4911c85e86
Add space between slash for ALTER TABLE / ADD .... Regression and *.po updates to follow.
249 lines
7.0 KiB
Plaintext
249 lines
7.0 KiB
Plaintext
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$Header: /cvsroot/pgsql/doc/src/sgml/ref/cluster.sgml,v 1.12 2001/10/31 04:49:43 momjian Exp $
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Postgres documentation
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-->
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<refentry id="SQL-CLUSTER">
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<refmeta>
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<refentrytitle id="sql-cluster-title">
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CLUSTER
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</refentrytitle>
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<refmiscinfo>SQL - Language Statements</refmiscinfo>
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</refmeta>
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<refnamediv>
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<refname>
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CLUSTER
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</refname>
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<refpurpose>
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cluster a table according to an index
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</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<refsynopsisdivinfo>
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<date>1999-07-20</date>
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</refsynopsisdivinfo>
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<synopsis>
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CLUSTER <replaceable class="PARAMETER">indexname</replaceable> ON <replaceable class="PARAMETER">tablename</replaceable>
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</synopsis>
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<refsect2 id="R2-SQL-CLUSTER-1">
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<refsect2info>
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<date>1998-09-08</date>
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</refsect2info>
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<title>
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Inputs
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</title>
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<para>
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</para>
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<variablelist>
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<varlistentry>
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<term><replaceable class="PARAMETER">indexname</replaceable></term>
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<listitem>
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<para>
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The name of an index.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="PARAMETER">table</replaceable></term>
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<listitem>
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<para>
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The name of a table.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</refsect2>
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<refsect2 id="R2-SQL-CLUSTER-2">
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<refsect2info>
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<date>1998-09-08</date>
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</refsect2info>
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<title>
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Outputs
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</title>
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<para>
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<variablelist>
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<varlistentry>
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<term><computeroutput>
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CLUSTER
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</computeroutput></term>
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<listitem>
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<para>
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The clustering was done successfully.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><computeroutput>
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ERROR: relation <<replaceable class="PARAMETER">tablerelation_number</replaceable>> inherits "<replaceable class="PARAMETER">table</replaceable>"
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</computeroutput></term>
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<listitem>
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<para>
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<comment>
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This is not documented anywhere. It seems not to be possible to
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cluster a table that is inherited.
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</comment>
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><computeroutput>
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ERROR: Relation <replaceable class="PARAMETER">table</replaceable> does not exist!
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</computeroutput></term>
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<listitem>
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<para>
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<comment>
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The specified relation was not shown in the error message,
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which contained a random string instead of the relation name.
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</comment>
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</para>
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</refsect2>
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</refsynopsisdiv>
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<refsect1 id="R1-SQL-CLUSTER-1">
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<refsect1info>
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<date>1998-09-08</date>
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</refsect1info>
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<title>
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Description
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</title>
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<para>
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<command>CLUSTER</command> instructs <productname>Postgres</productname>
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to cluster the table specified
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by <replaceable class="parameter">table</replaceable> approximately
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based on the index specified by
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<replaceable class="parameter">indexname</replaceable>. The index must
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already have been defined on
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<replaceable class="parameter">tablename</replaceable>.
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</para>
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<para>
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When a table is clustered, it is physically reordered
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based on the index information. The clustering is static.
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In other words, as the table is updated, the changes are
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not clustered. No attempt is made to keep new instances or
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updated tuples clustered. If one wishes, one can
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re-cluster manually by issuing the command again.
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</para>
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<refsect2 id="R2-SQL-CLUSTER-3">
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<refsect2info>
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<date>1998-09-08</date>
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</refsect2info>
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<title>
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Notes
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</title>
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<para>
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The table is actually copied to a temporary table in index
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order, then renamed back to the original name. For this
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reason, all grant permissions and other indexes are lost
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when clustering is performed.
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</para>
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<para>
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In cases where you are accessing single rows randomly
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within a table, the actual order of the data in the heap
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table is unimportant. However, if you tend to access some
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data more than others, and there is an index that groups
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them together, you will benefit from using <command>CLUSTER</command>.
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</para>
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<para>
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Another place where <command>CLUSTER</command> is helpful is in
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cases where you use an
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index to pull out several rows from a table. If you are
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requesting a range of indexed values from a table, or a
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single indexed value that has multiple rows that match,
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<command>CLUSTER</command> will help because once the index identifies the
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heap page for the first row that matches, all other rows
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that match are probably already on the same heap page,
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saving disk accesses and speeding up the query.
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</para>
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<para>
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There are two ways to cluster data. The first is with the
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<command>CLUSTER</command> command, which reorders the original table with
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the ordering of the index you specify. This can be slow
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on large tables because the rows are fetched from the heap
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in index order, and if the heap table is unordered, the
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entries are on random pages, so there is one disk page
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retrieved for every row moved. <productname>Postgres</productname> has a cache,
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but the majority of a big table will not fit in the cache.
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</para>
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<para>
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Another way to cluster data is to use
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<programlisting>
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SELECT <replaceable class="parameter">columnlist</replaceable> INTO TABLE <replaceable class="parameter">newtable</replaceable>
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FROM <replaceable class="parameter">table</replaceable> ORDER BY <replaceable class="parameter">columnlist</replaceable>
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</programlisting>
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which uses the <productname>Postgres</productname> sorting code in
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the ORDER BY clause to match the index, and which is much faster for
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unordered data. You then drop the old table, use
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<command>ALTER TABLE...RENAME</command>
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to rename <replaceable class="parameter">temp</replaceable> to the old name, and
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recreate any indexes. The only problem is that <acronym>OID</acronym>s
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will not be preserved. From then on, <command>CLUSTER</command> should be
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fast because most of the heap data has already been
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ordered, and the existing index is used.
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</para>
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</refsect2>
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</refsect1>
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<refsect1 id="R1-SQL-CLUSTER-2">
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<title>
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Usage
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</title>
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<para>
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Cluster the employees relation on the basis of its salary attribute:
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</para>
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<programlisting>
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CLUSTER emp_ind ON emp;
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</programlisting>
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</refsect1>
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<refsect1 id="R1-SQL-CLUSTER-3">
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<title>
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Compatibility
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</title>
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<refsect2 id="R2-SQL-CLUSTER-4">
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<refsect2info>
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<date>1998-09-08</date>
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</refsect2info>
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<title>
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SQL92
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</title>
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<para>
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There is no <command>CLUSTER</command> statement in SQL92.
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</para>
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</refsect2>
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</refsect1>
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</refentry>
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<!-- Keep this comment at the end of the file
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Local variables:
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mode: sgml
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sgml-omittag:nil
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sgml-shorttag:t
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sgml-minimize-attributes:nil
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sgml-always-quote-attributes:t
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sgml-indent-step:1
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sgml-indent-data:t
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sgml-parent-document:nil
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sgml-default-dtd-file:"../reference.ced"
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sgml-exposed-tags:nil
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sgml-local-catalogs:"/usr/lib/sgml/catalog"
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sgml-local-ecat-files:nil
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End:
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-->
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