On Thu, May 27, 2010 at 2:17 PM, Josh Berkus <j...@agliodbs.com> wrote: > On 5/26/10 6:32 PM, Robert Haas wrote: >> Hmm, yeah. Maybe we should freeze when we set PD_ALL_VISIBLE; that >> might be just as good, and simpler. Assuming the visibility map is >> sufficiently crash-safe/non-buggy, we could then teach VACUUM that >> it's OK to advance relfrozenxid even when doing just a partial vacuum >> - because any pages that were skipped must contain only frozen tuples. >> Previously you've objected to proposals in this direction because >> they might destroy forensic information, but maybe we should do it >> anyway. > > It would be an improvement, and easier than the various ways of never > having to visit the pages, which are all fairly intensive. Given the > destruction of rollback information, though, we'd probably want a way to > switch this behaviour on and off as an autovacuum setting.
It's not going to destroy anything that is needed for rollback unless there's a bug - PD_ALL_VISIBLE only gets set when all tuples on the page are visible to all backends. That can't happen until all transactions that wrote the page, and all others that have a lower xmin, have committed. That having been said, if making it a GUC makes people less nervous about doing it, then +1 from me. > Does this send us down the wrong path, though? I thought we wanted to > think about removing hint bits so that we could implement things like > CRCs. No? PD_ALL_VISIBLE is a page-level bit, not a tuple-level bit, and I strongly suspect it's not going anywhere. It's critical infrastructure for index-only scans, among other things. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise Postgres Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
