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> [, ...] ]
VACUUM ( FULL [, ...] ) [ TABLESPACE <replaceable class="parameter">new_tablespace</replaceable> ] [ <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 [ <replaceable class="parameter">boolean</replaceable> ]
    TRUNCATE [ <replaceable class="parameter">boolean</replaceable> ]
    PARALLEL <replaceable class="parameter">integer</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 exclusive 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>
      Specifies that <command>VACUUM</command> should attempt to remove
      index entries pointing to dead tuples.  This is normally the desired
      behavior and is the default unless the
      <literal>vacuum_index_cleanup</literal> option has been set to false
      for the table to be vacuumed.  Setting this option to false may be
      useful when it is necessary to make vacuum run as quickly as possible,
      for example to avoid imminent transaction ID wraparound
      (see <xref linkend="vacuum-for-wraparound"/>).  However, 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.  This option has no effect
      for tables that do not have an index and is ignored if 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 vacuum index and cleanup index phases of <command>VACUUM</command>
      in parallel using <replaceable class="parameter">integer</replaceable>
      background workers (for the detail of each vacuum phases, please
      refer to <xref linkend="vacuum-phases"/>).  If the
      <literal>PARALLEL</literal> option is omitted, then
      <command>VACUUM</command> decides the number of workers based on number
      of indexes that support parallel vacuum operation on the relation which
      is further limited by <xref linkend="guc-max-parallel-workers-maintenance"/>.
      The index can participate in a 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 launches before starting 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><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">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>

   <varlistentry>
    <term><replaceable class="parameter">new_tablespace</replaceable></term>
    <listitem>
     <para>
      The name of a specific tablespace to write a new copy of the table.
     </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 active production databases be
    vacuumed frequently (at least nightly), in order to
    remove dead rows. After adding or deleting a large number
    of rows, it might be a good idea to issue a <command>VACUUM
    ANALYZE</command> command for the affected table. This will update the
    system catalogs with
    the results of all recent changes, and allow the
    <productname>PostgreSQL</productname> query planner to make better
    choices in planning queries.
   </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 purpose.
     Even if this option is specified with <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 proportional to the work done by that
    worker.  See <xref linkend="runtime-config-resource-vacuum-cost"/> for
    details.
   </para>

   <para>
    <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>
 </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>
  </simplelist>
 </refsect1>
</refentry>