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Postgres 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>
   Constructs a secondary index
  </refpurpose>
 </refnamediv>
 <refsynopsisdiv>
  <refsynopsisdivinfo>
   <date>1999-07-20</date>
  </refsynopsisdivinfo>
  <synopsis>
CREATE [ UNIQUE ] INDEX <replaceable class="parameter">index_name</replaceable> ON <replaceable class="parameter">table</replaceable>
    [ USING <replaceable class="parameter">acc_name</replaceable> ] ( <replaceable class="parameter">column</replaceable> [ <replaceable class="parameter">ops_name</replaceable> ] [, ...] )
CREATE [ UNIQUE ] INDEX <replaceable class="parameter">index_name</replaceable> ON <replaceable class="parameter">table</replaceable>
    [ USING <replaceable class="parameter">acc_name</replaceable> ] ( <replaceable class="parameter">func_name</replaceable>( <replaceable class="parameter">column</replaceable> [, ... ]) [ <replaceable class="parameter">ops_name</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.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">table</replaceable></term>
      <listitem>
       <para>
	The name of the table to be indexed.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">acc_name</replaceable></term>
      <listitem>
       <para>
	The name of the access method to be used for
	the index. The default access method is BTREE.
	Postgres provides three access methods for indexes:

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

	 <varlistentry>
	  <term>RTREE</term>
	  <listitem>
	   <para>implements standard rtrees using Guttman's
	    quadratic split algorithm.
	   </para>
	  </listitem>
	 </varlistentry>

	 <varlistentry>
	  <term>HASH</term>
	  <listitem>
	   <para>
	    an implementation of Litwin's linear hashing.
	   </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>
    </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
       </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
   multi-column 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 attributes of a single class.
   These <firstterm>functional indices</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.
  </para>

  <para>
   Postgres provides btree, rtree and hash access methods for
   indices.  The btree access method is an implementation of
   Lehman-Yao high-concurrency btrees.  The rtree access method
   implements standard rtrees 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>
   Use <xref linkend="sql-dropindex-title" 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>Postgres</productname>
    query optimizer will consider using a btree 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>Postgres</productname>
    query optimizer will consider using an rtree 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>Postgres</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>
    Currently, only the btree access method supports multi-column
    indexes. Up to 16 keys may be specified by default (this limit
    can be altered when building Postgres).
   </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 btree 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
   datatype is usually sufficient.  The main point of having operator classes
   is that for some datatypes, there could be more than one meaningful
   ordering.  For example, we might want to sort a complex-number datatype
   either by absolute value or by real part.  We could do this by defining
   two operator classes for the datatype 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 rtree indices on the
      <literal>box</literal> datatype.
      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.  If the field on which your rectangles lie
      is about 20,000 units square or larger, you should use
      <literal>bigbox_ops</literal>.
     </para>
    </listitem>

    <listitem>
     <para>
      The <literal>int24_ops</literal>
      operator class is useful for constructing indices on int2 data, and
      doing comparisons against int4 data in query qualifications.
      Similarly, <literal>int42_ops</literal>
      support indices on int4 data that is to be compared against int2 data
      in queries.
     </para>
    </listitem>
   </itemizedlist>
  </para>

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

    <programlisting>
SELECT am.amname AS acc_name,
       opc.opcname AS ops_name,
       opr.oprname AS ops_comp
    FROM pg_am am, pg_amop amop,
         pg_opclass opc, pg_operator opr
    WHERE amop.amopid = am.oid AND
          amop.amopclaid = opc.oid AND
          amop.amopopr = opr.oid
    ORDER BY acc_name, ops_name, ops_comp
    </programlisting>
   </para>
  </refsect2>
 </refsect1>

 <refsect1 id="R1-SQL-CREATEINDEX-2">
  <title>
   Usage
  </title>
  <para>To create a btree 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 rtree 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>Postgres</productname> language extension.
   </para>
   <para>
    There is no <command>CREATE INDEX</command> command in SQL92.
   </para>
  </refsect2>
 </refsect1>
</refentry>

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