url="https://www.unicode.org/reports/tr18/#Compatibility_Properties">Compatibility
        Properties</ulink>.  Behavior is efficient and stable within a
        <productname>Postgres</productname> major version.  It is only
        available for encoding <literal>UTF8</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>pg_c_utf8</literal></term>
      <listitem>
       <para>
        This collation sorts by Unicode code point values rather than natural
        language order.  For the functions <function>lower</function>,
        <function>initcap</function>, and <function>upper</function>, it uses
        Unicode simple case mapping.  For pattern matching (including regular
        expressions), it uses the POSIX Compatible variant of Unicode <ulink
        url="https://www.unicode.org/reports/tr18/#Compatibility_Properties">Compatibility
        Properties</ulink>.  Behavior is efficient and stable within a
        <productname>PostgreSQL</productname> major version.  This collation is
        only available for encoding <literal>UTF8</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>C</literal> (equivalent to <literal>POSIX</literal>)</term>
      <listitem>
       <para>
        The <literal>C</literal> and <literal>POSIX</literal> collations are
        based on <quote>traditional C</quote> behavior.  They sort by byte
        values rather than natural language order, and only the ASCII letters
        <quote><literal>A</literal></quote> through
        <quote><literal>Z</literal></quote> are treated as letters.  The
        behavior is efficient and stable across all versions for a given
        database encoding, but behavior may vary between different database
        encodings.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>default</literal></term>
      <listitem>
       <para>
        The <literal>default</literal> collation selects the locale specified
        at database creation time.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>

   <para>
    Additional collations may be available depending on operating system
    support.  The efficiency and stability of these additional collations
    depend on the collation provider, the provider version, and the locale.
   </para>
  </sect3>

  <sect3 id="collation-managing-predefined">
   <title>Predefined Collations</title>

   <para>
    If the operating system provides support for using multiple locales
    within a single program (<function>newlocale</function> and related functions),
    or if support for ICU is configured,
    then when a database cluster is initialized, <command>initdb</command>
    populates the system catalog <literal>pg_collation</literal> with
    collations based on all the locales it finds in the operating
    system at the time.
   </para>

   <para>
    To inspect the currently available locales, use the query <literal>SELECT
    * FROM pg_collation</literal>, or the command <command>\dOS+</command>
    in <application>psql</application>.
   </para>

  <sect4 id="collation-managing-predefined-libc">
   <title>libc Collations</title>

   <para>
    For example, the operating system might
    provide a locale named <literal>de_DE.utf8</literal>.
    <command>initdb</command> would then create a collation named
    <literal>de_DE.utf8</literal> for encoding <literal>UTF8</literal>
    that has both <symbol>LC_COLLATE</symbol> and
    <symbol>LC_CTYPE</symbol> set to <literal>de_DE.utf8</literal>.
    It will also create a collation with the <literal>.utf8</literal>
    tag stripped off the name.  So you could also use the collation
    under the name <literal>de_DE</literal>, which is less cumbersome
    to write and makes the name less encoding-dependent.  Note that,
    nevertheless, the initial set of collation names is
    platform-dependent.