doc/src/sgml/ref/create_index.sgml - hawq - Git at Google

 <!--
 $PostgreSQL: pgsql/doc/src/sgml/ref/create_index.sgml,v 1.58 2006/09/16 00:30:17 momjian Exp $
 PostgreSQL documentation
 -->

 <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 name ON table
        [USING btree|bitmap|gist]
        ( {column | (expression)} [opclass] [, ...] )
        [ WITH ( FILLFACTOR = value ) ]
        [TABLESPACE tablespace]
        [WHERE predicate]


 </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><literal>CONCURRENTLY</literal></term>
       <listitem>
        <para>
         When this option is used, <productname>PostgreSQL</> will build the
         index without taking any locks that prevent concurrent inserts,
         updates, or deletes on the table; whereas a standard index build
         locks out writes (but not reads) on the table until it's done.
         There are several caveats to be aware of when using this option
         &mdash; see <xref linkend="SQL-CREATEINDEX-CONCURRENTLY"
         endterm="SQL-CREATEINDEX-CONCURRENTLY-title">.
        </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, leaf 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 10 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 varies between methods.
      </para>
     </listitem>
    </varlistentry>

    </variablelist>

   </refsect2>

   <refsect2 id="SQL-CREATEINDEX-CONCURRENTLY">
    <title id="SQL-CREATEINDEX-CONCURRENTLY-title">Building Indexes Concurrently</title>

    <indexterm zone="SQL-CREATEINDEX-CONCURRENTLY">
    <primary>index</primary>
    <secondary>building concurrently</secondary>
    </indexterm>

    <para>
     Creating an index can interfere with regular operation of a database.
     Normally <productname>PostgreSQL</> locks the table to be indexed against
     writes and performs the entire index build with a single scan of the
     table. Other transactions can still read the table, but if they try to
     insert, update, or delete rows in the table they will block until the
     index build is finished. This could have a severe effect if the system is
     a live production database. Large tables can take many hours to be
     indexed, and even for smaller tables, an index build can lock out writers
     for periods that are unacceptably long for a production system.
    </para>

    <para>
     <productname>PostgreSQL</> supports building indexes without locking
     out writes.  This method is invoked by specifying the
     <literal>CONCURRENTLY</> option of <command>CREATE INDEX</>.
     When this option is used,
     <productname>PostgreSQL</> must perform two scans of the table, and in
     addition it must wait for all existing transactions to terminate.  Thus
     this method requires more total work than a standard index build and takes
     significantly longer to complete.  However, since it allows normal
     operations to continue while the index is built, this method is useful for
     adding new indexes in a production environment.  Of course, the extra CPU
     and I/O load imposed by the index creation may slow other operations.
    </para>

    <para>
     If a problem arises during the second scan of the table, such as a
     uniqueness violation in a unique index, the <command>CREATE INDEX</>
     command will fail but leave behind an <quote>invalid</> index. This index
     will be ignored for querying purposes because it may be incomplete;
     however it will still consume update overhead.  The recommended recovery
     method in such cases is to drop the index and try again to perform
     <command>CREATE INDEX CONCURRENTLY</>.  (Another possibility is to rebuild
     the index with <command>REINDEX</>.  However, since <command>REINDEX</>
     does not support concurrent builds, this option is unlikely to seem
     attractive.)
    </para>

    <para>
     Another caveat when building a unique index concurrently is that the
     uniqueness constraint is already being enforced against other transactions
     when the second table scan begins.  This means that constraint violations
     could be reported in other queries prior to the index becoming available
     for use, or even in cases where the index build eventually fails.  Also,
     if a failure does occur in the second scan, the <quote>invalid</> index
     continues to enforce its uniqueness constraint afterwards.
    </para>

    <para>
     Concurrent builds of expression indexes and partial indexes are supported.
     Errors occurring in the evaluation of these expressions could cause
     behavior similar to that described above for unique constraint violations.
    </para>

    <para>
     Regular index builds permit other regular index builds on the
     same table to occur in parallel, but only one concurrent index build
     can occur on a table at a time.  In both cases, no other types of schema
     modification on the table are allowed meanwhile.  Another difference
     is that a regular <command>CREATE INDEX</> command can be performed within
     a transaction block, but <command>CREATE INDEX CONCURRENTLY</> cannot.
    </para>
   </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:
   <programlisting>
 CREATE INDEX pointloc
     ON points USING GIST (point2box(location) box_ops);
 SELECT * FROM points
     WHERE point2box(points.pointloc) = boxes.box;
   </programlisting>
   </para>
 -->

   <para>
    To create an index without locking out writes to the table:
 <programlisting>
 CREATE INDEX CONCURRENTLY sales_quantity_index ON sales_table (quantity);
 </programlisting>
   </para>

