2 $PostgreSQL: pgsql/doc/src/sgml/ref/create_index.sgml,v 1.58 2006/09/16 00:30:17 momjian 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 [ CONCURRENTLY ] <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><literal>CONCURRENTLY</literal></term>
117 When this option is used, <productname>PostgreSQL</> will build the
118 index without taking any locks that prevent concurrent inserts,
119 updates, or deletes on the table; whereas a standard index build
120 locks out writes (but not reads) on the table until it's done.
121 There are several caveats to be aware of when using this option
122 — see <xref linkend="SQL-CREATEINDEX-CONCURRENTLY"
123 endterm="SQL-CREATEINDEX-CONCURRENTLY-title">.
129 <term><replaceable class="parameter">name</replaceable></term>
132 The name of the index to be created. No schema name can be included
133 here; the index is always created in the same schema as its parent
140 <term><replaceable class="parameter">table</replaceable></term>
143 The name (possibly schema-qualified) of the table to be indexed.
149 <term><replaceable class="parameter">method</replaceable></term>
152 The name of the index method to be used. Choices are
153 <literal>btree</literal>, <literal>hash</literal>,
154 <literal>gist</literal>, and <literal>gin</>. The
155 default method is <literal>btree</literal>.
161 <term><replaceable class="parameter">column</replaceable></term>
164 The name of a column of the table.
170 <term><replaceable class="parameter">expression</replaceable></term>
173 An expression based on one or more columns of the table. The
174 expression usually must be written with surrounding parentheses,
175 as shown in the syntax. However, the parentheses may be omitted
176 if the expression has the form of a function call.
182 <term><replaceable class="parameter">opclass</replaceable></term>
185 The name of an operator class. See below for details.
191 <term><replaceable class="parameter">storage_parameter</replaceable></term>
194 The name of an index-method-specific storage parameter. See
201 <term><replaceable class="parameter">tablespace</replaceable></term>
204 The tablespace in which to create the index. If not specified,
205 <xref linkend="guc-default-tablespace"> is used, or the database's
206 default tablespace if <varname>default_tablespace</> is an empty
213 <term><replaceable class="parameter">predicate</replaceable></term>
216 The constraint expression for a partial index.
223 <refsect2 id="SQL-CREATEINDEX-storage-parameters">
224 <title id="SQL-CREATEINDEX-storage-parameters-title">Index Storage Parameters</title>
227 The <literal>WITH</> clause can specify <firstterm>storage parameters</>
228 for indexes. Each index method can have its own set of allowed storage
229 parameters. The built-in index methods all accept a single parameter:
235 <term><literal>FILLFACTOR</></term>
238 The fillfactor for an index is a percentage that determines how full
239 the index method will try to pack index pages. For B-trees, leaf pages
240 are filled to this percentage during initial index build, and also
241 when extending the index at the right (largest key values). If pages
242 subsequently become completely full, they will be split, leading to
243 gradual degradation in the index's efficiency. B-trees use a default
244 fillfactor of 90, but any value from 10 to 100 can be selected.
245 If the table is static then fillfactor 100 is best to minimize the
246 index's physical size, but for heavily updated tables a smaller
247 fillfactor is better to minimize the need for page splits. The
248 other index methods use fillfactor in different but roughly analogous
249 ways; the default fillfactor varies between methods.
258 <refsect2 id="SQL-CREATEINDEX-CONCURRENTLY">
259 <title id="SQL-CREATEINDEX-CONCURRENTLY-title">Building Indexes Concurrently</title>
261 <indexterm zone="SQL-CREATEINDEX-CONCURRENTLY">
262 <primary>index</primary>
263 <secondary>building concurrently</secondary>
267 Creating an index can interfere with regular operation of a database.
268 Normally <productname>PostgreSQL</> locks the table to be indexed against
269 writes and performs the entire index build with a single scan of the
270 table. Other transactions can still read the table, but if they try to
271 insert, update, or delete rows in the table they will block until the
272 index build is finished. This could have a severe effect if the system is
273 a live production database. Large tables can take many hours to be
274 indexed, and even for smaller tables, an index build can lock out writers
275 for periods that are unacceptably long for a production system.
279 <productname>PostgreSQL</> supports building indexes without locking
280 out writes. This method is invoked by specifying the
281 <literal>CONCURRENTLY</> option of <command>CREATE INDEX</>.
282 When this option is used,
283 <productname>PostgreSQL</> must perform two scans of the table, and in
284 addition it must wait for all existing transactions to terminate. Thus
285 this method requires more total work than a standard index build and takes
286 significantly longer to complete. However, since it allows normal
287 operations to continue while the index is built, this method is useful for
288 adding new indexes in a production environment. Of course, the extra CPU
289 and I/O load imposed by the index creation may slow other operations.
