Make 'col IS NULL' clauses be indexable conditions.

[pg-rex/syncrep.git] / doc / src / sgml / ref / create_index.sgml

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$PostgreSQL: pgsql/doc/src/sgml/ref/create_index.sgml,v 1.62 2007/04/06 22:33:41 tgl Exp $
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<refentry id="SQL-CREATEINDEX">
 <refmeta>
  <refentrytitle id="sql-createindex-title">CREATE INDEX</refentrytitle>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

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

 <indexterm zone="sql-createindex">
  <primary>CREATE INDEX</primary>
 </indexterm>

 <refsynopsisdiv>
<synopsis>
CREATE [ UNIQUE ] INDEX [ CONCURRENTLY ] <replaceable class="parameter">name</replaceable> ON <replaceable class="parameter">table</replaceable> [ USING <replaceable class="parameter">method</replaceable> ]
    ( { <replaceable class="parameter">column</replaceable> | ( <replaceable class="parameter">expression</replaceable> ) } [ <replaceable class="parameter">opclass</replaceable> ] [ ASC | DESC ] [ NULLS { FIRST | LAST } ] [, ...] )
    [ WITH ( <replaceable class="PARAMETER">storage_parameter</replaceable> = <replaceable class="PARAMETER">value</replaceable> [, ... ] ) ]
    [ TABLESPACE <replaceable class="parameter">tablespace</replaceable> ]
    [ WHERE <replaceable class="parameter">predicate</replaceable> ]
</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 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</>, 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 can 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><literal>ASC</></term>
      <listitem>
       <para>
        Specifies ascending sort order (which is the default).
       </para>
      </listitem>
     </varlistentry>

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

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

     <varlistentry>
      <term><literal>NULLS LAST</></term>
      <listitem>
       <para>
        Specifies that nulls sort after non-nulls.  This is the default
        when <literal>DESC</> 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
        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 might 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 might be incomplete;
    however it will still consume update overhead. The <application>psql</>
    <command>\d</> command will mark such an index as <literal>INVALID</>:

<programlisting>
postgres=# \d tab
       Table "public.tab"
 Column |  Type   | Modifiers 
--------+---------+-----------
 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</>.  (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 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>
   For index methods that support ordered scans (currently, only B-tree),
   the optional clauses <literal>ASC</>, <literal>DESC</>, <literal>NULLS
   FIRST</>, and/or <literal>NULLS LAST</> can be specified to reverse
   the normal sort direction 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</> 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</>.  The <literal>NULLS</> options are useful if you need to support
   <quote>nulls sort low</> behavior, rather than the default <quote>nulls
   sort high</>, in queries that depend on indexes to avoid sorting steps.
  </para>

  <para>
   Use <xref linkend="sql-dropindex" endterm="sql-dropindex-title">
   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</> 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 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 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>