2 $PostgreSQL: pgsql/doc/src/sgml/ref/create_index.sgml,v 1.54 2006/07/04 18:07:24 tgl Exp $
3 PostgreSQL documentation
6 <refentry id="SQL-CREATEINDEX">
8 <refentrytitle id="sql-createindex-title">CREATE INDEX</refentrytitle>
9 <refmiscinfo>SQL - Language Statements</refmiscinfo>
13 <refname>CREATE INDEX</refname>
14 <refpurpose>define a new index</refpurpose>
17 <indexterm zone="sql-createindex">
18 <primary>CREATE INDEX</primary>
23 CREATE [ UNIQUE ] INDEX <replaceable class="parameter">name</replaceable> ON <replaceable class="parameter">table</replaceable> [ USING <replaceable class="parameter">method</replaceable> ]
24 ( { <replaceable class="parameter">column</replaceable> | ( <replaceable class="parameter">expression</replaceable> ) } [ <replaceable class="parameter">opclass</replaceable> ] [, ...] )
25 [ WITH ( <replaceable class="PARAMETER">storage_parameter</replaceable> = <replaceable class="PARAMETER">value</replaceable> [, ... ] ) ]
26 [ TABLESPACE <replaceable class="parameter">tablespace</replaceable> ]
27 [ WHERE <replaceable class="parameter">predicate</replaceable> ]
32 <title>Description</title>
35 <command>CREATE INDEX</command> constructs an index <replaceable
36 class="parameter">index_name</replaceable> on the specified table.
37 Indexes are primarily used to enhance database performance (though
38 inappropriate use can result in slower performance).
42 The key field(s) for the index are specified as column names,
43 or alternatively as expressions written in parentheses.
44 Multiple fields can be specified if the index method supports
49 An index field can be an expression computed from the values of
50 one or more columns of the table row. This feature can be used
51 to obtain fast access to data based on some transformation of
52 the basic data. For example, an index computed on
53 <literal>upper(col)</> would allow the clause
54 <literal>WHERE upper(col) = 'JIM'</> to use an index.
58 <productname>PostgreSQL</productname> provides the index methods
59 B-tree, hash, GiST, and GIN. Users can also define their own index
60 methods, but that is fairly complicated.
64 When the <literal>WHERE</literal> clause is present, a
65 <firstterm>partial index</firstterm> is created.
66 A partial index is an index that contains entries for only a portion of
67 a table, usually a portion that is more useful for indexing than the
68 rest of the table. For example, if you have a table that contains both
69 billed and unbilled orders where the unbilled orders take up a small
70 fraction of the total table and yet that is an often used section, you
71 can improve performance by creating an index on just that portion.
72 Another possible application is to use <literal>WHERE</literal> with
73 <literal>UNIQUE</literal> to enforce uniqueness over a subset of a
74 table. See <xref linkend="indexes-partial"> for more discussion.
78 The expression used in the <literal>WHERE</literal> clause may refer
79 only to columns of the underlying table, but it can use all columns,
80 not just the ones being indexed. Presently, subqueries and
81 aggregate expressions are also forbidden in <literal>WHERE</literal>.
82 The same restrictions apply to index fields that are expressions.
86 All functions and operators used in an index definition must be
87 <quote>immutable</>, that is, their results must depend only on
88 their arguments and never on any outside influence (such as
89 the contents of another table or the current time). This restriction
90 ensures that the behavior of the index is well-defined. To use a
91 user-defined function in an index expression or <literal>WHERE</literal>
92 clause, remember to mark the function immutable when you create it.
97 <title>Parameters</title>
101 <term><literal>UNIQUE</literal></term>
104 Causes the system to check for
105 duplicate values in the table when the index is created (if data
106 already exist) and each time data is added. Attempts to
107 insert or update data which would result in duplicate entries
108 will generate an error.
114 <term><replaceable class="parameter">name</replaceable></term>
117 The name of the index to be created. No schema name can be included
118 here; the index is always created in the same schema as its parent
125 <term><replaceable class="parameter">table</replaceable></term>
128 The name (possibly schema-qualified) of the table to be indexed.
134 <term><replaceable class="parameter">method</replaceable></term>
137 The name of the index method to be used. Choices are
138 <literal>btree</literal>, <literal>hash</literal>,
139 <literal>gist</literal>, and <literal>gin</>. The
140 default method is <literal>btree</literal>.
146 <term><replaceable class="parameter">column</replaceable></term>
149 The name of a column of the table.
155 <term><replaceable class="parameter">expression</replaceable></term>
158 An expression based on one or more columns of the table. The
159 expression usually must be written with surrounding parentheses,
160 as shown in the syntax. However, the parentheses may be omitted
161 if the expression has the form of a function call.