293 If a problem arises during the second scan of the table, such as a
294 uniqueness violation in a unique index, the <command>CREATE INDEX</>
295 command will fail but leave behind an <quote>invalid</> index. This index
296 will be ignored for querying purposes because it may be incomplete;
297 however it will still consume update overhead. The recommended recovery
298 method in such cases is to drop the index and try again to perform
299 <command>CREATE INDEX CONCURRENTLY</>. (Another possibility is to rebuild
300 the index with <command>REINDEX</>. However, since <command>REINDEX</>
301 does not support concurrent builds, this option is unlikely to seem
306 Another caveat when building a unique index concurrently is that the
307 uniqueness constraint is already being enforced against other transactions
308 when the second table scan begins. This means that constraint violations
309 could be reported in other queries prior to the index becoming available
310 for use, or even in cases where the index build eventually fails. Also,
311 if a failure does occur in the second scan, the <quote>invalid</> index
312 continues to enforce its uniqueness constraint afterwards.
316 Concurrent builds of expression indexes and partial indexes are supported.
317 Errors occurring in the evaluation of these expressions could cause
318 behavior similar to that described above for unique constraint violations.
322 Regular index builds permit other regular index builds on the
323 same table to occur in parallel, but only one concurrent index build
324 can occur on a table at a time. In both cases, no other types of schema
325 modification on the table are allowed meanwhile. Another difference
326 is that a regular <command>CREATE INDEX</> command can be performed within
327 a transaction block, but <command>CREATE INDEX CONCURRENTLY</> cannot.
336 See <xref linkend="indexes"> for information about when indexes can
337 be used, when they are not used, and in which particular situations
342 Currently, only the B-tree and GiST index methods support
343 multicolumn indexes. Up to 32 fields may be specified by default.
344 (This limit can be altered when building
345 <productname>PostgreSQL</productname>.) Only B-tree currently
346 supports unique indexes.
350 An <firstterm>operator class</firstterm> can be specified for each
351 column of an index. The operator class identifies the operators to be
352 used by the index for that column. For example, a B-tree index on
353 four-byte integers would use the <literal>int4_ops</literal> class;
354 this operator class includes comparison functions for four-byte
355 integers. In practice the default operator class for the column's data
356 type is usually sufficient. The main point of having operator classes
357 is that for some data types, there could be more than one meaningful
358 ordering. For example, we might want to sort a complex-number data
359 type either by absolute value or by real part. We could do this by
360 defining two operator classes for the data type and then selecting
361 the proper class when making an index. More information about
362 operator classes is in <xref linkend="indexes-opclass"> and in <xref
367 Use <xref linkend="sql-dropindex" endterm="sql-dropindex-title">
372 Indexes are not used for <literal>IS NULL</> clauses by default.
373 The best way to use indexes in such cases is to create a partial index
374 using an <literal>IS NULL</> predicate.
378 Prior releases of <productname>PostgreSQL</productname> also had an
379 R-tree index method. This method has been removed because
380 it had no significant advantages over the GiST method.
381 If <literal>USING rtree</> is specified, <command>CREATE INDEX</>
382 will interpret it as <literal>USING gist</>, to simplify conversion
383 of old databases to GiST.
388 <title>Examples</title>
391 To create a B-tree index on the column <literal>title</literal> in
392 the table <literal>films</literal>:
394 CREATE UNIQUE INDEX title_idx ON films (title);
399 To create an index on the expression <literal>lower(title)</>,
400 allowing efficient case-insensitive searches:
402 CREATE INDEX lower_title_idx ON films ((lower(title)));
407 To create an index with non-default fill factor:
409 CREATE UNIQUE INDEX title_idx ON films (title) WITH (fillfactor = 70);
414 To create an index on the column <literal>code</> in the table
415 <literal>films</> and have the index reside in the tablespace
416 <literal>indexspace</>:
418 CREATE INDEX code_idx ON films(code) TABLESPACE indexspace;
424 Is this example correct?
427 To create a GiST index on a point attribute so that we
428 can efficiently use box operators on the result of the
431 CREATE INDEX pointloc
432 ON points USING GIST (point2box(location) box_ops);
434 WHERE point2box(points.pointloc) = boxes.box;
440 To create an index without locking out writes to the table:
442 CREATE INDEX CONCURRENTLY sales_quantity_index ON sales_table (quantity);
449 <title>Compatibility</title>
452 <command>CREATE INDEX</command> is a
453 <productname>PostgreSQL</productname> language extension. There
454 are no provisions for indexes in the SQL standard.
459 <title>See Also</title>
461 <simplelist type="inline">
462 <member><xref linkend="sql-alterindex" endterm="sql-alterindex-title"></member>
463 <member><xref linkend="sql-dropindex" endterm="sql-dropindex-title"></member>