<!--
$Header: /cvsroot/pgsql/doc/src/sgml/maintenance.sgml,v 1.4 2001/09/23 04:06:24 momjian Exp $
-->

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

 <sect1 id="maintenance-general">
  <title>General Discussion</Title>

  <para>
   There are a few routine maintenance chores that must be performed on
   a regular basis to keep a <productname>PostgreSQL</productname>
   installation running smoothly.  The tasks discussed here are repetitive
   in nature and can easily be automated using standard Unix tools such
   as <filename>cron</filename> 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>
  It is also necessary to rotate the database server log file that is specified 
  when the <application>postmaster</application> is started.  If you are using
  <application>syslog</application>, you can send a <literal>SIGHUP</literal>
  signal to the syslog daemon to force it to start writing a new log file.
  </para>

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

 </sect1>

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

  <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 <command>VACUUM</>s performed for each of
   these reasons will vary depending on the needs of each installation.
   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 <command>VACUUM</> command 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 schemas).
   Routine vacuuming is therefore not nearly as intrusive as it was in prior
   releases, and it's not as critical to try to schedule it at low-usage
   times of day.
  </para>

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

   <para>
    In normal <productname>PostgreSQL</productname> operation, an UPDATE or
    DELETE of a row does not immediately remove the old <firstterm>tuple</>
    (version of the row).  This approach is necessary to gain the benefits
    of multiversion concurrency control (see the User's Guide): the tuple
    must not be deleted while 
    it is still potentially visible to other transactions.  But eventually,
    an outdated or deleted tuple is no longer of interest to any transaction.
    The space it occupies must be reclaimed for reuse by new tuples, 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 vacuum 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 --- the extra cost of vacuuming a small table
    isn't enough to be worth worrying about.
   </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.  The standard
    form finds old tuples and makes their space available for re-use within
    the table, but it does not try very hard to shorten the table file and
    return disk space to the operating system.  If you need to return disk
    space to the operating system you can use <command>VACUUM FULL</> ---
    but what's the point of releasing disk space that will only have to be
    allocated again soon?  Moderately frequent standard <command>VACUUM</>s
    are a better approach than infrequent <command>VACUUM FULL</>s 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.
    (If you have multiple databases in an installation, don't forget to
    vacuum each one; the <filename>vacuumdb</> script may be helpful.)
    Use plain <command>VACUUM</>, not <command>VACUUM FULL</>, for routine
    vacuuming for space recovery.
   </para>

   <para>
    <command>VACUUM FULL</> is recommended for cases where you know you have
    deleted the majority of tuples in a table, so that the steady-state size
    of the table can be shrunk substantially with <command>VACUUM FULL</>'s
    more aggressive approach.
   </para>

   <para>
    If you have a table whose contents are deleted completely every so often,
    consider doing it with <command>TRUNCATE</> rather than using
    <command>DELETE</> followed by <command>VACUUM</>.
   </para>
  </sect2>

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

   <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
    just specific columns of a table, so the flexibility exists to update some
    statistics more frequently than others if your application requires it.
    In practice, however, the usefulness of this feature is doubtful.
    Beginning in <productname>PostgreSQL</productname> 7.2,
    <command>ANALYZE</> is a fairly fast operation even on large tables,
    because it uses a statistical random sampling of the rows of a table
    rather than reading every single row.  So it's probably much simpler
    to just run it over the whole database every so often.
   </para>

   <tip>
    <para>
     Although per-column tweaking of <command>ANALYZE</> frequency may not be
     very productive, you may well find it worthwhile to do per-column
     adjustment of the level of detail of the statistics collected by
     <command>ANALYZE</>.  Columns that are heavily used in WHERE clauses
     and have highly irregular data distributions may require a finer-grain
     data histogram than other columns.  See <command>ALTER TABLE SET
     STATISTICS</>.
    </para>
   </tip>

   <para>
    Recommended practice for most sites is to schedule a database-wide
    <command>ANALYZE</> once a day at a low-usage time of day; this can
    usefully be combined with a nightly <command>VACUUM</>.  However,
    sites with relatively slowly changing table statistics may find that
    this is overkill, and that less-frequent <command>ANALYZE</> runs
    are sufficient.
   </para>
  </sect2>

  <sect2 id="vacuum-for-wraparound">
   <title>Preventing transaction ID wraparound failures</title>

   <para>
    <productname>PostgreSQL</productname>'s MVCC transaction semantics
    depend on being able to compare transaction ID (<firstterm>XID</>)
    numbers: a tuple with an insertion XID newer than the current
    transaction's XID is <quote>in the future</> and should not be visible
    to the current transaction.  But since transaction IDs have limited size
    (32 bits at this writing) an installation that runs for a long time (more
    than 4 billion transactions) will suffer <firstterm>transaction ID
    wraparound</>: the XID counter wraps around to zero, and all of a sudden
    transactions that were in the past appear to be in the future --- which
    means their outputs become invisible.  In short, catastrophic data loss.
    (Actually the data is still there, but that's cold comfort if you can't
    get at it.)
   </para>

   <para>
    Prior to <productname>PostgreSQL</productname> 7.2, the only defense
    against XID wraparound was to re-<command>initdb</> at least every 4 billion
    transactions.  This of course was not very satisfactory for high-traffic
    sites, so a better solution has been devised.  The new approach allows an
    installation to remain up indefinitely, without <command>initdb</> or any sort of
    restart.  The price is this maintenance requirement:
    <emphasis>every table in the database must be vacuumed at least once every
    billion transactions</emphasis>.
   </para>

