204 lines
6.6 KiB
Plaintext
204 lines
6.6 KiB
Plaintext
<!-- doc/src/sgml/uuid-ossp.sgml -->
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<sect1 id="uuid-ossp" xreflabel="uuid-ossp">
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<title>uuid-ossp</title>
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<indexterm zone="uuid-ossp">
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<primary>uuid-ossp</primary>
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</indexterm>
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<para>
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The <filename>uuid-ossp</> module provides functions to generate universally
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unique identifiers (UUIDs) using one of several standard algorithms. There
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are also functions to produce certain special UUID constants.
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</para>
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<sect2>
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<title><literal>uuid-ossp</literal> Functions</title>
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<para>
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<xref linkend="uuid-ossp-functions"> shows the functions available to
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generate UUIDs.
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The relevant standards ITU-T Rec. X.667, ISO/IEC 9834-8:2005, and RFC
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4122 specify four algorithms for generating UUIDs, identified by the
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version numbers 1, 3, 4, and 5. (There is no version 2 algorithm.)
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Each of these algorithms could be suitable for a different set of
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applications.
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</para>
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<table id="uuid-ossp-functions">
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<title>Functions for UUID Generation</title>
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<tgroup cols="2">
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<thead>
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<row>
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<entry>Function</entry>
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<entry>Description</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry><function>uuid_generate_v1()</function><indexterm><primary>uuid_generate_v1</primary></indexterm></entry>
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<entry>
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<para>
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This function generates a version 1 UUID. This involves the MAC
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address of the computer and a time stamp. Note that UUIDs of this
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kind reveal the identity of the computer that created the identifier
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and the time at which it did so, which might make it unsuitable for
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certain security-sensitive applications.
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</para>
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</entry>
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</row>
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<row>
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<entry><function>uuid_generate_v1mc()</function><indexterm><primary>uuid_generate_v1mc</primary></indexterm></entry>
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<entry>
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<para>
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This function generates a version 1 UUID but uses a random multicast
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MAC address instead of the real MAC address of the computer.
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</para>
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</entry>
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</row>
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<row>
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<entry><function>uuid_generate_v3(namespace uuid, name text)</function><indexterm><primary>uuid_generate_v3</primary></indexterm></entry>
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<entry>
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<para>
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This function generates a version 3 UUID in the given namespace using
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the specified input name. The namespace should be one of the special
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constants produced by the <function>uuid_ns_*()</> functions shown
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in <xref linkend="uuid-ossp-constants">. (It could be any UUID in theory.) The name is an identifier
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in the selected namespace.
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</para>
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<para>
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For example:
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<programlisting>
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SELECT uuid_generate_v3(uuid_ns_url(), 'http://www.postgresql.org');
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</programlisting>
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The name parameter will be MD5-hashed, so the cleartext cannot be
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derived from the generated UUID.
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The generation of UUIDs by this method has no random or
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environment-dependent element and is therefore reproducible.
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</para>
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</entry>
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</row>
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<row>
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<entry><literal>uuid_generate_v4()</literal></entry>
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<entry>
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<para>
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This function generates a version 4 UUID, which is derived entirely
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from random numbers.
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</para>
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</entry>
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</row>
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<row>
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<entry><literal>uuid_generate_v5(namespace uuid, name text)</literal></entry>
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<entry>
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<para>
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This function generates a version 5 UUID, which works like a version 3
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UUID except that SHA-1 is used as a hashing method. Version 5 should
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be preferred over version 3 because SHA-1 is thought to be more secure
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than MD5.
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</para>
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</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<table id="uuid-ossp-constants">
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<title>Functions Returning UUID Constants</title>
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<tgroup cols="2">
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<tbody>
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<row>
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<entry><literal>uuid_nil()</literal></entry>
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<entry>
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<para>
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A <quote>nil</> UUID constant, which does not occur as a real UUID.
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</para>
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</entry>
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</row>
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<row>
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<entry><literal>uuid_ns_dns()</literal></entry>
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<entry>
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<para>
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Constant designating the DNS namespace for UUIDs.
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</para>
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</entry>
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</row>
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<row>
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<entry><literal>uuid_ns_url()</literal></entry>
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<entry>
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<para>
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Constant designating the URL namespace for UUIDs.
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</para>
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</entry>
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</row>
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<row>
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<entry><literal>uuid_ns_oid()</literal></entry>
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<entry>
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<para>
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Constant designating the ISO object identifier (OID) namespace for
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UUIDs. (This pertains to ASN.1 OIDs, which are unrelated to the OIDs
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used in <productname>PostgreSQL</>.)
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</para>
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</entry>
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</row>
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<row>
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<entry><literal>uuid_ns_x500()</literal></entry>
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<entry>
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<para>
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Constant designating the X.500 distinguished name (DN) namespace for
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UUIDs.
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</para>
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</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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</sect2>
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<sect2>
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<title>Building <filename>uuid-ossp</></title>
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<para>
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Historically this module depended on the OSSP UUID library, which accounts
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for the module's name. While the OSSP UUID library can still be found
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at <ulink url="http://www.ossp.org/pkg/lib/uuid/"></ulink>, it is not well
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maintained, and is becoming increasingly difficult to port to newer
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platforms. <filename>uuid-ossp</> can now be built without the OSSP
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library on some platforms. On FreeBSD, NetBSD, and some other BSD-derived
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platforms, suitable UUID creation functions are included in the
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core <filename>libc</> library. On Linux, macOS, and some other
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platforms, suitable functions are provided in the <filename>libuuid</>
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library, which originally came from the <literal>e2fsprogs</> project
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(though on modern Linux it is considered part
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of <literal>util-linux-ng</>). When invoking <filename>configure</>,
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specify <option>--with-uuid=bsd</option> to use the BSD functions,
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or <option>--with-uuid=e2fs</option> to
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use <literal>e2fsprogs</>' <filename>libuuid</>, or
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<option>--with-uuid=ossp</option> to use the OSSP UUID library.
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More than one of these libraries might be available on a particular
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machine, so <filename>configure</> does not automatically choose one.
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</para>
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<note>
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<para>
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If you only need randomly-generated (version 4) UUIDs,
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consider using the <function>gen_random_uuid()</> function
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from the <xref linkend="pgcrypto"> module instead.
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</para>
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</note>
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</sect2>
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<sect2>
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<title>Author</title>
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<para>
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Peter Eisentraut <email>peter_e@gmx.net</email>
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</para>
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</sect2>
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</sect1>
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