create_index.sgml

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$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_index.sgml,v 1.22 2001/08/21 16:35:59 tgl Exp $
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>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.
       </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_method</replaceable></term>
      <listitem>
       <para>
	The name of the access method to be used for
	the index. The default access method is BTREE.
	Postgres provides four 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>

	 <varlistentry>
	  <term>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
       </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 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.
  </para>

  <para>
   Postgres provides btree, rtree, hash, and GiST access methods for
   indexes.  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>
    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, sub-SELECTs and
    aggregate expressions are also forbidden in <command>WHERE</command>.
  </para>

  <para>
   All functions and operators used in an index definition must be
   <firstterm>cachable</>, 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 cachable
   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>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 and gist access methods support multi-column
    indexes. Up to 16 keys may be specified by default (this limit
    can be altered when building Postgres).  Only btree 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 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
   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 rtree 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,
       opr.oprname AS ops_comp
    FROM pg_am am, pg_opclass opc, pg_amop amop, pg_operator opr
    WHERE opc.opcamid = am.oid AND
          amop.amopclaid = opc.oid AND
          amop.amopopr = opr.oid
    ORDER BY acc_method, 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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