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
|
<?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>11.3. Multicolumn Indexes</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 Vsnapshot" /><link rel="prev" href="indexes-types.html" title="11.2. Index Types" /><link rel="next" href="indexes-ordering.html" title="11.4. Indexes and ORDER BY" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">11.3. Multicolumn Indexes</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="indexes-types.html" title="11.2. Index Types">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="indexes.html" title="Chapter 11. Indexes">Up</a></td><th width="60%" align="center">Chapter 11. Indexes</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.7 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="indexes-ordering.html" title="11.4. Indexes and ORDER BY">Next</a></td></tr></table><hr /></div><div class="sect1" id="INDEXES-MULTICOLUMN"><div class="titlepage"><div><div><h2 class="title" style="clear: both">11.3. Multicolumn Indexes</h2></div></div></div><a id="id-1.5.10.6.2" class="indexterm"></a><p>
An index can be defined on more than one column of a table. For example, if
you have a table of this form:
</p><pre class="programlisting">
CREATE TABLE test2 (
major int,
minor int,
name varchar
);
</pre><p>
(say, you keep your <code class="filename">/dev</code>
directory in a database...) and you frequently issue queries like:
</p><pre class="programlisting">
SELECT name FROM test2 WHERE major = <em class="replaceable"><code>constant</code></em> AND minor = <em class="replaceable"><code>constant</code></em>;
</pre><p>
then it might be appropriate to define an index on the columns
<code class="structfield">major</code> and
<code class="structfield">minor</code> together, e.g.:
</p><pre class="programlisting">
CREATE INDEX test2_mm_idx ON test2 (major, minor);
</pre><p>
</p><p>
Currently, only the B-tree, GiST, GIN, and BRIN index types support
multiple-key-column indexes. Whether there can be multiple key
columns is independent of whether <code class="literal">INCLUDE</code> columns
can be added to the index. Indexes can have up to 32 columns,
including <code class="literal">INCLUDE</code> columns. (This limit can be
altered when building <span class="productname">PostgreSQL</span>; see the
file <code class="filename">pg_config_manual.h</code>.)
</p><p>
A multicolumn B-tree index can be used with query conditions that
involve any subset of the index's columns, but the index is most
efficient when there are constraints on the leading (leftmost) columns.
The exact rule is that equality constraints on leading columns, plus
any inequality constraints on the first column that does not have an
equality constraint, will be used to limit the portion of the index
that is scanned. Constraints on columns to the right of these columns
are checked in the index, so they save visits to the table proper, but
they do not reduce the portion of the index that has to be scanned.
For example, given an index on <code class="literal">(a, b, c)</code> and a
query condition <code class="literal">WHERE a = 5 AND b >= 42 AND c < 77</code>,
the index would have to be scanned from the first entry with
<code class="literal">a</code> = 5 and <code class="literal">b</code> = 42 up through the last entry with
<code class="literal">a</code> = 5. Index entries with <code class="literal">c</code> >= 77 would be
skipped, but they'd still have to be scanned through.
This index could in principle be used for queries that have constraints
on <code class="literal">b</code> and/or <code class="literal">c</code> with no constraint on <code class="literal">a</code>
— but the entire index would have to be scanned, so in most cases
the planner would prefer a sequential table scan over using the index.
</p><p>
A multicolumn GiST index can be used with query conditions that
involve any subset of the index's columns. Conditions on additional
columns restrict the entries returned by the index, but the condition on
the first column is the most important one for determining how much of
the index needs to be scanned. A GiST index will be relatively
ineffective if its first column has only a few distinct values, even if
there are many distinct values in additional columns.
</p><p>
A multicolumn GIN index can be used with query conditions that
involve any subset of the index's columns. Unlike B-tree or GiST,
index search effectiveness is the same regardless of which index column(s)
the query conditions use.
</p><p>
A multicolumn BRIN index can be used with query conditions that
involve any subset of the index's columns. Like GIN and unlike B-tree or
GiST, index search effectiveness is the same regardless of which index
column(s) the query conditions use. The only reason to have multiple BRIN
indexes instead of one multicolumn BRIN index on a single table is to have
a different <code class="literal">pages_per_range</code> storage parameter.
</p><p>
Of course, each column must be used with operators appropriate to the index
type; clauses that involve other operators will not be considered.
</p><p>
Multicolumn indexes should be used sparingly. In most situations,
an index on a single column is sufficient and saves space and time.
Indexes with more than three columns are unlikely to be helpful
unless the usage of the table is extremely stylized. See also
<a class="xref" href="indexes-bitmap-scans.html" title="11.5. Combining Multiple Indexes">Section 11.5</a> and
<a class="xref" href="indexes-index-only-scans.html" title="11.9. Index-Only Scans and Covering Indexes">Section 11.9</a> for some discussion of the
merits of different index configurations.
</p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="indexes-types.html" title="11.2. Index Types">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="indexes.html" title="Chapter 11. Indexes">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="indexes-ordering.html" title="11.4. Indexes and ORDER BY">Next</a></td></tr><tr><td width="40%" align="left" valign="top">11.2. Index Types </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.7 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 11.4. Indexes and <code class="literal">ORDER BY</code></td></tr></table></div></body></html>
|
