cluster.sgml

<!--
doc/src/sgml/ref/cluster.sgml
PostgreSQL documentation
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<refentry id="SQL-CLUSTER">
 <refmeta>
  <refentrytitle>CLUSTER</refentrytitle>
  <manvolnum>7</manvolnum>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>CLUSTER</refname>
  <refpurpose>cluster a table according to an index</refpurpose>
 </refnamediv>

 <indexterm zone="sql-cluster">
  <primary>CLUSTER</primary>
 </indexterm>

 <refsynopsisdiv>
<synopsis>
CLUSTER [VERBOSE] <replaceable class="PARAMETER">table_name</replaceable> [ USING <replaceable class="PARAMETER">index_name</replaceable> ]
CLUSTER [VERBOSE]
</synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   <command>CLUSTER</command> instructs <productname>PostgreSQL</productname>
   to cluster the table specified
   by <replaceable class="parameter">table_name</replaceable>
   based on the index specified by
   <replaceable class="parameter">index_name</replaceable>. The index must
   already have been defined on
   <replaceable class="parameter">table_name</replaceable>.
  </para>

  <para>
   When a table is clustered, it is physically reordered
   based on the index information. Clustering is a one-time operation:
   when the table is subsequently updated, the changes are
   not clustered.  That is, no attempt is made to store new or
   updated rows according to their index order.  (If one wishes, one can
   periodically recluster by issuing the command again.  Also, setting
   the table's <literal>FILLFACTOR</literal> storage parameter to less than
   100% can aid in preserving cluster ordering during updates, since updated
   rows are kept on the same page if enough space is available there.)
  </para>

  <para>
   When a table is clustered, <productname>PostgreSQL</productname>
   remembers which index it was clustered by.  The form
   <command>CLUSTER <replaceable class="parameter">table_name</replaceable></command>
   reclusters the table using the same index as before.  You can also
   use the <literal>CLUSTER</literal> or <literal>SET WITHOUT CLUSTER</literal>
   forms of <xref linkend="SQL-ALTERTABLE"> to set the index to be used for
   future cluster operations, or to clear any previous setting.
  </para>

  <para>
   <command>CLUSTER</command> without any parameter reclusters all the
   previously-clustered tables in the current database that the calling user
   owns, or all such tables if called by a superuser.  This
   form of <command>CLUSTER</command> cannot be executed inside a transaction
   block.
  </para>

  <para>
   When a table is being clustered, an <literal>ACCESS
   EXCLUSIVE</literal> lock is acquired on it. This prevents any other
   database operations (both reads and writes) from operating on the
   table until the <command>CLUSTER</command> is finished.
  </para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

  <variablelist>
   <varlistentry>
    <term><replaceable class="PARAMETER">table_name</replaceable></term>
    <listitem>
     <para>
      The name (possibly schema-qualified) of a table.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="PARAMETER">index_name</replaceable></term>
    <listitem>
     <para>
      The name of an index.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>VERBOSE</literal></term>
    <listitem>
     <para>
      Prints a progress report as each table is clustered.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </refsect1>

 <refsect1>
  <title>Notes</title>

   <para>
    In cases where you are accessing single rows randomly
    within a table, the actual order of the data in the
    table is unimportant. However, if you tend to access some
    data more than others, and there is an index that groups
    them together, you will benefit from using <command>CLUSTER</command>.
    If you are requesting a range of indexed values from a table, or a
    single indexed value that has multiple rows that match,
    <command>CLUSTER</command> will help because once the index identifies the
    table page for the first row that matches, all other rows
    that match are probably already on the same table page,
    and so you save disk accesses and speed up the query.
   </para>

   <para>
    <command>CLUSTER</> can re-sort the table using either an index scan
    on the specified index, or (if the index is a b-tree) a sequential
    scan followed by sorting.  It will attempt to choose the method that
    will be faster, based on planner cost parameters and available statistical
    information.
   </para>

   <para>
    When an index scan is used, a temporary copy of the table is created that
    contains the table data in the index order.  Temporary copies of each
    index on the table are created as well.  Therefore, you need free space on
    disk at least equal to the sum of the table size and the index sizes.
   </para>

   <para>
    When a sequential scan and sort is used, a temporary sort file is
    also created, so that the peak temporary space requirement is as much
    as double the table size, plus the index sizes.  This method is often
    faster than the index scan method, but if the disk space requirement is
    intolerable, you can disable this choice by temporarily setting <xref
    linkend="guc-enable-sort"> to <literal>off</>.
   </para>

   <para>
    It is advisable to set <xref linkend="guc-maintenance-work-mem"> to
    a reasonably large value (but not more than the amount of RAM you can
    dedicate to the <command>CLUSTER</> operation) before clustering.
   </para>

   <para>
    Because the planner records statistics about the ordering of
    tables, it is advisable to run <xref linkend="sql-analyze">
    on the newly clustered table.
    Otherwise, the planner might make poor choices of query plans.
   </para>

   <para>
    Because <command>CLUSTER</command> remembers which indexes are clustered,
    one can cluster the tables one wants clustered manually the first time,
    then set up a periodic maintenance script that executes
    <command>CLUSTER</> without any parameters, so that the desired tables
    are periodically reclustered.
   </para>

 </refsect1>

 <refsect1>
  <title>Examples</title>

  <para>
   Cluster the table <literal>employees</literal> on the basis of
   its index <literal>employees_ind</literal>:
<programlisting>
CLUSTER employees USING employees_ind;
</programlisting>
  </para>

  <para>
   Cluster the <literal>employees</literal> table using the same
   index that was used before:
<programlisting>
CLUSTER employees;
</programlisting>
  </para>

  <para>
   Cluster all tables in the database that have previously been clustered:
<programlisting>
CLUSTER;
</programlisting></para>
 </refsect1>

 <refsect1>
  <title>Compatibility</title>

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

  <para>
   The syntax
<synopsis>
CLUSTER <replaceable class="PARAMETER">index_name</replaceable> ON <replaceable class="PARAMETER">table_name</replaceable>
</synopsis>
  is also supported for compatibility with pre-8.3 <productname>PostgreSQL</>
  versions.
  </para>
 </refsect1>

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

  <simplelist type="inline">
   <member><xref linkend="app-clusterdb"></member>
  </simplelist>
 </refsect1>
</refentry>