Per-table freeze limit proposal

Hackers,

As you've probably heard too many times already, I'm thinking in
improving vacuum, so we can keep track of the freeze Xid on a table
level, rather than database level. Hopefully this will eliminate the
need for database-wide vacuums.

In fact this seems pretty easy to do. Add a field to pg_class, tell
VACUUM to update it using the determined freezeLimit, and that's it.
(Note that if we ever implement partial vacuum, it won't be able to
update the freeze point. But that was true before anyway.)

We also need to teach autovacuum to update pg_database.datfreezexid,
using the minimum from pg_class. (I don't think it's a good idea to
seqscan pg_class to find out the minimum on each VACUUM call.) So, an
autovacuum iteration would issue all needed VACUUM/ANALYZE calls, then
get the minimum freezexid from pg_class to update pg_database. This
way, GetNewTransactionId can continue checking pg_database.datfreezexid
as the hard limit for issuing warnings for Xid wraparound.

Does anyone see a need for anything other than the autovacuum process to
be updating pg_database.datfreezexid? Of course, if autovacuum is not
in use, things would continue as now, that is, manual database-wide
VACUUM calls updating pg_database.datfreezexid. But note that you can
mark all tables as disabled on pg_autovacuum, issue your manuals VACUUM
calls as needed (from cron or whatever), and use autovacuum to set
pg_database.datfreezexid -- so autovacuum would in fact do nothing
except set the freeze limit.

The problem is, this seems so awfully simple that I fear I am missing
something ... Otherwise, does this sound like a plan?

--
Alvaro Herrera -- Valdivia, Chile Architect, www.EnterpriseDB.com
The easiest way to resolve [trivial code guidelines disputes] is to fire
one or both of the people involved. (Damian Conway)