On Fri, Mar 13, 2020 at 10:48:27PM +0100, Laurenz Albe wrote: > On Fri, 2020-03-13 at 13:44 -0500, Justin Pryzby wrote: > > Possible it would be better to run VACUUM *without* freeze_min_age=0 ? (I > > get > > confused and have to spend 20min re-reading the vacuum GUC docs every time I > > deal with this stuff, so maybe I'm off). > > > > As I understand, the initial motivation of this patch was to avoid > > disruptive > > anti-wraparound vacuums on insert-only table. But if vacuum were triggered > > at > > all, it would freeze the oldest tuples, which is all that's needed; > > especially > > since fd31cd2651 "Don't vacuum all-frozen pages.", those pages would never > > need > > to be vacuumed again. Recently written tuples wouldn't be frozen, which is > > ok, > > they're handled next time. > > Freezing tuples too early is wasteful if the tuples get updated or deleted > soon after, but based on the assumption that an autovacuum triggered by insert > is dealing with an insert-mostly table, it is not that wasteful.
You're right that it's not *that* wasteful. If it's a table that gets 90% inserts/10% updates, then only 10% of its tuples will be frozen. In the worst case, it's the same tuples every time, and that's somewhat wasteful. In the best case, those tuples are clustered on a small number of pages. -- Justin
