<!--
doc/src/sgml/ref/create_index.sgml
PostgreSQL documentation
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<refentry id="sql-createindex">
 <indexterm zone="sql-createindex">
  <primary>CREATE INDEX</primary>
 </indexterm>

 <refmeta>
  <refentrytitle>CREATE INDEX</refentrytitle>
  <manvolnum>7</manvolnum>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>CREATE INDEX</refname>
  <refpurpose>define a new index</refpurpose>
 </refnamediv>

 <refsynopsisdiv>
<synopsis>
CREATE [ UNIQUE ] INDEX [ CONCURRENTLY ] [ [ IF NOT EXISTS ] <replaceable class="parameter">name</replaceable> ] ON [ ONLY ] <replaceable class="parameter">table_name</replaceable> [ USING <replaceable class="parameter">method</replaceable> ]
    ( { <replaceable class="parameter">column_name</replaceable> | ( <replaceable class="parameter">expression</replaceable> ) } [ COLLATE <replaceable class="parameter">collation</replaceable> ] [ <replaceable class="parameter">opclass</replaceable> ] [ ASC | DESC ] [ NULLS { FIRST | LAST } ] [, ...] )
    [ INCLUDE ( <replaceable class="parameter">column_name</replaceable> [, ...] ) ]
    [ WITH ( <replaceable class="parameter">storage_parameter</replaceable> = <replaceable class="parameter">value</replaceable> [, ... ] ) ]
    [ TABLESPACE <replaceable class="parameter">tablespace_name</replaceable> ]
    [ WHERE <replaceable class="parameter">predicate</replaceable> ]
</synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   <command>CREATE INDEX</command> constructs an index on the specified column(s)
   of the specified relation, which can be a table or a materialized view.
   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)</literal> would allow the clause
   <literal>WHERE upper(col) = 'JIM'</literal> to use an index.
  </para>

  <para>
   <productname>PostgreSQL</productname> provides the index methods
   B-tree, hash, GiST, SP-GiST, GIN, and BRIN.  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 can 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</quote>, 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>

       <para>
        Additional restrictions apply when unique indexes are applied to
        partitioned tables; see <xref linkend="sql-createtable" />.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>CONCURRENTLY</literal></term>
      <listitem>
       <para>
        When this option is used, <productname>PostgreSQL</productname> 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><literal>IF NOT EXISTS</literal></term>
      <listitem>
       <para>
        Do not throw an error if a relation with the same name already exists.
        A notice is issued in this case. Note that there is no guarantee that
        the existing index is anything like the one that would have been created.
        Index name is required when <literal>IF NOT EXISTS</literal> is specified.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>INCLUDE</literal></term>
      <listitem>
       <para>
        The optional <literal>INCLUDE</literal> clause specifies a
        list of columns which will be included in the index
        as <firstterm>non-key</firstterm> columns.  A non-key column cannot
        be used in an index scan search qualification, and it is disregarded
        for purposes of any uniqueness or exclusion constraint enforced by
        the index.  However, an index-only scan can return the contents of
        non-key columns without having to visit the index's table, since
        they are available directly from the index entry.  Thus, addition of
        non-key columns allows index-only scans to be used for queries that
        otherwise could not use them.
       </para>

       <para>
        It's wise to be conservative about adding non-key columns to an
        index, especially wide columns.  If an index tuple exceeds the
        maximum size allowed for the index type, data insertion will fail.
        In any case, non-key columns duplicate data from the index's table
        and bloat the size of the index, thus potentially slowing searches.
       </para>

       <para>
        Columns listed in the <literal>INCLUDE</literal> clause don't need
        appropriate operator classes; the clause can include
        columns whose data types don't have operator classes defined for
        a given access method.
       </para>

       <para>
        Expressions are not supported as included columns since they cannot be
        used in index-only scans.
       </para>

       <para>
        Currently, only the B-tree index access method supports this feature.
        In B-tree indexes, the values of columns listed in the
        <literal>INCLUDE</literal> clause are included in leaf tuples which
        correspond to heap tuples, but are not included in upper-level
        index entries used for tree navigation.
       </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.  If the name is omitted, <productname>PostgreSQL</productname> chooses a
        suitable name based on the parent table's name and the indexed column
        name(s).
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>ONLY</literal></term>
      <listitem>
       <para>
        Indicates not to recurse creating indexes on partitions, if the
        table is partitioned.  The default is to recurse.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">table_name</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>, <literal>spgist</literal>, <literal>gin</literal>, and
        <literal>brin</literal>.
        The default method is <literal>btree</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">column_name</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 can be omitted
        if the expression has the form of a function call.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">collation</replaceable></term>
      <listitem>
       <para>
        The name of the collation to use for the index.  By default,
        the index uses the collation declared for the column to be
        indexed or the result collation of the expression to be
        indexed.  Indexes with non-default collations can be useful for
        queries that involve expressions using non-default collations.
       </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><literal>ASC</literal></term>
      <listitem>
       <para>
        Specifies ascending sort order (which is the default).
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>DESC</literal></term>
      <listitem>
       <para>
        Specifies descending sort order.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>NULLS FIRST</literal></term>
      <listitem>
       <para>
        Specifies that nulls sort before non-nulls.  This is the default
        when <literal>DESC</literal> is specified.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>NULLS LAST</literal></term>
      <listitem>
       <para>
        Specifies that nulls sort after non-nulls.  This is the default
        when <literal>DESC</literal> is not specified.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">storage_parameter</replaceable></term>
      <listitem>
       <para>
        The name of an index-method-specific storage parameter.  See
        <xref linkend="sql-createindex-storage-parameters" endterm="sql-createindex-storage-parameters-title"/>
        for details.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">tablespace_name</replaceable></term>
      <listitem>
       <para>
        The tablespace in which to create the index.  If not specified,
        <xref linkend="guc-default-tablespace"/> is consulted, or
        <xref linkend="guc-temp-tablespaces"/> for indexes on temporary
        tables.
       </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 optional <literal>WITH</literal> clause specifies <firstterm>storage
    parameters</firstterm> for the index.  Each index method has its own set of allowed
    storage parameters.  All indexes accept the following parameter:
   </para>

   <variablelist>
   <varlistentry>
    <term><literal>recheck_on_update</literal></term>
    <listitem>
     <para>
       Specifies whether to recheck a functional index value to see whether
       we can use a HOT update or not. The default value is on for functional
       indexes with an total expression cost less than 1000, otherwise off.
       You might decide to turn this off if you knew that a function used in
       an index is unlikely to return the same value when one of the input
       columns is updated and so the recheck is not worth the additional cost
       of executing the function.
     </para>

     <para>
       Functional indexes are used frequently for the case where the function
       returns a subset of the argument. Examples of this would be accessing
       part of a string with <literal>SUBSTR()</literal> or accessing a single
       field in a JSON document using an expression such as
       <literal>(bookinfo-&gt;&gt;'isbn')</literal>. In this example, the JSON
       document might be updated frequently, yet it is uncommon for the ISBN
       field for a book to change so we would keep the parameter set to on
       for that index. A more frequently changing field might have an index
       with this parameter turned off, while very frequently changing fields
       might be better to avoid indexing at all under high load.
     </para>
    </listitem>
   </varlistentry>
   </variablelist>

   <para>
     The B-tree, hash, GiST and SP-GiST index methods all accept this parameter:
   </para>

   <variablelist>
   <varlistentry>
    <term><literal>fillfactor</literal></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 (adding new 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 integer 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>

   <para>
    B-tree indexes additionally accept this parameter:
   </para>

   <variablelist>
   <varlistentry>
    <term><literal>vacuum_cleanup_index_scale_factor</literal></term>
    <listitem>
    <para>
      Per-index value for <xref linkend="guc-vacuum-cleanup-index-scale-factor"/>.
    </para>
    </listitem>
   </varlistentry>
   </variablelist>

   <para>
    GiST indexes additionally accept this parameter:
   </para>

   <variablelist>
   <varlistentry>
    <term><literal>buffering</literal></term>
    <listitem>
    <para>
     Determines whether the buffering build technique described in
     <xref linkend="gist-buffering-build"/> is used to build the index. With
     <literal>OFF</literal> it is disabled, with <literal>ON</literal> it is enabled, and
     with <literal>AUTO</literal> it is initially disabled, but turned on
     on-the-fly once the index size reaches <xref linkend="guc-effective-cache-size"/>. The default is <literal>AUTO</literal>.
    </para>
    </listitem>
   </varlistentry>
   </variablelist>

   <para>
    GIN indexes accept different parameters:
   </para>

   <variablelist>
   <varlistentry>
    <term><literal>fastupdate</literal></term>
    <listitem>
    <para>
     This setting controls usage of the fast update technique described in
     <xref linkend="gin-fast-update"/>.  It is a Boolean parameter:
     <literal>ON</literal> enables fast update, <literal>OFF</literal> disables it.
     (Alternative spellings of <literal>ON</literal> and <literal>OFF</literal> are
     allowed as described in <xref linkend="config-setting"/>.)  The
     default is <literal>ON</literal>.
    </para>

