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$PostgreSQL: pgsql/doc/src/sgml/ref/create_index.sgml,v 1.54 2006/07/04 18:07:24 tgl Exp $
PostgreSQL documentation
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<refentry id="SQL-CREATEINDEX">
<refmeta>
<refentrytitle id="sql-createindex-title">CREATE INDEX</refentrytitle>
<refmiscinfo>SQL - Language Statements</refmiscinfo>
</refmeta>
<refnamediv>
<refname>CREATE INDEX</refname>
<refpurpose>define a new index</refpurpose>
</refnamediv>
<indexterm zone="sql-createindex">
<primary>CREATE INDEX</primary>
</indexterm>
<refsynopsisdiv>
<synopsis>
CREATE [ UNIQUE ] INDEX <replaceable class="parameter">name</replaceable> ON <replaceable class="parameter">table</replaceable> [ USING <replaceable class="parameter">method</replaceable> ]
( { <replaceable class="parameter">column</replaceable> | ( <replaceable class="parameter">expression</replaceable> ) } [ <replaceable class="parameter">opclass</replaceable> ] [, ...] )
[ WITH ( <replaceable class="PARAMETER">storage_parameter</replaceable> = <replaceable class="PARAMETER">value</replaceable> [, ... ] ) ]
[ TABLESPACE <replaceable class="parameter">tablespace</replaceable> ]
[ WHERE <replaceable class="parameter">predicate</replaceable> ]
</synopsis>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para>
<command>CREATE INDEX</command> constructs an index <replaceable
class="parameter">index_name</replaceable> on the specified table.
Indexes are primarily used to enhance database performance (though
inappropriate use can result in slower performance).
</para>
<para>
The key field(s) for the index are specified as column names,
or alternatively as expressions written in parentheses.
Multiple fields can be specified if the index method supports
multicolumn indexes.
</para>
<para>
An index field can be an expression computed from the values of
one or more columns of the table row. This feature can be used
to obtain fast access to data based on some transformation of
the basic data. For example, an index computed on
<literal>upper(col)</> would allow the clause
<literal>WHERE upper(col) = 'JIM'</> to use an index.
</para>
<para>
<productname>PostgreSQL</productname> provides the index methods
B-tree, hash, GiST, and GIN. Users can also define their own index
methods, but that is fairly complicated.
</para>
<para>
When the <literal>WHERE</literal> clause is present, a
<firstterm>partial index</firstterm> is created.
A partial index is an index that contains entries for only a portion of
a table, usually a portion that is more useful for indexing than the
rest of the table. For example, if you have a table that contains both
billed and unbilled orders where the unbilled orders take up a small
fraction of the total table and yet that is an often used section, you
can improve performance by creating an index on just that portion.
Another possible application is to use <literal>WHERE</literal> with
<literal>UNIQUE</literal> to enforce uniqueness over a subset of a
table. See <xref linkend="indexes-partial"> for more discussion.
</para>
<para>
The expression used in the <literal>WHERE</literal> clause may refer
only to columns of the underlying table, but it can use all columns,
not just the ones being indexed. Presently, subqueries and
aggregate expressions are also forbidden in <literal>WHERE</literal>.
The same restrictions apply to index fields that are expressions.
</para>
<para>
All functions and operators used in an index definition must be
<quote>immutable</>, that is, their results must depend only on
their arguments and never on any outside influence (such as
the contents of another table or the current time). This restriction
ensures that the behavior of the index is well-defined. To use a
user-defined function in an index expression or <literal>WHERE</literal>
clause, remember to mark the function immutable when you create it.
</para>
</refsect1>
<refsect1>
<title>Parameters</title>
<variablelist>
<varlistentry>
<term><literal>UNIQUE</literal></term>
<listitem>
<para>
Causes the system to check for
duplicate values in the table when the index is created (if data
already exist) and each time data is added. Attempts to
insert or update data which would result in duplicate entries
will generate an error.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="parameter">name</replaceable></term>
<listitem>
<para>
The name of the index to be created. No schema name can be included
here; the index is always created in the same schema as its parent
table.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="parameter">table</replaceable></term>
<listitem>
<para>
The name (possibly schema-qualified) of the table to be indexed.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="parameter">method</replaceable></term>
<listitem>
<para>
The name of the index method to be used. Choices are
<literal>btree</literal>, <literal>hash</literal>,
<literal>gist</literal>, and <literal>gin</>. The
default method is <literal>btree</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="parameter">column</replaceable></term>
<listitem>
<para>
The name of a column of the table.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="parameter">expression</replaceable></term>
<listitem>
<para>
An expression based on one or more columns of the table. The
expression usually must be written with surrounding parentheses,
as shown in the syntax. However, the parentheses may be omitted
if the expression has the form of a function call.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="parameter">opclass</replaceable></term>
<listitem>
<para>
The name of an operator class. See below for details.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="parameter">storage_parameter</replaceable></term>
<listitem>
<para>
The name of an index-method-specific storage parameter. See
below for details.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="parameter">tablespace</replaceable></term>
<listitem>
<para>
The tablespace in which to create the index. If not specified,
<xref linkend="guc-default-tablespace"> is used, or the database's
default tablespace if <varname>default_tablespace</> is an empty
string.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="parameter">predicate</replaceable></term>
<listitem>
<para>
The constraint expression for a partial index.
