UUID Generation Functions
=========================
Peter Eisentraut <peter_e@gmx.net>

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.


Installation
------------

The extra library required can be found at
<http://www.ossp.org/pkg/lib/uuid/>.


UUID Generation
---------------

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.

uuid_generate_v1()
~~~~~~~~~~~~~~~~~~

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.

uuid_generate_v1mc()
~~~~~~~~~~~~~~~~~~~~

This function generates a version 1 UUID but uses a random multicast
MAC address instead of the real MAC address of the computer.

uuid_generate_v3(namespace uuid, name text)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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:

  uuid_generate_v3(uuid_ns_url(), 'http://www.postgresql.org')

The name parameter will be MD5-hashed, so the cleartext cannot be
derived from the generated UUID.

The generation of UUIDs by this method has no random or
environment-dependent element and is therefore reproducible.

uuid_generate_v4()
~~~~~~~~~~~~~~~~~~

This function generates a version 4 UUID, which is derived entirely
from random numbers.

uuid_generate_v5(namespace uuid, name text)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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.


UUID Constants
--------------

  uuid_nil()

A "nil" UUID constant, which does not occur as a real UUID.

  uuid_ns_dns()

Constant designating the DNS namespace for UUIDs.

  uuid_ns_url()

Constant designating the URL namespace for UUIDs.

  uuid_ns_oid()

Constant designating the ISO object identifier (OID) namespace for
UUIDs.  (This pertains to ASN.1 OIDs, unrelated to the OIDs used in
PostgreSQL.)

  uuid_ns_x500()

Constant designating the X.500 distinguished name (DN) namespace for
UUIDs.