<!--
doc/src/sgml/ref/alter_index.sgml
PostgreSQL documentation
-->

<refentry id="sql-alterindex">
 <indexterm zone="sql-alterindex">
  <primary>ALTER INDEX</primary>
 </indexterm>

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

 <refnamediv>
  <refname>ALTER INDEX</refname>
  <refpurpose>change the definition of an index</refpurpose>
 </refnamediv>

 <refsynopsisdiv>
<synopsis>
ALTER INDEX [ IF EXISTS ] <replaceable class="parameter">name</replaceable> RENAME TO <replaceable class="parameter">new_name</replaceable>
ALTER INDEX [ IF EXISTS ] <replaceable class="parameter">name</replaceable> SET TABLESPACE <replaceable class="parameter">tablespace_name</replaceable>
ALTER INDEX <replaceable class="parameter">name</replaceable> DEPENDS ON EXTENSION <replaceable class="parameter">extension_name</replaceable>
ALTER INDEX [ IF EXISTS ] <replaceable class="parameter">name</replaceable> SET ( <replaceable class="parameter">storage_parameter</replaceable> = <replaceable class="parameter">value</replaceable> [, ... ] )
ALTER INDEX [ IF EXISTS ] <replaceable class="parameter">name</replaceable> RESET ( <replaceable class="parameter">storage_parameter</replaceable> [, ... ] )
ALTER INDEX [ IF EXISTS ] <replaceable class="parameter">name</replaceable> ALTER [ COLUMN ] <replaceable class="parameter">column_number</replaceable>
    SET STATISTICS <replaceable class="parameter">integer</replaceable>
ALTER INDEX ALL IN TABLESPACE <replaceable class="parameter">name</replaceable> [ OWNED BY <replaceable class="parameter">role_name</replaceable> [, ... ] ]
    SET TABLESPACE <replaceable class="parameter">new_tablespace</replaceable> [ NOWAIT ]
</synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   <command>ALTER INDEX</command> changes the definition of an existing index.
   There are several subforms:

  <variablelist>

   <varlistentry>
    <term><literal>RENAME</literal></term>
    <listitem>
     <para>
      The <literal>RENAME</literal> form changes the name of the index.
      There is no effect on the stored data.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>SET TABLESPACE</literal></term>
    <listitem>
     <para>
      This form changes the index's tablespace to the specified tablespace and
      moves the data file(s) associated with the index to the new tablespace.
      To change the tablespace of an index, you must own the index and have
      <literal>CREATE</literal> privilege on the new tablespace.
      All indexes in the current database in a tablespace can be moved by using
      the <literal>ALL IN TABLESPACE</literal> form, which will lock all
      indexes to be moved and then move each one.  This form also supports
      <literal>OWNED BY</literal>, which will only move indexes owned by the
      roles specified.  If the <literal>NOWAIT</literal> option is specified
      then the command will fail if it is unable to acquire all of the locks
      required immediately.  Note that system catalogs will not be moved by
      this command, use <command>ALTER DATABASE</command> or explicit
      <command>ALTER INDEX</command> invocations instead if desired.
      See also
      <xref linkend="sql-createtablespace"/>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>DEPENDS ON EXTENSION</literal></term>
    <listitem>
     <para>
      This form marks the index as dependent on the extension, such that if the
      extension is dropped, the index will automatically be dropped as well.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>SET ( <replaceable class="parameter">storage_parameter</replaceable> = <replaceable class="parameter">value</replaceable> [, ... ] )</literal></term>
    <listitem>
     <para>
      This form changes one or more index-method-specific storage parameters
      for the index.  See
      <xref linkend="sql-createindex"/>
      for details on the available parameters.  Note that the index contents
      will not be modified immediately by this command; depending on the
      parameter you might need to rebuild the index with
      <xref linkend="sql-reindex"/>
      to get the desired effects.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>RESET ( <replaceable class="parameter">storage_parameter</replaceable> [, ... ] )</literal></term>
    <listitem>
     <para>
      This form resets one or more index-method-specific storage parameters to
      their defaults.  As with <literal>SET</literal>, a <literal>REINDEX</literal>
      might be needed to update the index entirely.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>ALTER [ COLUMN ] <replaceable class="parameter">column_number</replaceable> SET STATISTICS <replaceable class="parameter">integer</replaceable></literal></term>
    <listitem>
     <para>
      This form sets the per-column statistics-gathering target for
      subsequent <xref linkend="sql-analyze"/> operations, though can
      be used only on index columns that are defined as an expression.
      Since expressions lack a unique name, we refer to them using the
      ordinal number of the index column. 
      The target can be set in the range 0 to 10000; alternatively, set it
      to -1 to revert to using the system default statistics
      target (<xref linkend="guc-default-statistics-target"/>).
      For more information on the use of statistics by the
      <productname>PostgreSQL</productname> query planner, refer to
      <xref linkend="planner-stats"/>.
     </para>
    </listitem>
   </varlistentry>

