vacuum.sgml

PostgreSQL 8.1.4的源码 适用于Linux下的开源数据库系统

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$PostgreSQL: pgsql/doc/src/sgml/ref/vacuum.sgml,v 1.40 2005/11/01 21:09:51 tgl Exp $
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<refentry id="SQL-VACUUM">
 <refmeta>
  <refentrytitle id="sql-vacuum-title">VACUUM</refentrytitle>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

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

 <indexterm zone="sql-vacuum">
  <primary>VACUUM</primary>
 </indexterm>

 <refsynopsisdiv>
<synopsis>
VACUUM [ FULL | FREEZE ] [ VERBOSE ] [ <replaceable class="PARAMETER">table</replaceable> ]
VACUUM [ FULL | FREEZE ] [ VERBOSE ] ANALYZE [ <replaceable class="PARAMETER">table</replaceable> [ (<replaceable class="PARAMETER">column</replaceable> [, ...] ) ] ]
</synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   <command>VACUUM</command> reclaims storage occupied by deleted 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>
   With no parameter, <command>VACUUM</command> processes every table in the
   current database.  With a parameter, <command>VACUUM</command> processes
   only that table.
  </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" endterm="sql-analyze-title">
   for more details about its processing.
  </para>

  <para>
   Plain <command>VACUUM</command> (without <literal>FULL</>) 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. <command>VACUUM
   FULL</command> does more extensive processing, including moving of tuples
   across blocks to try to compact the table to the minimum number of disk
   blocks.  This form is much slower and requires an exclusive lock on each
   table while it is being processed.
  </para>

  <para>
   <literal>FREEZE</literal> is a special-purpose option that
   causes tuples to be marked <quote>frozen</quote> as soon as possible,
   rather than waiting until they are quite old.  If this is done when there
   are no other open transactions in the same database, then it is guaranteed
   that all tuples in the database are <quote>frozen</> and will not be
   subject to transaction ID wraparound problems, no matter how long the
   database is left unvacuumed.
   <literal>FREEZE</literal> is not recommended for routine use.  Its only
   intended usage is in connection with preparation of user-defined template
   databases, or other databases that are completely read-only and will not
   receive routine maintenance <command>VACUUM</> operations.
   See <xref linkend="maintenance"> for details.
  </para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

  <variablelist>
   <varlistentry>
    <term><literal>FULL</literal></term>
    <listitem>
     <para>
      Selects <quote>full</quote> vacuum, which may reclaim more
      space, but takes much longer and exclusively locks the table.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>FREEZE</literal></term>
    <listitem>
     <para>
      Selects aggressive <quote>freezing</quote> of tuples.
     </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><replaceable class="PARAMETER">table</replaceable></term>
    <listitem>
     <para>
      The name (optionally schema-qualified) of a specific table to
      vacuum. Defaults to all tables in the current database.
     </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>
 </refsect1>

 <refsect1>
  <title>Outputs</title>

   <para>
    When <literal>VERBOSE</> is specified, <command>VACUUM</> 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>
    We recommend that active production databases be
    vacuumed frequently (at least nightly), in order to
    remove expired rows. After adding or deleting a large number
    of rows, it may 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 may be useful in special cases.  An example is when you have deleted
    most of the rows in a table and would like the table to physically shrink
    to occupy less disk space.  <command>VACUUM FULL</command> will usually
    shrink the table more than a plain <command>VACUUM</command> would.
   </para>

   <para>
    <command>VACUUM</command> causes a substantial increase in I/O traffic,
    which can cause poor performance for other active sessions.  Therefore,
    it is sometimes advisable to use the cost-based vacuum delay feature.
    See <xref linkend="runtime-config-resource-vacuum-cost"> for details.
   </para>
 </refsect1>

 <refsect1>
  <title>Examples</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;
INFO:  vacuuming "public.onek"
INFO:  index "onek_unique1" now contains 1000 tuples in 14 pages
DETAIL:  3000 index tuples were removed.
0 index pages have been deleted, 0 are currently reusable.
CPU 0.01s/0.08u sec elapsed 0.18 sec.
INFO:  index "onek_unique2" now contains 1000 tuples in 16 pages
DETAIL:  3000 index tuples were removed.
0 index pages have been deleted, 0 are currently reusable.
CPU 0.00s/0.07u sec elapsed 0.23 sec.
INFO:  index "onek_hundred" now contains 1000 tuples in 13 pages
DETAIL:  3000 index tuples were removed.
0 index pages have been deleted, 0 are currently reusable.
CPU 0.01s/0.08u sec elapsed 0.17 sec.
INFO:  index "onek_stringu1" now contains 1000 tuples in 48 pages
DETAIL:  3000 index tuples were removed.
0 index pages have been deleted, 0 are currently reusable.
CPU 0.01s/0.09u sec elapsed 0.59 sec.
INFO:  "onek": removed 3000 tuples in 108 pages
DETAIL:  CPU 0.01s/0.06u sec elapsed 0.07 sec.
INFO:  "onek": found 3000 removable, 1000 nonremovable tuples in 143 pages
DETAIL:  0 dead tuples cannot be removed yet.
There were 0 unused item pointers.
0 pages are entirely empty.
CPU 0.07s/0.39u sec elapsed 1.56 sec.
INFO:  analyzing "public.onek"
INFO:  "onek": 36 pages, 1000 rows sampled, 1000 estimated total rows
VACUUM
</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" endterm="app-vacuumdb-title"></member>
   <member><xref linkend="runtime-config-resource-vacuum-cost" endterm="runtime-config-resource-vacuum-cost-title"></member>
  </simplelist>
 </refsect1>
</refentry>

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