uuid-ossp The uuid-ossp 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 PostgreSQL. See for built-in ways to generate UUIDs. This module is considered trusted, that is, it can be installed by non-superusers who have CREATE privilege on the current database. shows the functions available to generate UUIDs. 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. Function Description uuid_generate_v1 uuid_generate_v1 () uuid 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 uuid_generate_v1mc () uuid Generates a version 1 UUID, but uses a random multicast MAC address instead of the real MAC address of the computer. uuid_generate_v3 uuid_generate_v3 ( namespace uuid, name text ) uuid 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 in . (It could be any UUID in theory.) The name is an identifier in the selected namespace. For example: SELECT 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 () uuid Generates a version 4 UUID, which is derived entirely from random numbers. uuid_generate_v5 ( namespace uuid, name text ) uuid 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. Function Description uuid_nil () uuid Returns a nil UUID constant, which does not occur as a real UUID. uuid_ns_dns () uuid Returns a constant designating the DNS namespace for UUIDs. uuid_ns_url () uuid Returns a constant designating the URL namespace for UUIDs. uuid_ns_oid () uuid 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 PostgreSQL.) uuid_ns_x500 () uuid Returns a constant designating the X.500 distinguished name (DN) namespace for UUIDs. 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 , it is not well maintained, and is becoming increasingly difficult to port to newer platforms. uuid-ossp 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 libc library. On Linux, macOS, and some other platforms, suitable functions are provided in the libuuid library, which originally came from the e2fsprogs project (though on modern Linux it is considered part of util-linux-ng). When invoking configure, specify --with-uuid=bsd to use the BSD functions, or --with-uuid=e2fs to use e2fsprogs' libuuid, or --with-uuid=ossp to use the OSSP UUID library. More than one of these libraries might be available on a particular machine, so configure does not automatically choose one. Peter Eisentraut peter_e@gmx.net