postgresql/doc/src/sgml/ref/cluster.sgml

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PostgreSQL documentation
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<refentry id="SQL-CLUSTER">
 <refmeta>
  <refentrytitle id="sql-cluster-title">CLUSTER</refentrytitle>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>
 <refnamediv>
  <refname>
   CLUSTER
  </refname>
  <refpurpose>
   cluster a table according to an index
  </refpurpose>
 </refnamediv>
 <refsynopsisdiv>
  <refsynopsisdivinfo>
   <date>1999-07-20</date>
  </refsynopsisdivinfo>
  <synopsis>
CLUSTER <replaceable class="PARAMETER">indexname</replaceable> ON <replaceable class="PARAMETER">tablename</replaceable>
CLUSTER <replaceable class="PARAMETER">tablename</replaceable>
CLUSTER ALL
  </synopsis>

  <refsect2 id="R2-SQL-CLUSTER-1">
   <refsect2info>
    <date>1998-09-08</date>
   </refsect2info>
   <title>
    Inputs
   </title>
   <para>
   </para>
   <variablelist>
    <varlistentry>
     <term><replaceable class="PARAMETER">indexname</replaceable></term>
     <listitem>
      <para>
       The name of an index.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><replaceable class="PARAMETER">table</replaceable></term>
     <listitem>
      <para>
       The name (possibly schema-qualified) of a table.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </refsect2>

  <refsect2 id="R2-SQL-CLUSTER-2">
   <refsect2info>
    <date>1998-09-08</date>
   </refsect2info>
   <title>
    Outputs
   </title>
   <para>

    <variablelist>
     <varlistentry>
      <term><computeroutput>
CLUSTER
       </computeroutput></term>
      <listitem>
       <para>
	The clustering was done successfully.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </refsect2>
 </refsynopsisdiv>

 <refsect1 id="R1-SQL-CLUSTER-1">
  <refsect1info>
   <date>1998-09-08</date>
  </refsect1info>
  <title>
   Description
  </title>
  <para>
   <command>CLUSTER</command> instructs <productname>PostgreSQL</productname> 
   to cluster the table specified
   by <replaceable class="parameter">table</replaceable>
   based on the index specified by
   <replaceable class="parameter">indexname</replaceable>. The index must
   already have been defined on 
   <replaceable class="parameter">tablename</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 tuples according to their index order.  If one wishes, one can
   periodically re-cluster by issuing the command again.
  </para>

  <para>
   When a table is clustered, <productname>PostgreSQL</productname>
   remembers on which index it was clustered.  In calls to
   <command>CLUSTER <replaceable class="parameter">tablename</replaceable></command>,
   the table is clustered on the same index that it was clustered before.
  </para>

  <para>
   In calls to <command>CLUSTER ALL</command>, all the tables in the database
   that the calling user owns are clustered using the saved information.  This
   form of <command>CLUSTER</command> cannot be called from inside a
   transaction or function.
  </para>

  <refsect2 id="R2-SQL-CLUSTER-3">
   <refsect2info>
    <date>1998-09-08</date>
   </refsect2info>
   <title>
    Notes
   </title>

   <para>
    In cases where you are accessing single rows randomly
    within a table, the actual order of the data in the heap
    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>.
   </para>

   <para> 
    Another place where <command>CLUSTER</command> is helpful is in
    cases where you use an
    index to pull out several rows from a table. 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
    heap page for the first row that matches, all other rows
    that match are probably already on the same heap page,
    saving disk accesses and speeding up the query.
   </para>

   <para>
    During the cluster operation, 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>
	<command>CLUSTER</command> preserves GRANT, inheritance, index, foreign
	key, and other ancillary information about the table.
   </para>

   <para>
	Because <command>CLUSTER</command> remembers the clustering information,
	one can cluster the tables one wants clustered manually the first time, and
	setup a timed event similar to <command>VACUUM</command> so that the tables
	are periodically and automatically clustered.
   </para>

   <para>
    Because the optimizer records statistics about the ordering of tables, it
    is advisable to run <command>ANALYZE</command> on the newly clustered
    table.  Otherwise, the optimizer may make poor choices of query plans.
   </para>

   <para>
    There is another way to cluster data. The
    <command>CLUSTER</command> command reorders the original table using
    the ordering of the index you specify. This can be slow
    on large tables because the rows are fetched from the heap
    in index order, and if the heap table is unordered, the
    entries are on random pages, so there is one disk page
    retrieved for every row moved. (<productname>PostgreSQL</productname> has a cache,
    but the majority of a big table will not fit in the cache.)
    The other way to cluster a table is to use

    <programlisting>
SELECT <replaceable class="parameter">columnlist</replaceable> INTO TABLE <replaceable class="parameter">newtable</replaceable>
     FROM <replaceable class="parameter">table</replaceable> ORDER BY <replaceable class="parameter">columnlist</replaceable>
    </programlisting>

    which uses the <productname>PostgreSQL</productname> sorting code in 
    the ORDER BY clause to create the desired order; this is usually much
    faster than an index scan for
    unordered data. You then drop the old table, use
    <command>ALTER TABLE...RENAME</command>
    to rename <replaceable class="parameter">newtable</replaceable> to the old name, and
    recreate the table's indexes. However, this approach does not preserve
    OIDs, constraints, foreign key relationships, granted privileges, and
    other ancillary properties of the table --- all such items must be
    manually recreated.
   </para>

  </refsect2>
 </refsect1>

 <refsect1 id="R1-SQL-CLUSTER-2">
  <title>
   Usage
  </title>
  <para>
   Cluster the employees relation on the basis of its ID attribute:
  </para>
  <programlisting>
CLUSTER emp_ind ON emp;
  </programlisting>
  <para>
   Cluster the employees relation using the same index that was used before:
  </para>
  <programlisting>
CLUSTER emp;
  </programlisting>
  <para>
   Cluster all the tables on the database that have previously been clustered:
  </para>
  <programlisting>
CLUSTER ALL;
  </programlisting>
 </refsect1>

 <refsect1 id="R1-SQL-CLUSTER-3">
  <title>
   Compatibility
  </title>

  <refsect2 id="R2-SQL-CLUSTER-4">
   <refsect2info>
    <date>1998-09-08</date>
   </refsect2info>
   <title>
    SQL92
   </title>
   <para>
    There is no <command>CLUSTER</command> statement in SQL92.
   </para>
  </refsect2>
 </refsect1>
</refentry>

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