167 <term><replaceable class="parameter">opclass</replaceable></term>
170 The name of an operator class. See below for details.
176 <term><replaceable class="parameter">storage_parameter</replaceable></term>
179 The name of an index-method-specific storage parameter. See
186 <term><replaceable class="parameter">tablespace</replaceable></term>
189 The tablespace in which to create the index. If not specified,
190 <xref linkend="guc-default-tablespace"> is used, or the database's
191 default tablespace if <varname>default_tablespace</> is an empty
198 <term><replaceable class="parameter">predicate</replaceable></term>
201 The constraint expression for a partial index.
208 <refsect2 id="SQL-CREATEINDEX-storage-parameters">
209 <title id="SQL-CREATEINDEX-storage-parameters-title">Index Storage Parameters</title>
212 The <literal>WITH</> clause can specify <firstterm>storage parameters</>
213 for indexes. Each index method can have its own set of allowed storage
214 parameters. The built-in index methods all accept a single parameter:
220 <term><literal>FILLFACTOR</></term>
223 The fillfactor for an index is a percentage that determines how full
224 the index method will try to pack index pages. For B-trees, pages
225 are filled to this percentage during initial index build, and also
226 when extending the index at the right (largest key values). If pages
227 subsequently become completely full, they will be split, leading to
228 gradual degradation in the index's efficiency. B-trees use a default
229 fillfactor of 90, but any value from 70 to 100 can be selected.
230 If the table is static then fillfactor 100 is best to minimize the
231 index's physical size, but for heavily updated tables a smaller
232 fillfactor is better to minimize the need for page splits. The
233 other index methods use fillfactor in different but roughly analogous
234 ways; the default fillfactor and allowed range varies.
248 See <xref linkend="indexes"> for information about when indexes can
249 be used, when they are not used, and in which particular situations
254 Currently, only the B-tree and GiST index methods support
255 multicolumn indexes. Up to 32 fields may be specified by default.
256 (This limit can be altered when building
257 <productname>PostgreSQL</productname>.) Only B-tree currently
258 supports unique indexes.
262 An <firstterm>operator class</firstterm> can be specified for each
263 column of an index. The operator class identifies the operators to be
264 used by the index for that column. For example, a B-tree index on
265 four-byte integers would use the <literal>int4_ops</literal> class;
266 this operator class includes comparison functions for four-byte
267 integers. In practice the default operator class for the column's data
268 type is usually sufficient. The main point of having operator classes
269 is that for some data types, there could be more than one meaningful
270 ordering. For example, we might want to sort a complex-number data
271 type either by absolute value or by real part. We could do this by
272 defining two operator classes for the data type and then selecting
273 the proper class when making an index. More information about
274 operator classes is in <xref linkend="indexes-opclass"> and in <xref
279 Use <xref linkend="sql-dropindex" endterm="sql-dropindex-title">
284 Indexes are not used for <literal>IS NULL</> clauses by default.
285 The best way to use indexes in such cases is to create a partial index
286 using an <literal>IS NULL</> predicate.
290 Prior releases of <productname>PostgreSQL</productname> also had an
291 R-tree index method. This method has been removed because
292 it had no significant advantages over the GiST method.
293 If <literal>USING rtree</> is specified, <command>CREATE INDEX</>
294 will interpret it as <literal>USING gist</>, to simplify conversion
295 of old databases to GiST.
300 <title>Examples</title>
303 To create a B-tree index on the column <literal>title</literal> in
304 the table <literal>films</literal>:
306 CREATE UNIQUE INDEX title_idx ON films (title);
311 To create an index on the expression <literal>lower(title)</>,
312 allowing efficient case-insensitive searches:
314 CREATE INDEX lower_title_idx ON films ((lower(title)));
319 To create an index with non-default fill factor:
321 CREATE UNIQUE INDEX title_idx ON films (title) WITH (fillfactor = 70);
326 To create an index on the column <literal>code</> in the table
327 <literal>films</> and have the index reside in the tablespace
328 <literal>indexspace</>:
330 CREATE INDEX code_idx ON films(code) TABLESPACE indexspace;
336 Is this example correct?
339 To create a GiST index on a point attribute so that we
340 can efficiently use box operators on the result of the
344 CREATE INDEX pointloc
345 ON points USING GIST (point2box(location) box_ops);
347 WHERE point2box(points.pointloc) = boxes.box;
354 <title>Compatibility</title>
357 <command>CREATE INDEX</command> is a
358 <productname>PostgreSQL</productname> language extension. There
359 are no provisions for indexes in the SQL standard.
364 <title>See Also</title>
366 <simplelist type="inline">
367 <member><xref linkend="sql-alterindex" endterm="sql-alterindex-title"></member>
368 <member><xref linkend="sql-dropindex" endterm="sql-dropindex-title"></member>
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