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$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_index.sgml,v 1.37 2002/09/21 18:32:54 petere 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>
 <refsynopsisdiv>
  <refsynopsisdivinfo>
   <date>2001-07-15</date>
  </refsynopsisdivinfo>
  <synopsis>
CREATE [ UNIQUE ] INDEX <replaceable class="parameter">index_name</replaceable> ON <replaceable class="parameter">table</replaceable>
    [ USING <replaceable class="parameter">acc_method</replaceable> ] ( <replaceable class="parameter">column</replaceable> [ <replaceable class="parameter">ops_name</replaceable> ] [, ...] )
    [ WHERE <replaceable class="parameter">predicate</replaceable> ]
CREATE [ UNIQUE ] INDEX <replaceable class="parameter">index_name</replaceable> ON <replaceable class="parameter">table</replaceable>
    [ USING <replaceable class="parameter">acc_method</replaceable> ] ( <replaceable class="parameter">func_name</replaceable>( <replaceable class="parameter">column</replaceable> [, ... ]) [ <replaceable class="parameter">ops_name</replaceable> ] )
    [ WHERE <replaceable class="parameter">predicate</replaceable> ]
  </synopsis>

  <refsect2 id="R2-SQL-CREATEINDEX-1">
   <refsect2info>
    <date>1998-09-09</date>
   </refsect2info>
   <title>
    Inputs
   </title>
   <para>

    <variablelist>
     <varlistentry>
      <term>UNIQUE</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">index_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">acc_method</replaceable></term>
      <listitem>
       <para>
	The name of the access method to be used for the index. The
	default access method is <literal>BTREE</literal>.
	<application>PostgreSQL</application> provides four access
	methods for indexes:

	<variablelist>
	 <varlistentry>
	  <term><literal>BTREE</></term>
	  <listitem>
	   <para>
	    an implementation of Lehman-Yao
	    high-concurrency B-trees.
	   </para>
	  </listitem>
	 </varlistentry>

	 <varlistentry>
	  <term><literal>RTREE</></term>
	  <listitem>
	   <para>implements standard R-trees using Guttman's
	    quadratic split algorithm.
	   </para>
	  </listitem>
	 </varlistentry>

	 <varlistentry>
	  <term><literal>HASH</></term>
	  <listitem>
	   <para>
	    an implementation of Litwin's linear hashing.
	   </para>
	  </listitem>
	 </varlistentry>

	 <varlistentry>
	  <term><literal>GIST</></term>
	  <listitem>
	   <para>
	    Generalized Index Search Trees.
	   </para>
	  </listitem>
	 </varlistentry>
	</variablelist>
       </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">ops_name</replaceable></term>
      <listitem>
       <para>
	An associated operator class. See below for details.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">func_name</replaceable></term>
      <listitem>
       <para>
	A function, which returns a value that can be indexed.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">predicate</replaceable></term>
      <listitem>
       <para>
	Defines the constraint expression for a partial index.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </refsect2>

  <refsect2 id="R2-SQL-CREATEINDEX-2">
   <refsect2info>
    <date>1998-09-09</date>
   </refsect2info>
   <title>
    Outputs
   </title>
   <para>

    <variablelist>
     <varlistentry>
      <term><computeroutput>
CREATE INDEX
       </computeroutput></term>
      <listitem>
       <para>
	The message returned if the index is successfully created.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><computeroutput>
ERROR: Cannot create index: 'index_name' already exists.
       </computeroutput></term>
      <listitem>
       <para>
	This error occurs if it is impossible to create the index.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </refsect2>
 </refsynopsisdiv>

 <refsect1 id="R1-SQL-CREATEINDEX-1">
  <refsect1info>
   <date>1998-09-09</date>
  </refsect1info>
  <title>
   Description
  </title>
  <para>
   <command>CREATE INDEX</command> constructs an index 
   <replaceable class="parameter">index_name</replaceable>
   on the specified <replaceable class="parameter">table</replaceable>.

   <tip>
    <para>
     Indexes are primarily used to enhance database performance.
     But inappropriate use will result in slower performance.
    </para>
   </tip>
  </para>

  <para>
   In the first syntax shown above, the key field(s) for the
   index are specified as column names.
   Multiple fields can be specified if the index access method supports
   multicolumn indexes.
  </para>

  <para>
   In the second syntax shown above, an index is defined on the result
   of a user-specified function <replaceable
   class="parameter">func_name</replaceable> applied to one or more
   columns of a single table. These <firstterm>functional
   indexes</firstterm> can be used to obtain fast access to data based
   on operators that would normally require some transformation to apply
   them to the base data. For example, a functional index on
   <literal>upper(col)</> would allow the clause
   <literal>WHERE upper(col) = 'JIM'</> to use an index.
  </para>

