<!-- doc/src/sgml/uuid-ossp.sgml -->
<sect1 id="uuid-ossp" xreflabel="uuid-ossp">
<title>uuid-ossp &mdash; a UUID generator</title>
<indexterm zone="uuid-ossp">
<primary>uuid-ossp</primary>
</indexterm>
<para>
The <filename>uuid-ossp</filename> module provides functions to generate universally
unique identifiers (UUIDs) using one of several standard algorithms. There
are also functions to produce certain special UUID constants.
This module is only necessary for special requirements beyond what is
available in core <productname>PostgreSQL</productname>. See <xref
linkend="functions-uuid"/> for built-in ways to generate UUIDs.
</para>
<para>
This module is considered <quote>trusted</quote>, that is, it can be
installed by non-superusers who have <literal>CREATE</literal> privilege
on the current database.
</para>
<sect2 id="uuid-ossp-functions-sect">
<title><literal>uuid-ossp</literal> Functions</title>
<para>
<xref linkend="uuid-ossp-functions"/> shows the functions available to
generate UUIDs.
The relevant standards ITU-T Rec. X.667, ISO/IEC 9834-8:2005, and
<ulink url="https://datatracker.ietf.org/doc/html/rfc4122">RFC 4122</ulink>
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 id="uuid-ossp-functions">
<title>Functions for UUID Generation</title>
<tgroup cols="1">
<thead>
<row>
<entry role="func_table_entry"><para role="func_signature">
Function
</para>
<para>
Description
</para></entry>
</row>
</thead>
<tbody>
<row>
<entry role="func_table_entry"><para role="func_signature">
<indexterm><primary>uuid_generate_v1</primary></indexterm>
<function>uuid_generate_v1</function> ()
<returnvalue>uuid</returnvalue>
</para>
<para>
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 role="func_table_entry"><para role="func_signature">
<indexterm><primary>uuid_generate_v1mc</primary></indexterm>
<function>uuid_generate_v1mc</function> ()
<returnvalue>uuid</returnvalue>
</para>
<para>
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 role="func_table_entry"><para role="func_signature">
<indexterm><primary>uuid_generate_v3</primary></indexterm>
<function>uuid_generate_v3</function> ( <parameter>namespace</parameter> <type>uuid</type>, <parameter>name</parameter> <type>text</type> )
<returnvalue>uuid</returnvalue>
</para>
<para>
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 <function>uuid_ns_*()</function> functions
shown in <xref linkend="uuid-ossp-constants"/>. (It could be any UUID
in theory.) The name is an identifier in the selected namespace.
</para>
<para>
For example:
<programlisting>
SELECT uuid_generate_v3(uuid_ns_url(), 'http://www.postgresql.org');
</programlisting>
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.
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<function>uuid_generate_v4</function> ()
<returnvalue>uuid</returnvalue>
</para>
<para>
Generates a version 4 UUID, which is derived entirely
from random numbers.
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<function>uuid_generate_v5</function> ( <parameter>namespace</parameter> <type>uuid</type>, <parameter>name</parameter> <type>text</type> )
<returnvalue>uuid</returnvalue>
</para>
<para>
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 id="uuid-ossp-constants">
<title>Functions Returning UUID Constants</title>
<tgroup cols="1">
<thead>
<row>
<entry role="func_table_entry"><para role="func_signature">
Function
</para>
<para>
Description
</para></entry>
</row>
</thead>
<tbody>
<row>
<entry role="func_table_entry"><para role="func_signature">
<function>uuid_nil</function> ()
<returnvalue>uuid</returnvalue>
</para>
<para>
Returns a <quote>nil</quote> UUID constant, which does not occur as a
real UUID.
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<function>uuid_ns_dns</function> ()
<returnvalue>uuid</returnvalue>
</para>
<para>
Returns a constant designating the DNS namespace for UUIDs.
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<function>uuid_ns_url</function> ()
<returnvalue>uuid</returnvalue>
</para>
<para>
Returns a constant designating the URL namespace for UUIDs.
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<function>uuid_ns_oid</function> ()
<returnvalue>uuid</returnvalue>
</para>
<para>
Returns a constant designating the ISO object identifier (OID) namespace for
UUIDs. (This pertains to ASN.1 OIDs, which are unrelated to the OIDs
used in <productname>PostgreSQL</productname>.)
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<function>uuid_ns_x500</function> ()
<returnvalue>uuid</returnvalue>
</para>
<para>
Returns a constant designating the X.500 distinguished name (DN)
namespace for UUIDs.
</para></entry>
</row>
</tbody>
</tgroup>
</table>
</sect2>
<sect2 id="uuid-ossp-building">
<title>Building <filename>uuid-ossp</filename></title>
<para>
Historically this module depended on the OSSP UUID library, which accounts
for the module's name. While the OSSP UUID library can still be found
at <ulink url="http://www.ossp.org/pkg/lib/uuid/"></ulink>, it is not well
maintained, and is becoming increasingly difficult to port to newer
platforms. <filename>uuid-ossp</filename> can now be built without the OSSP
library on some platforms. On FreeBSD and some other BSD-derived
platforms, suitable UUID creation functions are included in the
core <filename>libc</filename> library. On Linux, macOS, and some other
platforms, suitable functions are provided in the <filename>libuuid</filename>
library, which originally came from the <literal>e2fsprogs</literal> project
(though on modern Linux it is considered part
of <literal>util-linux-ng</literal>). When invoking <filename>configure</filename>,
specify <option>--with-uuid=bsd</option> to use the BSD functions,
or <option>--with-uuid=e2fs</option> to
use <literal>e2fsprogs</literal>' <filename>libuuid</filename>, or
<option>--with-uuid=ossp</option> to use the OSSP UUID library.
More than one of these libraries might be available on a particular
machine, so <filename>configure</filename> does not automatically choose one.
</para>
</sect2>
<sect2 id="uuid-ossp-author">
<title>Author</title>
<para>
Peter Eisentraut <email>peter_e@gmx.net</email>
</para>
</sect2>
</sect1>