  </variablelist>
  </para>

 </refsect1>

 <refsect1>
  <title>Parameters</title>

    <variablelist>

     <varlistentry>
      <term><literal>IF EXISTS</literal></term>
      <listitem>
       <para>
        Do not throw an error if the index does not exist. A notice is issued
        in this case.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">column_number</replaceable></term>
      <listitem>
       <para>
        The ordinal number refers to the ordinal (left-to-right) position
        of the index column.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">name</replaceable></term>
      <listitem>
       <para>
        The name (possibly schema-qualified) of an existing index to
        alter.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">new_name</replaceable></term>
      <listitem>
       <para>
        The new name for the index.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">tablespace_name</replaceable></term>
      <listitem>
       <para>
        The tablespace to which the index will be moved.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">extension_name</replaceable></term>
      <listitem>
       <para>
        The name of the extension that the index is to depend on.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">storage_parameter</replaceable></term>
      <listitem>
       <para>
        The name of an index-method-specific storage parameter.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">value</replaceable></term>
      <listitem>
       <para>
        The new value for an index-method-specific storage parameter.
        This might be a number or a word depending on the parameter.
       </para>
      </listitem>
     </varlistentry>

    </variablelist>
 </refsect1>

 <refsect1>
  <title>Notes</title>

   <para>
    These operations are also possible using
    <xref linkend="sql-altertable"/>.
    <command>ALTER INDEX</command> is in fact just an alias for the forms
    of <command>ALTER TABLE</command> that apply to indexes.
   </para>

   <para>
    There was formerly an <command>ALTER INDEX OWNER</command> variant, but
    this is now ignored (with a warning).  An index cannot have an owner
    different from its table's owner.  Changing the table's owner
    automatically changes the index as well.
   </para>

   <para>
    Changing any part of a system catalog index is not permitted.
   </para>
 </refsect1>

 <refsect1>
  <title>Examples</title>
  <para>
   To rename an existing index:
<programlisting>
ALTER INDEX distributors RENAME TO suppliers;
</programlisting>
  </para>

  <para>
   To move an index to a different tablespace:
<programlisting>
ALTER INDEX distributors SET TABLESPACE fasttablespace;
</programlisting>
  </para>

  <para>
   To change an index's fill factor (assuming that the index method
   supports it):
<programlisting>
ALTER INDEX distributors SET (fillfactor = 75);
REINDEX INDEX distributors;
</programlisting></para>

  <para>
   Set the statistics-gathering target for an expression index:
<programlisting>
CREATE INDEX coord_idx ON measured (x, y, (z + t));
ALTER INDEX coord_idx ALTER COLUMN 3 SET STATISTICS 1000;
</programlisting>
  </para>

 </refsect1>

 <refsect1>
  <title>Compatibility</title>

  <para>
   <command>ALTER INDEX</command> is a <productname>PostgreSQL</productname>
   extension.
  </para>
 </refsect1>


 <refsect1>
  <title>See Also</title>

  <simplelist type="inline">
   <member><xref linkend="sql-createindex"/></member>
   <member><xref linkend="sql-reindex"/></member>
  </simplelist>
 </refsect1>
</refentry>
