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$PostgreSQL: pgsql/doc/src/sgml/ref/create_index.sgml,v 1.44 2003/11/29 19:51:38 pgsql Exp $
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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> ] [, ...] )
    [ 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 will 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, R-tree, hash, and GiST. The B-tree index method is an
   implementation of Lehman-Yao high-concurrency B-trees. The R-tree
   index method implements standard R-trees using Guttman's quadratic
   split algorithm. The hash index method is an implementation of
   Litwin's linear hashing.  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 somehow more interesting 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.
  </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 only the one(s) 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 method to be used for the index.  Choices are
        <literal>btree</literal>, <literal>hash</literal>,
        <literal>rtree</literal>, and <literal>gist</literal>.  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">predicate</replaceable></term>
      <listitem>
       <para>
	The constraint expression for a partial index.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
 </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
   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>
 </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>

<!--
<comment>
Is this example correct?
</comment>
  <para>
   To create a R-tree 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 RTREE (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>
</refentry>

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