> "Problem: As of 9.3, there's a significant benefit to vacuum freezing > tables early so that index-only scan is enabled, since freezing also > updates the visibility map. However, with default settings, such > freezing only happens for data which is very old. This means that > index-only scan is less effective than it could be for tables which > have relatively infrequent updates and deletes." > > Why specifically VACUUM FREEZE rather than regular VACUUM? I thought > regular VACUUM updated the visibility map too? And why as of 9.3 > instead of 9.2?
As of 9.2, that was a typo. Allvisible only gets set if there was some reason for VACUUM to visit the page anyway, no? Which means that an insert-only or insert-mostly table doesn't get set allvisible until FREEZE. And insert-only tables are usually very large, and thus really *need* index-only scan. Hmmm. I should rewrite that item entirely. It has nothing to do with FREEZE, really. -- Josh Berkus PostgreSQL Experts Inc. http://pgexperts.com -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
