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

<refentry id="sql-altercollation">
 <indexterm zone="sql-altercollation">
  <primary>ALTER COLLATION</primary>
 </indexterm>

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

 <refnamediv>
  <refname>ALTER COLLATION</refname>
  <refpurpose>change the definition of a collation</refpurpose>
 </refnamediv>

 <refsynopsisdiv>
<synopsis>
ALTER COLLATION <replaceable>name</replaceable> REFRESH VERSION

ALTER COLLATION <replaceable>name</replaceable> RENAME TO <replaceable>new_name</replaceable>
ALTER COLLATION <replaceable>name</replaceable> OWNER TO { <replaceable>new_owner</replaceable> | CURRENT_USER | SESSION_USER }
ALTER COLLATION <replaceable>name</replaceable> SET SCHEMA <replaceable>new_schema</replaceable>
</synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   <command>ALTER COLLATION</command> changes the definition of a
   collation.
  </para>

  <para>
   You must own the collation to use <command>ALTER COLLATION</command>.
   To alter the owner, you must also be a direct or indirect member of the new
   owning role, and that role must have <literal>CREATE</literal> privilege on
   the collation's schema.  (These restrictions enforce that altering the
   owner doesn't do anything you couldn't do by dropping and recreating the
   collation. However, a superuser can alter ownership of any collation
   anyway.)
  </para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

  <variablelist>
   <varlistentry>
    <term><replaceable class="parameter">name</replaceable></term>
    <listitem>
     <para>
      The name (optionally schema-qualified) of an existing collation.
     </para>
    </listitem>
   </varlistentry>

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

   <varlistentry>
    <term><replaceable class="parameter">new_owner</replaceable></term>
    <listitem>
     <para>
      The new owner of the collation.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">new_schema</replaceable></term>
    <listitem>
     <para>
      The new schema for the collation.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>REFRESH VERSION</literal></term>
    <listitem>
     <para>
      Update the collation's version.
      See <xref linkend="sql-altercollation-notes"/> below.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </refsect1>

 <refsect1 id="sql-altercollation-notes" xreflabel="Notes">
  <title>Notes</title>

  <para>
   When using collations provided by the ICU library, the ICU-specific version
   of the collator is recorded in the system catalog when the collation object
   is created.  When the collation is used, the current version is
   checked against the recorded version, and a warning is issued when there is
   a mismatch, for example:
<screen>
WARNING:  collation "xx-x-icu" has version mismatch
DETAIL:  The collation in the database was created using version 1.2.3.4, but the operating system provides version 2.3.4.5.
HINT:  Rebuild all objects affected by this collation and run ALTER COLLATION pg_catalog."xx-x-icu" REFRESH VERSION, or build PostgreSQL with the right library version.
</screen>
   A change in collation definitions can lead to corrupt indexes and other
   problems because the database system relies on stored objects having a
   certain sort order.  Generally, this should be avoided, but it can happen
   in legitimate circumstances, such as when
   using <command>pg_upgrade</command> to upgrade to server binaries linked
   with a newer version of ICU.  When this happens, all objects depending on
   the collation should be rebuilt, for example,
   using <command>REINDEX</command>.  When that is done, the collation version
   can be refreshed using the command <literal>ALTER COLLATION ... REFRESH
   VERSION</literal>.  This will update the system catalog to record the
   current collator version and will make the warning go away.  Note that this
   does not actually check whether all affected objects have been rebuilt
   correctly.
  </para>
  <para>
   When using collations provided by <literal>libc</literal> and
   <productname>PostgreSQL</productname> was built with the GNU C library, the
   C library's version is used as a collation version.  Since collation
   definitions typically change only with GNU C library releases, this provides
   some defense against corruption, but it is not completely reliable.
  </para>
  <para>
   Currently, there is no version tracking for the database default collation.
  </para>

  <para>
   The following query can be used to identify all collations in the current
   database that need to be refreshed and the objects that depend on them:
<programlisting><![CDATA[
SELECT pg_describe_object(refclassid, refobjid, refobjsubid) AS "Collation",
       pg_describe_object(classid, objid, objsubid) AS "Object"
  FROM pg_depend d JOIN pg_collation c
       ON refclassid = 'pg_collation'::regclass AND refobjid = c.oid
  WHERE c.collversion <> pg_collation_actual_version(c.oid)
  ORDER BY 1, 2;
]]></programlisting>
  </para>
 </refsect1>

 <refsect1>
  <title>Examples</title>

  <para>
   To rename the collation <literal>de_DE</literal> to
   <literal>german</literal>:
<programlisting>
ALTER COLLATION "de_DE" RENAME TO german;
</programlisting>
  </para>

  <para>
   To change the owner of the collation <literal>en_US</literal> to
   <literal>joe</literal>:
<programlisting>
ALTER COLLATION "en_US" OWNER TO joe;
</programlisting></para>
 </refsect1>

 <refsect1>
  <title>Compatibility</title>

  <para>
   There is no <command>ALTER COLLATION</command> statement in the SQL
   standard.
  </para>
 </refsect1>

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

  <simplelist type="inline">
   <member><xref linkend="sql-createcollation"/></member>
   <member><xref linkend="sql-dropcollation"/></member>
  </simplelist>
 </refsect1>
</refentry>
