Re: [PATCH] Implement uuid_version()

On Mon, Apr 8, 2019 at 11:04 PM Peter Eisentraut
<peter(dot)eisentraut(at)2ndquadrant(dot)com> wrote:
> Yeah, I think implementing a v4 generator in core would be trivial and
> address almost everyone's requirements.

FWIW, I think that we could do better with nbtree page splits given
sequential UUIDs of one form or another [1]. We could teach
nbtsplitloc.c to pack leaf pages full of UUIDs in the event of the
user using sequential UUIDs. With a circular UUID prefix, I think
you'll run into an issue similar to the issue that was addressed by
the "split after new tuple" optimization -- most leaf pages end up 50%
full. I've not verified this, but I can't see why it would be any
different to other multimodal key space with sequential insertions
that are grouped. Detecting this in UUIDs may or may not require
opclass infrastructure. Either way, I'm not likely to work on it until
there is a clear target, such as a core or contrib sequential UUID
generator. Though I am looking at various ways to improve
nbtsplitloc.c for Postgres 13 -- I suspect that additional wins are
possible.

Any sequential UUID scheme will already have far fewer problems with
indexing today, since random UUIDs are *dreadful*, but I can imagine
doing quite a lot better still. Application developers love UUIDs. We
should try to meet them where they are.

[1] https://www.2ndquadrant.com/en/blog/sequential-uuid-generators/
--
Peter Geoghegan