Stephen Frost <sfr...@snowman.net> writes: > * Hannu Krosing (hkros...@gmail.com) wrote: >> Is there any theoretical obstacle which would make it impossible to >> teach VACUUM not to hold back the whole vacuum horizon, but just >> to leave a single transaction alone in case of a long-running >> REPEATABLE READ transaction ?
> I've looked into this a couple of times and I believe it's possible to > calculate what records have to remain available for the long-running > transaction, but it's far from trivial. I think it'd become a lot easier if we went over to representing snapshots as LSN positions (and, concomitantly, had an inexpensive way to translate XIDs to their commit LSNs). That would mean that (1) a backend's snapshot state could be fully exposed in PGPROC, at least up to some small number of active snapshots; (2) it'd be fairly cheap for VACUUM to detect that a dead tuple's XMIN and XMAX are either both before or both after each live snapshot. Someone (Heikki, I think) has been working on this but I've not seen any patch yet. regards, tom lane -- Sent via pgsql-hackers mailing list (firstname.lastname@example.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers