sort order. Generally, this should be avoided, but it can happen
in legitimate circumstances, such as when upgrading the operating system
to a new major version or 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 collation 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>, version
information is recorded on systems using the GNU C library (most Linux
systems), FreeBSD and Windows. When using collations provided by ICU, the
version information is provided by the ICU library and is available on all
platforms.
</para>
<note>
<para>
When using the GNU C library for collations, the C library's version
is used as a proxy for the collation version. Many Linux distributions
change collation definitions only when upgrading the C library, but this
approach is imperfect as maintainers are free to back-port newer
collation definitions to older C library releases.
</para>
<para>
When using Windows for collations, version information is only available
for collations defined with BCP 47 language tags such as
<literal>en-US</literal>.
</para>
</note>
<para>
For the database default collation, there is an analogous command
<literal>ALTER DATABASE ... REFRESH COLLATION VERSION</literal>.
</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>