
<sect1 id="uuid-ossp">
 <title>uuid-ossp</title>
 
 <indexterm zone="uuid-ossp">
  <primary>uuid-ossp</primary>
 </indexterm>

 <para>
  This module provides functions to generate universally unique
  identifiers (UUIDs) using one of the several standard algorithms, as
  well as functions to produce certain special UUID constants.
 </para>

 <sect2>
  <title>UUID Generation</title>
  <para>
   The relevant standards ITU-T Rec. X.667, ISO/IEC 9834-8:2005, and RFC
   4122 specify four algorithms for generating UUIDs, identified by the
   version numbers 1, 3, 4, and 5.  (There is no version 2 algorithm.)
   Each of these algorithms could be suitable for a different set of
   applications.
  </para>

  <table>
   <title><literal>uuid-ossp</literal> functions</title>
   <tgroup cols="2">
    <thead>
     <row>
      <entry>Function</entry>
      <entry>Description</entry>
     </row>
    </thead>
    <tbody>
     <row>
      <entry><literal>uuid_generate_v1()</literal></entry>
      <entry>
       <para>
        This function generates a version 1 UUID.  This involves the MAC
        address of the computer and a time stamp.  Note that UUIDs of this
        kind reveal the identity of the computer that created the identifier
        and the time at which it did so, which might make it unsuitable for
        certain security-sensitive applications.
       </para>
      </entry>
     </row>
     <row>
      <entry><literal>uuid_generate_v1mc()</literal></entry>
      <entry>
       <para>
        This function generates a version 1 UUID but uses a random multicast
        MAC address instead of the real MAC address of the computer.
       </para>
      </entry>
     </row>
     <row>
      <entry><literal>uuid_generate_v3(namespace uuid, name text)</literal></entry>
      <entry>
       <para>
        This function generates a version 3 UUID in the given namespace using
        the specified input name.  The namespace should be one of the special
        constants produced by the uuid_ns_*() functions shown below.  (It
        could be any UUID in theory.)  The name is an identifier in the
        selected namespace.  For example:
       </para>
      </entry>
     </row>
     <row>
      <entry><literal>uuid_generate_v3(uuid_ns_url(), 'http://www.postgresql.org')</literal></entry>
      <entry>
       <para>
        The name parameter will be MD5-hashed, so the cleartext cannot be
        derived from the generated UUID.
       </para>
       <para>
        The generation of UUIDs by this method has no random or
        environment-dependent element and is therefore reproducible.
       </para>
      </entry>
     </row>
     <row>
      <entry><literal>uuid_generate_v4()</literal></entry>
      <entry>
       <para>
        This function generates a version 4 UUID, which is derived entirely
        from random numbers.
       </para>
      </entry>
     </row>
     <row>
      <entry><literal>uuid_generate_v5(namespace uuid, name text)</literal></entry>
      <entry>
       <para>
        This function generates a version 5 UUID, which works like a version 3
        UUID except that SHA-1 is used as a hashing method.  Version 5 should
        be preferred over version 3 because SHA-1 is thought to be more secure
        than MD5.
       </para>
      </entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <table>
   <title>UUID Constants</title>
   <tgroup cols="2">
    <tbody>
     <row>
      <entry><literal>uuid_nil()</literal></entry>
      <entry>
       <para>
        A "nil" UUID constant, which does not occur as a real UUID.
       </para>
      </entry>
     </row>
     <row>
      <entry><literal>uuid_ns_dns()</literal></entry>
      <entry>
       <para>
        Constant designating the DNS namespace for UUIDs.
       </para>
      </entry>
     </row>
     <row>
      <entry><literal>uuid_ns_url()</literal></entry>
      <entry>
       <para>
        Constant designating the URL namespace for UUIDs.
       </para>
      </entry>
     </row>
     <row>
      <entry><literal>uuid_ns_oid()</literal></entry>
      <entry>
       <para>
        Constant designating the ISO object identifier (OID) namespace for
        UUIDs.  (This pertains to ASN.1 OIDs, unrelated to the OIDs used in
        PostgreSQL.)
       </para>
      </entry>
     </row>
     <row>
      <entry><literal>uuid_ns_x500()</literal></entry>
      <entry>
       <para>
        Constant designating the X.500 distinguished name (DN) namespace for
        UUIDs.
       </para>
      </entry>
     </row>
    </tbody>
   </tgroup>
  </table>
 </sect2>
 <sect2>
  <title>Author</title>
  <para>
   Peter Eisentraut <email>peter_e@gmx.net</email>
  </para>
 </sect2>
</sect1>

