maintenance.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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<chapter id="maintenance">
 <title>Routine Database Maintenance Tasks</title>

 <indexterm zone="maintenance">
  <primary>maintenance</primary>
 </indexterm>

  <para>
   There are a few routine maintenance chores that must be performed on
   a regular basis to keep a <productname>PostgreSQL</productname>
   server running smoothly.  The tasks discussed here are repetitive
   in nature and can easily be automated using standard Unix tools such
   as <application>cron</application> scripts.  But it is the database
   administrator's responsibility to set up appropriate scripts, and to
   check that they execute successfully.
  </para>

  <para>
   One obvious maintenance task is creation of backup copies of the data on a
   regular schedule.  Without a recent backup, you have no chance of recovery
   after a catastrophe (disk failure, fire, mistakenly dropping a critical
   table, etc.).  The backup and recovery mechanisms available in
   <productname>PostgreSQL</productname> are discussed at length in
   <xref linkend="backup">.
  </para>

  <para>
   The other main category of maintenance task is periodic <quote>vacuuming</>
   of the database.  This activity is discussed in
   <xref linkend="routine-vacuuming">.
  </para>

  <para>
   Something else that might need periodic attention is log file management.
   This is discussed in <xref linkend="logfile-maintenance">.
  </para>

  <para>
   <productname>PostgreSQL</productname> is low-maintenance compared
   to some other database management systems.  Nonetheless,
   appropriate attention to these tasks will go far towards ensuring a
   pleasant and productive experience with the system.
  </para>

 <sect1 id="routine-vacuuming">
  <title>Routine Vacuuming</title>

  <indexterm zone="routine-vacuuming">
   <primary>vacuum</primary>
  </indexterm>

  <para>
   <productname>PostgreSQL</productname>'s <command>VACUUM</> command
   must be run on a regular basis for several reasons:

    <orderedlist>
     <listitem>
      <simpara>To recover disk space occupied by updated or deleted
      rows.</simpara>
     </listitem>

     <listitem>
      <simpara>To update data statistics used by the
      <productname>PostgreSQL</productname> query planner.</simpara>
     </listitem>

     <listitem>
      <simpara>To protect against loss of very old data due to
      <firstterm>transaction ID wraparound</>.</simpara>
     </listitem>
    </orderedlist>

   The frequency and scope of the <command>VACUUM</> operations
   performed for each of these reasons will vary depending on the
   needs of each site.  Therefore, database administrators must
   understand these issues and develop an appropriate maintenance
   strategy.  This section concentrates on explaining the high-level
   issues; for details about command syntax and so on, see the <xref
   linkend="sql-vacuum" endterm="sql-vacuum-title"> reference page.
  </para>

  <para>
   Beginning in <productname>PostgreSQL</productname> 7.2, the standard form
   of <command>VACUUM</> can run in parallel with normal database operations
   (selects, inserts, updates, deletes, but not changes to table definitions).
   Routine vacuuming is therefore not nearly as intrusive as it was in prior
   releases, and it is not as critical to try to schedule it at low-usage
   times of day.
  </para>

  <para>
   Beginning in <productname>PostgreSQL</productname> 8.0, there are
   configuration parameters that can be adjusted to further reduce the
   performance impact of background vacuuming.  See
   <xref linkend="runtime-config-resource-vacuum-cost">.
  </para>

  <para>
   An automated mechanism for performing the necessary <command>VACUUM</>
   operations has been added in <productname>PostgreSQL</productname> 8.1.
   See <xref linkend="autovacuum">.
  </para>

  <sect2 id="vacuum-for-space-recovery">
   <title>Recovering disk space</title>

   <indexterm zone="vacuum-for-space-recovery">
    <primary>disk space</primary>
   </indexterm>

   <para>
    In normal <productname>PostgreSQL</productname> operation, an
    <command>UPDATE</> or <command>DELETE</> of a row does not
    immediately remove the old version of the row.
    This approach is necessary to gain the benefits of multiversion
    concurrency control (see <xref linkend="mvcc">): the row version
    must not be deleted while it is still potentially visible to other
    transactions. But eventually, an outdated or deleted row version is no
    longer of interest to any transaction. The space it occupies must be
    reclaimed for reuse by new rows, to avoid infinite growth of disk
    space requirements. This is done by running <command>VACUUM</>.
   </para>

   <para>
    Clearly, a table that receives frequent updates or deletes will need
    to be vacuumed more often than tables that are seldom updated. It
    may be useful to set up periodic <application>cron</> tasks that
    <command>VACUUM</command> only selected tables, skipping tables that are known not to
    change often. This is only likely to be helpful if you have both
    large heavily-updated tables and large seldom-updated tables &mdash; the
    extra cost of vacuuming a small table isn't enough to be worth
    worrying about.
   </para>