</para>
</listitem>
</varlistentry>
</variablelist>
<refsect2 id="SQL-CREATEINDEX-storage-parameters">
<title id="SQL-CREATEINDEX-storage-parameters-title">Index Storage Parameters</title>
<para>
The <literal>WITH</> clause can specify <firstterm>storage parameters</>
for indexes. Each index method can have its own set of allowed storage
parameters. The built-in index methods all accept a single parameter:
</para>
<variablelist>
<varlistentry>
<term><literal>FILLFACTOR</></term>
<listitem>
<para>
The fillfactor for an index is a percentage that determines how full
the index method will try to pack index pages. For B-trees, pages
are filled to this percentage during initial index build, and also
when extending the index at the right (largest key values). If pages
subsequently become completely full, they will be split, leading to
gradual degradation in the index's efficiency. B-trees use a default
fillfactor of 90, but any value from 70 to 100 can be selected.
If the table is static then fillfactor 100 is best to minimize the
index's physical size, but for heavily updated tables a smaller
fillfactor is better to minimize the need for page splits. The
other index methods use fillfactor in different but roughly analogous
ways; the default fillfactor and allowed range varies.
</para>
</listitem>
</varlistentry>
</variablelist>
</refsect2>
</refsect1>
<refsect1>
<title>Notes</title>
<para>
See <xref linkend="indexes"> for information about when indexes can
be used, when they are not used, and in which particular situations
they can be useful.
</para>
<para>
Currently, only the B-tree and GiST index methods support
multicolumn indexes. Up to 32 fields may be specified by default.
(This limit can be altered when building
<productname>PostgreSQL</productname>.) Only B-tree currently
supports unique indexes.
</para>
<para>
An <firstterm>operator class</firstterm> can be specified for each
column of an index. The operator class identifies the operators to be
used by the index for that column. For example, a B-tree index on
four-byte integers would use the <literal>int4_ops</literal> class;
this operator class includes comparison functions for four-byte
integers. In practice the default operator class for the column's data
type is usually sufficient. The main point of having operator classes
is that for some data types, there could be more than one meaningful
ordering. For example, we might want to sort a complex-number data
type either by absolute value or by real part. We could do this by
defining two operator classes for the data type and then selecting
the proper class when making an index. More information about
operator classes is in <xref linkend="indexes-opclass"> and in <xref
linkend="xindex">.
</para>
<para>
Use <xref linkend="sql-dropindex" endterm="sql-dropindex-title">
to remove an index.
</para>
<para>
Indexes are not used for <literal>IS NULL</> clauses by default.
The best way to use indexes in such cases is to create a partial index
using an <literal>IS NULL</> predicate.
</para>
<para>
Prior releases of <productname>PostgreSQL</productname> also had an
R-tree index method. This method has been removed because
it had no significant advantages over the GiST method.
If <literal>USING rtree</> is specified, <command>CREATE INDEX</>
will interpret it as <literal>USING gist</>, to simplify conversion
of old databases to GiST.
</para>
</refsect1>
<refsect1>
<title>Examples</title>
<para>
To create a B-tree index on the column <literal>title</literal> in
the table <literal>films</literal>:
<programlisting>
CREATE UNIQUE INDEX title_idx ON films (title);
</programlisting>
</para>
<para>
To create an index on the expression <literal>lower(title)</>,
allowing efficient case-insensitive searches:
<programlisting>
CREATE INDEX lower_title_idx ON films ((lower(title)));
</programlisting>
</para>
<para>
To create an index with non-default fill factor:
<programlisting>
CREATE UNIQUE INDEX title_idx ON films (title) WITH (fillfactor = 70);
</programlisting>
</para>
<para>
To create an index on the column <literal>code</> in the table
<literal>films</> and have the index reside in the tablespace
<literal>indexspace</>:
<programlisting>
CREATE INDEX code_idx ON films(code) TABLESPACE indexspace;
</programlisting>
</para>
<!--
<comment>
Is this example correct?
</comment>
<para>
To create a GiST index on a point attribute so that we
can efficiently use box operators on the result of the
conversion function:
</para>
<programlisting>
CREATE INDEX pointloc
ON points USING GIST (point2box(location) box_ops);
SELECT * FROM points
WHERE point2box(points.pointloc) = boxes.box;
</programlisting>
-->
</refsect1>
<refsect1>
<title>Compatibility</title>
<para>
<command>CREATE INDEX</command> is a
<productname>PostgreSQL</productname> language extension. There
are no provisions for indexes in the SQL standard.
</para>
</refsect1>
<refsect1>
<title>See Also</title>
<simplelist type="inline">
<member><xref linkend="sql-alterindex" endterm="sql-alterindex-title"></member>
<member><xref linkend="sql-dropindex" endterm="sql-dropindex-title"></member>
</simplelist>
</refsect1>
</refentry>
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