<!--$Header: /cvsroot/pgsql/doc/src/sgml/ref/cluster.sgml,v 1.30 2003/11/02 12:59:54 petere Exp $PostgreSQL documentation--><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> <indexterm zone="sql-cluster">  <primary>CLUSTER</primary> </indexterm> <refsynopsisdiv><synopsis>CLUSTER <replaceable class="PARAMETER">indexname</replaceable> ON <replaceable class="PARAMETER">tablename</replaceable>CLUSTER <replaceable class="PARAMETER">tablename</replaceable>CLUSTER</synopsis> </refsynopsisdiv> <refsect1>  <title>Description</title>  <para>   <command>CLUSTER</command> instructs <productname>PostgreSQL</productname>    to cluster the table specified   by <replaceable class="parameter">tablename</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 rows according to their index order.  If one wishes, one can   periodically recluster by issuing the command again.  </para>  <para>   When a table is clustered, <productname>PostgreSQL</productname>   remembers on which index it was clustered.  The form   <command>CLUSTER <replaceable class="parameter">tablename</replaceable></command>,   reclusters the table on the same index that it was clustered before.  </para>  <para>   <command>CLUSTER</command> without any parameter reclusters all the tables   in the   current database that the calling user owns, or all tables if called   by a superuser.  (Never-clustered tables are not included.)  This   form of <command>CLUSTER</command> cannot be called from inside a   transaction or function.  </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">indexname</replaceable></term>    <listitem>     <para>      The name of an index.     </para>    </listitem>   </varlistentry>   <varlistentry>    <term><replaceable class="PARAMETER">tablename</replaceable></term>    <listitem>     <para>      The name (possibly schema-qualified) of a table.     </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    heap page for the first row that matches, all other rows    that match are probably already on the same heap page,    and so you save disk accesses and speed 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>    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 reclustered.   </para>   <para>    Because the planner records statistics about the ordering of tables, it    is advisable to run <command>ANALYZE</command> on the newly clustered    table.  Otherwise, the planner 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>CREATE TABLE <replaceable class="parameter">newtable</replaceable> AS    SELECT <replaceable class="parameter">columnlist</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 <literal>ORDER BY</literal> 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> </refsect1> <refsect1>  <title>Examples</title>  <para>   Cluster the table <literal>employees</literal> on the basis of   its index <literal>emp_ind</literal>:<programlisting>CLUSTER emp_ind ON emp;</programlisting>  </para>  <para>   Cluster the <literal>employees</literal> relation using the same   index that was used before:<programlisting>CLUSTER emp;</programlisting>  </para>  <para>   Cluster all the tables on 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> </refsect1> <refsect1>  <title>See Also</title>  <simplelist type="inline">   <member><xref linkend="app-clusterdb" endterm="app-clusterdb-title"></member>  </simplelist> </refsect1></refentry><!-- Keep this comment at the end of the fileLocal variables:mode: sgmlsgml-omittag:nilsgml-shorttag:tsgml-minimize-attributes:nilsgml-always-quote-attributes:tsgml-indent-step:1sgml-indent-data:tsgml-parent-document:nilsgml-default-dtd-file:"../reference.ced"sgml-exposed-tags:nilsgml-local-catalogs:"/usr/lib/sgml/catalog"sgml-local-ecat-files:nilEnd:-->