   <para>
    There are two variants of the <command>VACUUM</command>
    command. The first form, known as <quote>lazy vacuum</quote> or
    just <command>VACUUM</command>, marks expired data in tables and
    indexes for future reuse; it does <emphasis>not</emphasis> attempt
    to reclaim the space used by this expired data unless the space is
    at the end of the table and an exclusive table lock can be easily 
    obtained. Unused space at the start or middle of the file does
    not result in the file being shortened and space returned to the
    operating system. This variant of <command>VACUUM</command> can be
    run concurrently with normal database operations.
   </para>

   <para>
    The second form is the <command>VACUUM FULL</command>
    command. This uses a more aggressive algorithm for reclaiming the
    space consumed by expired row versions. Any space that is freed by
    <command>VACUUM FULL</command> is immediately returned to the
    operating system. Unfortunately, this variant of the
    <command>VACUUM</command> command acquires an exclusive lock on
    each table while <command>VACUUM FULL</command> is processing
    it. Therefore, frequently using <command>VACUUM FULL</command> can
    have an extremely negative effect on the performance of concurrent
    database queries.
   </para>

   <para>
    The standard form of <command>VACUUM</> is best used with the goal
    of maintaining a fairly level steady-state usage of disk space. If
    you need to return disk space to the operating system you can use
    <command>VACUUM FULL</> &mdash; but what's the point of releasing disk
    space that will only have to be allocated again soon?  Moderately
    frequent standard <command>VACUUM</> runs are a better approach
    than infrequent <command>VACUUM FULL</> runs for maintaining
    heavily-updated tables.
   </para>

   <para>
    Recommended practice for most sites is to schedule a database-wide
    <command>VACUUM</> once a day at a low-usage time of day,
    supplemented by more frequent vacuuming of heavily-updated tables
    if necessary. (Some installations with an extremely high
    rate of data modification <command>VACUUM</command> busy tables as
    often as once every few minutes.)  If you have multiple databases
    in a cluster, don't forget to <command>VACUUM</command> each one;
    the program <xref linkend="app-vacuumdb" endterm="app-vacuumdb-title">
    may be helpful.
   </para>

   <para>
    <command>VACUUM FULL</> is recommended for cases where you know
    you have deleted the majority of rows in a table, so that the
    steady-state size of the table can be shrunk substantially with
    <command>VACUUM FULL</>'s more aggressive approach.  Use plain
    <command>VACUUM</>, not <command>VACUUM FULL</>, for routine
    vacuuming for space recovery.
   </para>

   <para>
    If you have a table whose contents are deleted on a periodic
    basis, consider doing it with <command>TRUNCATE</command> rather
    than using <command>DELETE</command> followed by
    <command>VACUUM</command>. <command>TRUNCATE</command> removes the
    entire content of the table immediately, without requiring a
    subsequent <command>VACUUM</command> or <command>VACUUM
    FULL</command> to reclaim the now-unused disk space.
   </para>
  </sect2>

  <sect2 id="vacuum-for-statistics">
   <title>Updating planner statistics</title>

   <indexterm zone="vacuum-for-statistics">
    <primary>statistics</primary>
    <secondary>of the planner</secondary>
   </indexterm>

   <indexterm zone="vacuum-for-statistics">
    <primary>ANALYZE</primary>
   </indexterm>

   <para>
    The <productname>PostgreSQL</productname> query planner relies on
    statistical information about the contents of tables in order to
    generate good plans for queries.  These statistics are gathered by
    the <command>ANALYZE</> command, which can be invoked by itself or
    as an optional step in <command>VACUUM</>.  It is important to have
    reasonably accurate statistics, otherwise poor choices of plans may
    degrade database performance.
   </para>

   <para>
    As with vacuuming for space recovery, frequent updates of statistics
    are more useful for heavily-updated tables than for seldom-updated
    ones. But even for a heavily-updated table, there may be no need for
    statistics updates if the statistical distribution of the data is
    not changing much. A simple rule of thumb is to think about how much
    the minimum and maximum values of the columns in the table change.
    For example, a <type>timestamp</type> column that contains the time
    of row update will have a constantly-increasing maximum value as
    rows are added and updated; such a column will probably need more
    frequent statistics updates than, say, a column containing URLs for
    pages accessed on a website. The URL column may receive changes just
    as often, but the statistical distribution of its values probably
    changes relatively slowly.
   </para>

   <para>
    It is possible to run <command>ANALYZE</> on specific tables and even