  <para>
   <application>PostgreSQL</application> provides B-tree, R-tree, hash,
   and GiST access methods for indexes. The B-tree access method is an
   implementation of Lehman-Yao high-concurrency B-trees. The R-tree
   access method implements standard R-trees using Guttman's quadratic
   split algorithm. The hash access method is an implementation of
   Litwin's linear hashing. We mention the algorithms used solely to
   indicate that all of these access methods are fully dynamic and do
   not have to be optimized periodically (as is the case with, for
   example, static hash access methods).
  </para>

  <para>
    When the <command>WHERE</command> 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 <command>WHERE</command> with
    <command>UNIQUE</command> to enforce uniqueness over a subset of a
    table.
  </para>

  <para>
    The expression used in the <command>WHERE</command> 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 <command>WHERE</command>.
  </para>

  <para>
   All functions and operators used in an index definition must be
   <firstterm>immutable</>, that is, their results must depend only on
   their input 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, remember to mark the function immutable
   when you create it.
  </para>

  <para>
   Use <xref linkend="sql-dropindex" endterm="sql-dropindex-title">
   to remove an index.
  </para>

  <refsect2 id="R2-SQL-CREATEINDEX-3">
   <refsect2info>
    <date>1998-09-09</date>
   </refsect2info>
   <title>
    Notes
   </title>

   <para>
    The <productname>PostgreSQL</productname>
    query optimizer will consider using a B-tree index whenever
    an indexed attribute is involved in a comparison using one of:

    <simplelist type="inline">
     <member>&lt;</member>
     <member>&lt;=</member>
     <member>=</member>
     <member>&gt;=</member>
     <member>&gt;</member>
    </simplelist>
   </para>

   <para>
    The <productname>PostgreSQL</productname>
    query optimizer will consider using an R-tree index whenever
    an indexed attribute is involved in a comparison using one of:

    <simplelist type="inline">
     <member>&lt;&lt;</member>
     <member>&amp;&lt;</member>
     <member>&amp;&gt;</member>
     <member>&gt;&gt;</member>
     <member>@</member>
     <member>~=</member>
     <member>&amp;&amp;</member>
    </simplelist>
   </para>

   <para>
    The <productname>PostgreSQL</productname>
    query optimizer will consider using a hash index whenever
    an indexed attribute is involved in a comparison using
    the <literal>=</literal> operator.
   </para>
   <para>
     Testing has shown PostgreSQL's hash indexes to be similar or slower
     than B-tree indexes, and the index size and build time for hash
     indexes is much worse. Hash indexes also suffer poor performance
     under high concurrency. For these reasons, hash index use is
     discouraged.
   </para>

   <para>
    Currently, only the B-tree and gist access methods support multicolumn
    indexes. Up to 32 keys may be specified by default (this limit
    can be altered when building
    <application>PostgreSQL</application>).  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 field'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. There are also some operator
   classes with special purposes:

   <itemizedlist>
    <listitem>
     <para>
      The operator classes <literal>box_ops</literal> and
      <literal>bigbox_ops</literal> both support R-tree indexes on the
      <literal>box</literal> data type.
      The difference between them is that <literal>bigbox_ops</literal>
      scales box coordinates down, to avoid floating-point exceptions from
      doing multiplication, addition, and subtraction on very large
      floating-point coordinates.  (Note: this was true some time ago,
      but currently the two operator classes both use floating point
      and are effectively identical.)
     </para>
    </listitem>
   </itemizedlist>
  </para>

   <para>
    The following query shows all defined operator classes:

    <programlisting>
SELECT am.amname AS acc_method,
       opc.opcname AS ops_name
    FROM pg_am am, pg_opclass opc
    WHERE opc.opcamid = am.oid
    ORDER BY acc_method, ops_name;
    </programlisting>
   </para>
  </refsect2>
 </refsect1>

 <refsect1 id="R1-SQL-CREATEINDEX-2">
  <title>
   Usage
  </title>
  <para>To create a B-tree index on the field <literal>title</literal>
   in the table <literal>films</literal>:
  </para>
  <programlisting>
CREATE UNIQUE INDEX title_idx
    ON films (title);
  </programlisting>

<!--
<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 id="R1-SQL-CREATEINDEX-3">
  <title>
   Compatibility
  </title>
  
  <refsect2 id="R2-SQL-CREATEINDEX-4">
   <refsect2info>
    <date>1998-09-09</date>
   </refsect2info>
   <title>
    SQL92
   </title>
   <para>
    CREATE INDEX is a <productname>PostgreSQL</productname> language extension.
   </para>
   <para>
    There is no <command>CREATE INDEX</command> command in SQL92.
   </para>
  </refsect2>
 </refsect1>
</refentry>

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