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Postgres 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>
   Gives storage clustering advice to the server
  </refpurpose>
 </refnamediv>
 <refsynopsisdiv>
  <refsynopsisdivinfo>
   <date>1999-07-20</date>
  </refsynopsisdivinfo>
  <synopsis>
CLUSTER <replaceable class="PARAMETER">indexname</replaceable> ON <replaceable class="PARAMETER">table</replaceable>
  </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 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>
     <varlistentry>
      <term><computeroutput>
ERROR: relation &lt;<replaceable class="PARAMETER">tablerelation_number</replaceable>&gt; inherits "<replaceable class="PARAMETER">table</replaceable>"
       </computeroutput></term>
      <listitem>
       <para>
	<comment>
	 This is not documented anywhere. It seems not to be possible to
	 cluster a table that is inherited.
	</comment>
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><computeroutput>
ERROR: Relation <replaceable class="PARAMETER">table</replaceable> does not exist!
       </computeroutput></term>
      <listitem>
       <para>
	<comment>
	 The specified relation was not shown in the error message,
	 which contained a random string instead of the relation name.
	</comment>
       </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>Postgres</productname> 
   to cluster the class specified
   by <replaceable class="parameter">table</replaceable> approximately
   based on the index specified by
   <replaceable class="parameter">indexname</replaceable>. The index must
   already have been defined on 
   <replaceable class="parameter">classname</replaceable>.
  </para>

  <para>
   When a class is clustered, it is physically reordered
   based on the index information. The clustering is static.
   In other words, as the class is updated, the changes are
   not clustered. No attempt is made to keep new instances or
   updated tuples clustered.  If one wishes, one can
   re-cluster manually by issuing the command again.
  </para>

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

   <para>
    The table is actually copied to a temporary table in index
    order, then renamed back to the original name.  For this
    reason, all grant permissions and other indexes are lost
    when clustering is performed.
   </para>

   <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>
    There are two ways to cluster data. The first is with the
    <command>CLUSTER</command> command, which reorders the original table with
    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>Postgres</productname> has a cache,
    but the majority of a big table will not fit in the cache.
   </para>

   <para> 
    Another way to cluster data 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>Postgres</productname> sorting code in 
    the ORDER BY clause to match the index, and which is much faster for
    unordered data. You then drop the old table, use
    <command>ALTER TABLE/RENAME</command>
    to rename <replaceable class="parameter">temp</replaceable> to the old name, and
    recreate any indexes. The only problem is that <acronym>OID</acronym>s
    will not be preserved. From then on, <command>CLUSTER</command> should be
    fast because most of the heap data has already been
    ordered, and the existing index is used.
   </para>
  </refsect2>
 </refsect1>

 <refsect1 id="R1-SQL-CLUSTER-2">
  <title>
   Usage
  </title>
  <para>
   Cluster the employees relation on the basis of its salary attribute:
  </para>
  <programlisting>
CLUSTER emp_ind ON emp;
  </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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