>.  If <option>ANALYZE</option>
      is also specified, the <option>BUFFER_USAGE_LIMIT</option> value is used
      for both the vacuum and analyze stages.  This option can't be used with
      the <option>FULL</option> option except if <option>ANALYZE</option> is
      also specified.  When this option is not specified,
      <command>VACUUM</command> uses the value from
      <xref linkend="guc-vacuum-buffer-usage-limit"/>.  Higher settings can
      allow <command>VACUUM</command> to run more quickly, but having too
      large a setting may cause too many other useful pages to be evicted from
      shared buffers.  The minimum value is <literal>128 kB</literal> and the
      maximum value is <literal>16 GB</literal>.
     </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">integer</replaceable></term>
    <listitem>
     <para>
      Specifies a non-negative integer value passed to the selected option.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">size</replaceable></term>
    <listitem>
     <para>
      Specifies an amount of memory in kilobytes.  Sizes may also be specified
      as a string containing the numerical size followed by any one of the
      following memory units: <literal>B</literal> (bytes),
      <literal>kB</literal> (kilobytes), <literal>MB</literal> (megabytes),
      <literal>GB</literal> (gigabytes), or <literal>TB</literal> (terabytes).
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">table_name</replaceable></term>
    <listitem>
     <para>
      The name (optionally schema-qualified) of a specific table or
      materialized view to vacuum.  If the specified table is a partitioned
      table, all of its leaf partitions are vacuumed.
     </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.
      If a column list is specified, <literal>ANALYZE</literal> must also be
      specified.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </refsect1>

 <refsect1>
  <title>Outputs</title>

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

   <para>
    <command>VACUUM</command> cannot be executed inside a transaction block.
   </para>

   <para>
    For tables with <acronym>GIN</acronym> indexes, <command>VACUUM</command> (in
    any form) also completes any pending index insertions, by moving pending
    index entries to the appropriate places in the main <acronym>GIN</acronym> index
    structure.  See <xref linkend="gin-fast-update"/> for details.
   </para>

   <para>
    We recommend that all databases be vacuumed regularly in
    order to remove dead rows.  <productname>PostgreSQL</productname> includes
    an <quote>autovacuum</quote> facility which can automate routine vacuum
    maintenance.  For more information about automatic and manual vacuuming,
    see <xref linkend="routine-vacuuming"/>.
   </para>

   <para>
    The <option>FULL</option> option is not recommended for routine use,
    but might be useful in special cases.  An example is when you have deleted
    or updated most of the rows in a table and would like the table to
    physically shrink to occupy less disk space and allow faster table
    scans. <command>VACUUM FULL</command> will usually shrink the table
    more than a plain <command>VACUUM</command> would.
   </para>

   <para>
     The <option>PARALLEL</option> option is used only for vacuum purposes.
     If this option is specified with the <option>ANALYZE</option> option,
     it does not affect <option>ANALYZE</option>.
   </para>

   <para>
    <command>VACUUM</command> causes a substantial increase in I/O traffic,
    which might cause poor performance for other active sessions.  Therefore,
    it is sometimes advisable to use the cost-based vacuum delay feature.  For
    parallel vacuum, each worker sleeps in proportion to the work done by that
    worker.  See <xref linkend="runtime-config-resource-vacuum-cost"/> for
    details.
   </para>

   <para>
    Each backend running <command>VACUUM</command> without the
    <literal>FULL</literal> option will report its progress in the
    <structname>pg_stat_progress_vacuum</structname> view. Backends running
    <command>VACUUM FULL</command> will instead report their progress in the
    <structname>pg_stat_progress_cluster</structname> view. See
    <xref linkend="vacuum-progress-reporting"/> and
    <xref linkend="cluster-progress-reporting"/> for details.
   </para>
 </refsect1>

 <refsect1>
  <title>Examples</title>

  <para>
   To clean a single table <literal>onek</literal>, analyze it for
   the optimizer and print a detailed vacuum activity report:

<programlisting>
VACUUM (VERBOSE, ANALYZE) onek;
</programlisting></para>
 </refsect1>

 <refsect1>
  <title>Compatibility</title>

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

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

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