    <note>
     <para>
      Turning <literal>fastupdate</literal> off via <command>ALTER INDEX</command> prevents
      future insertions from going into the list of pending index entries,
      but does not in itself flush previous entries.  You might want to
      <command>VACUUM</command> the table or call <function>gin_clean_pending_list</function>
      function afterward to ensure the pending list is emptied.
     </para>
    </note>
    </listitem>
   </varlistentry>
   </variablelist>
   <variablelist>
   <varlistentry>
    <term><literal>gin_pending_list_limit</literal></term>
    <listitem>
    <para>
     Custom <xref linkend="guc-gin-pending-list-limit"/> parameter.
     This value is specified in kilobytes.
    </para>
    </listitem>
   </varlistentry>
   </variablelist>

   <para>
    <acronym>BRIN</acronym> indexes accept different parameters:
   </para>

   <variablelist>
   <varlistentry>
    <term><literal>pages_per_range</literal></term>
    <listitem>
    <para>
     Defines the number of table blocks that make up one block range for
     each entry of a <acronym>BRIN</acronym> index (see <xref linkend="brin-intro"/>
     for more details).  The default is <literal>128</literal>.
    </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>autosummarize</literal></term>
    <listitem>
    <para>
     Defines whether a summarization run is invoked for the previous page
     range whenever an insertion is detected on the next one.
    </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</productname> 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.  Very 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</productname> supports building indexes without locking
    out writes.  This method is invoked by specifying the
    <literal>CONCURRENTLY</literal> option of <command>CREATE INDEX</command>.
    When this option is used,
    <productname>PostgreSQL</productname> must perform two scans of the table, and in
    addition it must wait for all existing transactions that could potentially
    modify or use the index 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 might slow other operations.
   </para>

   <para>
    In a concurrent index build, the index is actually entered into
    the system catalogs in one transaction, then two table scans occur in
    two more transactions.  Before each table scan, the index build must
    wait for existing transactions that have modified the table to terminate.
    After the second scan, the index build must wait for any transactions
    that have a snapshot (see <xref linkend="mvcc"/>) predating the second
    scan to terminate.  Then finally the index can be marked ready for use,
    and the <command>CREATE INDEX</command> command terminates.
    Even then, however, the index may not be immediately usable for queries:
    in the worst case, it cannot be used as long as transactions exist that
    predate the start of the index build.
   </para>

   <para>
    If a problem arises while scanning the table, such as a deadlock or a
    uniqueness violation in a unique index, the <command>CREATE INDEX</command>
    command will fail but leave behind an <quote>invalid</quote> index. This index
    will be ignored for querying purposes because it might be incomplete;
    however it will still consume update overhead. The <application>psql</application>
    <command>\d</command> command will report such an index as <literal>INVALID</literal>:

<programlisting>
postgres=# \d tab
       Table "public.tab"
 Column |  Type   | Collation | Nullable | Default 
--------+---------+-----------+----------+---------
 col    | integer |           |          | 
Indexes:
    "idx" btree (col) INVALID
</programlisting>

    The recommended recovery
    method in such cases is to drop the index and try again to perform
    <command>CREATE INDEX CONCURRENTLY</command>.  (Another possibility is to rebuild
    the index with <command>REINDEX</command>.  However, since <command>REINDEX</command>
    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</quote> 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> command can be performed within
    a transaction block, but <command>CREATE INDEX CONCURRENTLY</command> 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, GiST, GIN, and BRIN index methods support
   multicolumn indexes. Up to 32 fields can 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>
   When <literal>CREATE INDEX</literal> is invoked on a partitioned
   table, the default behavior is to recurse to all partitions to ensure
   they all have matching indexes.
   Each partition is first checked to determine whether an equivalent
   index already exists, and if so, that index will become attached as a
   partition index to the index being created, which will become its
   parent index.
   If no matching index exists, a new index will be created and
   automatically attached; the name of the new index in each partition
   will be determined as if no index name had been specified in the
   command.
   If the <literal>ONLY</literal> option is specified, no recursion
   is done, and the index is marked invalid
   (<command>ALTER INDEX ... ATTACH PARTITION</command> turns the index
   valid, once all partitions acquire the index.)  Note, however, that
   any partition that is created in the future using
   <command>CREATE TABLE ... PARTITION OF</command> will automatically
   contain the index regardless of whether this option was specified.
  </para>

  <para>
   For index methods that support ordered scans (currently, only B-tree),
   the optional clauses <literal>ASC</literal>, <literal>DESC</literal>, <literal>NULLS
   FIRST</literal>, and/or <literal>NULLS LAST</literal> can be specified to modify
   the sort ordering of the index.  Since an ordered index can be
   scanned either forward or backward, it is not normally useful to create a
   single-column <literal>DESC</literal> index &mdash; that sort ordering is already
   available with a regular index.  The value of these options is that
   multicolumn indexes can be created that match the sort ordering requested
   by a mixed-ordering query, such as <literal>SELECT ... ORDER BY x ASC, y
   DESC</literal>.  The <literal>NULLS</literal> options are useful if you need to support
   <quote>nulls sort low</quote> behavior, rather than the default <quote>nulls
   sort high</quote>, in queries that depend on indexes to avoid sorting steps.
  </para>

