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diff --git a/doc/src/sgml/html/sql-cluster.html b/doc/src/sgml/html/sql-cluster.html new file mode 100644 index 0000000..93a2c62 --- /dev/null +++ b/doc/src/sgml/html/sql-cluster.html @@ -0,0 +1,117 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>CLUSTER</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets V1.79.1" /><link rel="prev" href="sql-close.html" title="CLOSE" /><link rel="next" href="sql-comment.html" title="COMMENT" /></head><body id="docContent" class="container-fluid col-10"><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">CLUSTER</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="sql-close.html" title="CLOSE">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="sql-commands.html" title="SQL Commands">Up</a></td><th width="60%" align="center">SQL Commands</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 13.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="sql-comment.html" title="COMMENT">Next</a></td></tr></table><hr></hr></div><div class="refentry" id="SQL-CLUSTER"><div class="titlepage"></div><a id="id-1.9.3.51.1" class="indexterm"></a><div class="refnamediv"><h2><span class="refentrytitle">CLUSTER</span></h2><p>CLUSTER — cluster a table according to an index</p></div><div class="refsynopsisdiv"><h2>Synopsis</h2><pre class="synopsis"> +CLUSTER [VERBOSE] <em class="replaceable"><code>table_name</code></em> [ USING <em class="replaceable"><code>index_name</code></em> ] +CLUSTER [VERBOSE] +</pre></div><div class="refsect1" id="id-1.9.3.51.5"><h2>Description</h2><p> + <code class="command">CLUSTER</code> instructs <span class="productname">PostgreSQL</span> + to cluster the table specified + by <em class="replaceable"><code>table_name</code></em> + based on the index specified by + <em class="replaceable"><code>index_name</code></em>. The index must + already have been defined on + <em class="replaceable"><code>table_name</code></em>. + </p><p> + 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 <code class="literal">fillfactor</code> 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.) + </p><p> + When a table is clustered, <span class="productname">PostgreSQL</span> + remembers which index it was clustered by. The form + <code class="command">CLUSTER <em class="replaceable"><code>table_name</code></em></code> + reclusters the table using the same index as before. You can also + use the <code class="literal">CLUSTER</code> or <code class="literal">SET WITHOUT CLUSTER</code> + forms of <a class="xref" href="sql-altertable.html" title="ALTER TABLE"><span class="refentrytitle">ALTER TABLE</span></a> to set the index to be used for + future cluster operations, or to clear any previous setting. + </p><p> + <code class="command">CLUSTER</code> 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 <code class="command">CLUSTER</code> cannot be executed inside a transaction + block. + </p><p> + When a table is being clustered, an <code class="literal">ACCESS + EXCLUSIVE</code> lock is acquired on it. This prevents any other + database operations (both reads and writes) from operating on the + table until the <code class="command">CLUSTER</code> is finished. + </p></div><div class="refsect1" id="id-1.9.3.51.6"><h2>Parameters</h2><div class="variablelist"><dl class="variablelist"><dt><span class="term"><em class="replaceable"><code>table_name</code></em></span></dt><dd><p> + The name (possibly schema-qualified) of a table. + </p></dd><dt><span class="term"><em class="replaceable"><code>index_name</code></em></span></dt><dd><p> + The name of an index. + </p></dd><dt><span class="term"><code class="literal">VERBOSE</code></span></dt><dd><p> + Prints a progress report as each table is clustered. + </p></dd></dl></div></div><div class="refsect1" id="id-1.9.3.51.7"><h2>Notes</h2><p> + 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 <code class="command">CLUSTER</code>. + If you are requesting a range of indexed values from a table, or a + single indexed value that has multiple rows that match, + <code class="command">CLUSTER</code> 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. + </p><p> + <code class="command">CLUSTER</code> 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. + </p><p> + 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. + </p><p> + 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 <a class="xref" href="runtime-config-query.html#GUC-ENABLE-SORT">enable_sort</a> to <code class="literal">off</code>. + </p><p> + It is advisable to set <a class="xref" href="runtime-config-resource.html#GUC-MAINTENANCE-WORK-MEM">maintenance_work_mem</a> to + a reasonably large value (but not more than the amount of RAM you can + dedicate to the <code class="command">CLUSTER</code> operation) before clustering. + </p><p> + Because the planner records statistics about the ordering of + tables, it is advisable to run <a class="xref" href="sql-analyze.html" title="ANALYZE"><span class="refentrytitle">ANALYZE</span></a> + on the newly clustered table. + Otherwise, the planner might make poor choices of query plans. + </p><p> + Because <code class="command">CLUSTER</code> 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 + <code class="command">CLUSTER</code> without any parameters, so that the desired tables + are periodically reclustered. + </p></div><div class="refsect1" id="id-1.9.3.51.8"><h2>Examples</h2><p> + Cluster the table <code class="literal">employees</code> on the basis of + its index <code class="literal">employees_ind</code>: +</p><pre class="programlisting"> +CLUSTER employees USING employees_ind; +</pre><p> + </p><p> + Cluster the <code class="literal">employees</code> table using the same + index that was used before: +</p><pre class="programlisting"> +CLUSTER employees; +</pre><p> + </p><p> + Cluster all tables in the database that have previously been clustered: +</p><pre class="programlisting"> +CLUSTER; +</pre></div><div class="refsect1" id="id-1.9.3.51.9"><h2>Compatibility</h2><p> + There is no <code class="command">CLUSTER</code> statement in the SQL standard. + </p><p> + The syntax +</p><pre class="synopsis"> +CLUSTER <em class="replaceable"><code>index_name</code></em> ON <em class="replaceable"><code>table_name</code></em> +</pre><p> + is also supported for compatibility with pre-8.3 <span class="productname">PostgreSQL</span> + versions. + </p></div><div class="refsect1" id="id-1.9.3.51.10"><h2>See Also</h2><span class="simplelist"><a class="xref" href="app-clusterdb.html" title="clusterdb"><span class="refentrytitle"><span class="application">clusterdb</span></span></a></span></div></div><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navfooter"><hr></hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="sql-close.html" title="CLOSE">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="sql-commands.html" title="SQL Commands">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="sql-comment.html" title="COMMENT">Next</a></td></tr><tr><td width="40%" align="left" valign="top">CLOSE </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 13.4 Documentation">Home</a></td><td width="40%" align="right" valign="top"> COMMENT</td></tr></table></div></body></html>
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