Excerpts from Robert Haas's message of mié jun 02 14:16:33 -0400 2010: > We could, but I think we'd be better off just freezing at the time we > mark the page PD_ALL_VISIBLE and then using the visibility map for > both purposes. Keeping around the old xmin values after every tuple > on the page is visible to every running transaction is useful only for > forensics, and building a whole new freeze map just to retain that > information longer (and eventually force a massive anti-wraparound > vacuum) seems like overkill.
Reducing the xid wraparound horizon "a bit" is reasonable, but moving it all the way forward to OldestXmin is a bit much, methinks. Besides, there's another argument for not freezing tuples immediately: they may be updated shortly thereafter, causing extra churn for no gain. I'd prefer a setting that would tell the system to freeze all tuples that fall within a safety range whenever any tuple in the page is frozen -- weren't you working on a patch to do this? (was it Jeff Davis?) (BTW maybe instead of separate visibility and freeze maps we could have two bits in the visibility map?) -- Álvaro Herrera <alvhe...@commandprompt.com> The PostgreSQL Company - Command Prompt, Inc. PostgreSQL Replication, Consulting, Custom Development, 24x7 support -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
