Add database to PGXACT / per database vacuuming

Hi,

For the logical decoding patch I added support for pegging
RecentGlobalXmin (and GetOldestXmin) to a lower value. To avoid causing
undue bloat & cpu overhead (hot pruning is friggin expensive) I split
RecentGlobalXmin into RecentGlobalXmin and RecentGlobalDataXmin where
the latter is the the xmin horizon used for non-shared, non-catalog
tables. That removed almost all overhead I could measure.

During that I was tinkering with the idea of reusing that split to
vacuum/prune user tables in a per db fashion. In a very quick and hacky
test that sped up the aggregate performance of concurrent pgbenches in
different databases by about 30%. So, somewhat worthwile ;).

The problem with that is that GetSnapshotData, which computes
RecentGlobalXmin, only looks at the PGXACT structures and not PGPROC
which contains the database oid. This is a recently added optimization
which made GetSnapshotData() quite a bit faster & scalable which is
important given the frequency it's called at.

What about moving/copying the database oid from PGPROC to PGXACT?
Currently a single PGXACT is 12 bytes which means we a) have several
entries in a single cacheline b) have ugly sharing because we will have
PGXACTs split over more than one cacheline.

Comments?

Greetings,

Andres Freund

-- Andres Freund                       http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
Services