   <para>
    In practice this isn't an onerous requirement, but since the consequences
    of failing to meet it can be complete data loss (not just wasted disk
    space or slow performance), some special provisions have been made to help
    database administrators keep track of the time since the last
    <command>VACUUM</>.  The remainder of this section gives the details.
   </para>

   <para>
    The new approach to XID comparison distinguishes two special XIDs, numbers
    1 and 2 (<literal>BootstrapXID</> and <literal>FrozenXID</>).  These two
    XIDs are always considered older than every normal XID.  Normal XIDs (those
    greater than 2) are compared using modulo-2^31 arithmetic.  This means
    that for every normal XID, there are two billion XIDs that are
    <quote>older</> and two billion that are <quote>newer</>; another way to
    say it is that the normal XID space is circular with no endpoint.
    Therefore, once a tuple has been created with a particular normal XID, the
    tuple will appear to be <quote>in the past</> for the next two billion 
    transactions, no matter which normal XID we are talking about.  If the
    tuple still exists after more than two billion transactions, it will
    suddenly appear to be in the future.  To prevent data loss, old tuples
    must be reassigned the XID <literal>FrozenXID</> sometime before they reach
    the two-billion-transactions-old mark.  Once they are assigned this
    special XID, they will appear to be <quote>in the past</> to all normal
    transactions regardless of wraparound issues, and so such tuples will be
    good until deleted, no matter how long that is.  This reassignment of
    XID is handled by <command>VACUUM</>.
   </para>

   <para>
    <command>VACUUM</>'s normal policy is to reassign <literal>FrozenXID</>
    to any tuple with a normal XID more than one billion transactions in the
    past.  This policy preserves the original insertion XID until it is not
    likely to be of interest anymore (in fact, most tuples will probably
    live and die without ever being <quote>frozen</>).  With this policy,
    the maximum safe interval between <command>VACUUM</>s of any table
    is exactly one billion transactions: if you wait longer, it's possible
    that a tuple that was not quite old enough to be reassigned last time
    is now more than two billion transactions old and has wrapped around
    into the future --- ie, is lost to you.  (Of course, it'll reappear
    after another two billion transactions, but that's no help.)
   </para>

   <para>
    Since periodic <command>VACUUM</>s are needed anyway for the reasons
    described earlier, it's unlikely that any table would not be vacuumed
    for as long as a billion transactions.  But to help administrators ensure
    this constraint is met, <command>VACUUM</> stores transaction ID
    statistics in the system table <filename>pg_database</>.  In particular,
    the <filename>datfrozenxid</> field of a database's
    <filename>pg_database</> row is updated at the completion of any
    database-wide vacuum operation (ie, <command>VACUUM</> that does not
    name a specific table).  The value stored in this field is the freeze
    cutoff XID that was used by that <command>VACUUM</> command.  All normal
    XIDs older than this cutoff XID are guaranteed to have been replaced by
    <literal>FrozenXID</> within that database.  A convenient way to
    examine this information is to execute the query
    <informalexample>
<programlisting>
select datname, age(datfrozenxid) from pg_database;
</programlisting>
    </informalexample>
    The <literal>age</> column measures the number of transactions from the
    cutoff XID to the current transaction's XID.
   </para>

   <para>
    With the standard freezing policy, the <literal>age</> column will start
    at one billion for a freshly-vacuumed database.  When the <literal>age</>
    approaches two billion, the database must be vacuumed again to avoid
    risk of wraparound failures.  Recommended practice is to vacuum each
    database at least once every half-a-billion (500 million) transactions,
    so as to provide plenty of safety margin.  To help meet this rule,
    each database-wide <command>VACUUM</> automatically delivers a warning
    if there are any <filename>pg_database</> entries showing an
    <literal>age</> of more than 1.5 billion transactions, for example:
    <informalexample>
<programlisting>
play=# vacuum;
NOTICE:  Some databases have not been vacuumed in 1613770184 transactions.
        Better vacuum them within 533713463 transactions,
        or you may have a wraparound failure.
VACUUM
</programlisting>
    </informalexample>
   </para>

   <para>
    <command>VACUUM</> with the <command>FREEZE</> option uses a more
    aggressive freezing policy: tuples are frozen if they are old enough
    to be considered good by all open transactions.  In particular, if
    a <command>VACUUM FREEZE</> is performed in an otherwise-idle database,
    it is guaranteed that <emphasis>all</> tuples in that database will be
    frozen.  Hence, as long as the database is not modified in any way, it
    will not need subsequent vacuuming to avoid transaction ID wraparound
    problems.  This technique is used by <filename>initdb</> to prepare the
    <filename>template0</> database.  It should also be used to prepare any
    user-created databases that are to be marked <literal>datallowconn</> =
    <literal>false</> in <filename>pg_database</>, since there isn't any
    convenient way to vacuum a database that you can't connect to.  Note
    that <command>VACUUM</command>'s automatic warning message about unvacuumed databases will
    ignore  <filename>pg_database</> entries with <literal>datallowconn</> =
    <literal>false</>, so as to avoid giving false warnings about these
    databases; therefore it's up to you to ensure that such databases are
    frozen correctly.
   </para>

  </sect2>
 </sect1>
</chapter>

<!-- 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:
-->
