dennis.jenkins.75@gmail.com (dennis jenkins) writes:
> The UUID itself is 128 bits. Some of those bits are pre-determined.
> I don't recall, but I think that a "normal" UUID has 121 bits of
> randomness.
That doesn't match RFC 4122 very well...
It indicates 5 forms of UUIDs:
1) Time-based, where about 1/2 the data comes from local timestamp data,
and 48 bits come from MAC address (or similar)
2) "DCE Security" (about which it says little)
3) Name-based, using MD5 hashing
4) Randomly generated UUIDs (which are quite likely what you're thinking
about) have 122 bits of random data
5) Name-based, using SHA-1 hashing
The reasonable choices for a would-be artificial primary key seem to be
1 and 3; in a distributed system, I'd expect to prefer 1, as the time +
host data are likely to eliminate the "oh, it might just randomly match"
problem.
Note the set of functions in the uuid-ossp contrib module fit this, down
to omitting Version 2 :-).
test@localhost-> \df+ public.uuid_generate*
List of functions
Schema | Name | Result data type | Argument data types | Type | Volatility | Owner | Language
| Source code | Description
--------+--------------------+------------------+---------------------------+--------+------------+----------+----------+--------------------+-------------
public | uuid_generate_v1 | uuid | | normal | volatile | postgres | c
|uuid_generate_v1 |
public | uuid_generate_v1mc | uuid | | normal | volatile | postgres | c
|uuid_generate_v1mc |
public | uuid_generate_v3 | uuid | namespace uuid, name text | normal | immutable | postgres | c
|uuid_generate_v3 |
public | uuid_generate_v4 | uuid | | normal | volatile | postgres | c
|uuid_generate_v4 |
public | uuid_generate_v5 | uuid | namespace uuid, name text | normal | immutable | postgres | c
|uuid_generate_v5 |
(5 rows)
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