1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
|
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>CLUSTER</title>
<link rel="stylesheet" href="stylesheet.css" type="text/css">
<link rev="made" href="pgsql-docs@postgresql.org">
<meta name="generator" content="DocBook XSL Stylesheets V1.70.0">
<link rel="start" href="index.html" title="PostgreSQL 8.1.4 Documentation">
<link rel="up" href="sql-commands.html" title="SQL Commands">
<link rel="prev" href="sql-close.html" title="CLOSE">
<link rel="next" href="sql-comment.html" title="COMMENT">
<link rel="copyright" href="ln-legalnotice.html" title="Legal Notice">
</head>
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="refentry" lang="en">
<a name="sql-cluster"></a><div class="titlepage"></div>
<div class="refnamediv">
<h2>Name</h2>
<p>CLUSTER — cluster a table according to an index</p>
</div>
<a name="id748887"></a><div class="refsynopsisdiv">
<h2>Synopsis</h2>
<pre class="synopsis">CLUSTER <em class="replaceable"><code>indexname</code></em> ON <em class="replaceable"><code>tablename</code></em>
CLUSTER <em class="replaceable"><code>tablename</code></em>
CLUSTER</pre>
</div>
<div class="refsect1" lang="en">
<a name="id748923"></a><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>tablename</code></em>
based on the index specified by
<em class="replaceable"><code>indexname</code></em>. The index must
already have been defined on
<em class="replaceable"><code>tablename</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.
</p>
<p> When a table is clustered, <span class="productname">PostgreSQL</span>
remembers on which index it was clustered. The form
<code class="command">CLUSTER <em class="replaceable"><code>tablename</code></em></code>
reclusters the table on the same index that it was clustered before.
</p>
<p> <code class="command">CLUSTER</code> 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 <code class="command">CLUSTER</code> cannot be called from inside a
transaction or function.
</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" lang="en">
<a name="id749037"></a><h2>Parameters</h2>
<div class="variablelist"><dl>
<dt><span class="term"><em class="replaceable"><code>indexname</code></em></span></dt>
<dd><p> The name of an index.
</p></dd>
<dt><span class="term"><em class="replaceable"><code>tablename</code></em></span></dt>
<dd><p> The name (possibly schema-qualified) of a table.
</p></dd>
</dl></div>
</div>
<div class="refsect1" lang="en">
<a name="id749067"></a><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
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.
</p>
<p> 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.
</p>
<p> Because <code class="command">CLUSTER</code> remembers the clustering information,
one can cluster the tables one wants clustered manually the first time, and
setup a timed event similar to <code class="command">VACUUM</code> so that the tables
are periodically reclustered.
</p>
<p> Because the planner records statistics about the ordering of
tables, it is advisable to run <a href="sql-analyze.html">ANALYZE</a> on the newly clustered table.
Otherwise, the planner may make poor choices of query plans.
</p>
<p> There is another way to cluster data. The
<code class="command">CLUSTER</code> 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. (<span class="productname">PostgreSQL</span> 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
</p>
<pre class="programlisting">CREATE TABLE <em class="replaceable"><code>newtable</code></em> AS
SELECT <em class="replaceable"><code>columnlist</code></em> FROM <em class="replaceable"><code>table</code></em> ORDER BY <em class="replaceable"><code>columnlist</code></em>;</pre>
<p>
which uses the <span class="productname">PostgreSQL</span> sorting code in
the <code class="literal">ORDER BY</code> 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
<code class="command">ALTER TABLE ... RENAME</code>
to rename <em class="replaceable"><code>newtable</code></em> 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 [mdash ] all such items must be
manually recreated.
</p>
</div>
<div class="refsect1" lang="en">
<a name="id749233"></a><h2>Examples</h2>
<p> Cluster the table <code class="literal">employees</code> on the basis of
its index <code class="literal">emp_ind</code>:
</p>
<pre class="programlisting">CLUSTER emp_ind ON emp;</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 emp;</pre>
<p>
</p>
<p> Cluster all tables in the database that have previously been clustered:
</p>
<pre class="programlisting">CLUSTER;</pre>
<p>
</p>
</div>
<div class="refsect1" lang="en">
<a name="id749288"></a><h2>Compatibility</h2>
<p> There is no <code class="command">CLUSTER</code> statement in the SQL standard.
</p>
</div>
<div class="refsect1" lang="en">
<a name="id749301"></a><h2>See Also</h2>
<span class="simplelist"><a href="app-clusterdb.html"><span class="application">clusterdb</span></a></span>
</div>
</div></body>
</html>
|