  <para>
   For most index methods, the speed of creating an index is
   dependent on the setting of <xref linkend="guc-maintenance-work-mem"/>.
   Larger values will reduce the time needed for index creation, so long
   as you don't make it larger than the amount of memory really available,
   which would drive the machine into swapping.
  </para>

  <para>
   <productname>PostgreSQL</productname> can build indexes while
   leveraging multiple CPUs in order to process the table rows faster.
   This feature is known as <firstterm>parallel index
   build</firstterm>.  For index methods that support building indexes
   in parallel (currently, only B-tree),
   <varname>maintenance_work_mem</varname> specifies the maximum
   amount of memory that can be used by each index build operation as
   a whole, regardless of how many worker processes were started.
   Generally, a cost model automatically determines how many worker
   processes should be requested, if any.
  </para>

  <para>
   Parallel index builds may benefit from increasing
   <varname>maintenance_work_mem</varname> where an equivalent serial
   index build will see little or no benefit.  Note that
   <varname>maintenance_work_mem</varname> may influence the number of
   worker processes requested, since parallel workers must have at
   least a <literal>32MB</literal> share of the total
   <varname>maintenance_work_mem</varname> budget.  There must also be
   a remaining <literal>32MB</literal> share for the leader process.
   Increasing <xref linkend="guc-max-parallel-workers-maintenance"/>
   may allow more workers to be used, which will reduce the time
   needed for index creation, so long as the index build is not
   already I/O bound.  Of course, there should also be sufficient
   CPU capacity that would otherwise lie idle.
  </para>

  <para>
   Setting a value for <literal>parallel_workers</literal> via <xref
   linkend="sql-altertable"/> directly controls how many parallel
   worker processes will be requested by a <command>CREATE
   INDEX</command> against the table.  This bypasses the cost model
   completely, and prevents <varname>maintenance_work_mem</varname>
   from affecting how many parallel workers are requested.  Setting
   <literal>parallel_workers</literal> to 0 via <command>ALTER
   TABLE</command> will disable parallel index builds on the table in
   all cases.
  </para>

  <tip>
   <para>
    You might want to reset <literal>parallel_workers</literal> after
    setting it as part of tuning an index build.  This avoids
    inadvertent changes to query plans, since
    <literal>parallel_workers</literal> affects
    <emphasis>all</emphasis> parallel table scans.
   </para>
  </tip>

  <para>
   While <command>CREATE INDEX</command> with the
   <literal>CONCURRENTLY</literal> option supports parallel builds
   without special restrictions, only the first table scan is actually
   performed in parallel.
  </para>

  <para>
   Use <xref linkend="sql-dropindex"/>
   to remove an index.
  </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</literal> is specified, <command>CREATE INDEX</command>
   will interpret it as <literal>USING gist</literal>, to simplify conversion
   of old databases to GiST.
  </para>
 </refsect1>

 <refsect1>
  <title>Examples</title>

  <para>
   To create a unique 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 a unique B-tree index on the column <literal>title</literal>
   with included columns <literal>director</literal>
   and <literal>rating</literal> in the table <literal>films</literal>:
<programlisting>
CREATE UNIQUE INDEX title_idx ON films (title) INCLUDE (director, rating);
</programlisting>
  </para>

  <para>
   To create an index on the expression <literal>lower(title)</literal>,
   allowing efficient case-insensitive searches:
<programlisting>
CREATE INDEX ON films ((lower(title)));
</programlisting>
   (In this example we have chosen to omit the index name, so the system
   will choose a name, typically <literal>films_lower_idx</literal>.)
  </para>

  <para>
   To create an index with non-default collation:
<programlisting>
CREATE INDEX title_idx_german ON films (title COLLATE "de_DE");
</programlisting>
  </para>

  <para>
   To create an index with non-default sort ordering of nulls:
<programlisting>
CREATE INDEX title_idx_nulls_low ON films (title NULLS FIRST);
</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 a <acronym>GIN</acronym> index with fast updates disabled:
<programlisting>
CREATE INDEX gin_idx ON documents_table USING GIN (locations) WITH (fastupdate = off);
</programlisting>
  </para>

  <para>
   To create an index on the column <literal>code</literal> in the table
   <literal>films</literal> and have the index reside in the tablespace
   <literal>indexspace</literal>:
<programlisting>
CREATE INDEX code_idx ON films (code) TABLESPACE indexspace;
</programlisting>
  </para>

  <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 (box(location,location));
SELECT * FROM points
    WHERE box(location,location) &amp;&amp; '(0,0),(1,1)'::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"/></member>
   <member><xref linkend="sql-dropindex"/></member>
  </simplelist>
 </refsect1>
</refentry>