  </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>
	<!--
	$PostgreSQL: pgsql/doc/src/sgml/ref/create_index.sgml,v 1.58 2006/09/16 00:30:17 momjian Exp $
	PostgreSQL documentation
	-->

	<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 name ON table
	[USING btree\|bitmap\|gist]
	( {column \| (expression)} [opclass] [, ...] )
	[ WITH ( FILLFACTOR = value ) ]
	[TABLESPACE tablespace]
	[WHERE predicate]


	</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><literal>CONCURRENTLY</literal></term>
	<listitem>
	<para>
	When this option is used, <productname>PostgreSQL</> will build the
	index without taking any locks that prevent concurrent inserts,
	updates, or deletes on the table; whereas a standard index build
	locks out writes (but not reads) on the table until it's done.
	There are several caveats to be aware of when using this option
	— see <xref linkend="SQL-CREATEINDEX-CONCURRENTLY"
	endterm="SQL-CREATEINDEX-CONCURRENTLY-title">.
	</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, leaf 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 10 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 varies between methods.
	</para>
	</listitem>
	</varlistentry>

	</variablelist>

	</refsect2>

	<refsect2 id="SQL-CREATEINDEX-CONCURRENTLY">
	<title id="SQL-CREATEINDEX-CONCURRENTLY-title">Building Indexes Concurrently</title>

	<indexterm zone="SQL-CREATEINDEX-CONCURRENTLY">
	<primary>index</primary>
	<secondary>building concurrently</secondary>
	</indexterm>

	<para>
	Creating an index can interfere with regular operation of a database.
	Normally <productname>PostgreSQL</> locks the table to be indexed against
	writes and performs the entire index build with a single scan of the
	table. Other transactions can still read the table, but if they try to
	insert, update, or delete rows in the table they will block until the
	index build is finished. This could have a severe effect if the system is
	a live production database. Large tables can take many hours to be
	indexed, and even for smaller tables, an index build can lock out writers
	for periods that are unacceptably long for a production system.
	</para>

	<para>
	<productname>PostgreSQL</> supports building indexes without locking
	out writes. This method is invoked by specifying the
	<literal>CONCURRENTLY</> option of <command>CREATE INDEX</>.
	When this option is used,
	<productname>PostgreSQL</> must perform two scans of the table, and in
	addition it must wait for all existing transactions to terminate. Thus
	this method requires more total work than a standard index build and takes
	significantly longer to complete. However, since it allows normal
	operations to continue while the index is built, this method is useful for
	adding new indexes in a production environment. Of course, the extra CPU
	and I/O load imposed by the index creation may slow other operations.
	</para>

	<para>
	If a problem arises during the second scan of the table, such as a
	uniqueness violation in a unique index, the <command>CREATE INDEX</>
	command will fail but leave behind an <quote>invalid</> index. This index
	will be ignored for querying purposes because it may be incomplete;
	however it will still consume update overhead. The recommended recovery
	method in such cases is to drop the index and try again to perform
	<command>CREATE INDEX CONCURRENTLY</>. (Another possibility is to rebuild
	the index with <command>REINDEX</>. However, since <command>REINDEX</>
	does not support concurrent builds, this option is unlikely to seem
	attractive.)
	</para>

	<para>
	Another caveat when building a unique index concurrently is that the
	uniqueness constraint is already being enforced against other transactions
	when the second table scan begins. This means that constraint violations
	could be reported in other queries prior to the index becoming available
	for use, or even in cases where the index build eventually fails. Also,
	if a failure does occur in the second scan, the <quote>invalid</> index
	continues to enforce its uniqueness constraint afterwards.
	</para>

	<para>
	Concurrent builds of expression indexes and partial indexes are supported.
	Errors occurring in the evaluation of these expressions could cause
	behavior similar to that described above for unique constraint violations.
	</para>

	<para>
	Regular index builds permit other regular index builds on the
	same table to occur in parallel, but only one concurrent index build
	can occur on a table at a time. In both cases, no other types of schema
	modification on the table are allowed meanwhile. Another difference
	is that a regular <command>CREATE INDEX</> command can be performed within
	a transaction block, but <command>CREATE INDEX CONCURRENTLY</> cannot.
	</para>
	</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:
	<programlisting>
	CREATE INDEX pointloc
	ON points USING GIST (point2box(location) box_ops);
	SELECT * FROM points
	WHERE point2box(points.pointloc) = boxes.box;
	</programlisting>
	</para>
	-->

	<para>
	To create an index without locking out writes to the table:
	<programlisting>
	CREATE INDEX CONCURRENTLY sales_quantity_index ON sales_table (quantity);
	</programlisting>
	</para>

	</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>