Alvaro Herrera <alvherre@commandprompt.com> writes:
> The problem is that vacuum doesn't know that a certain part of the table
> is already frozen. It needs to scan it completely anyways. If we had a
> "frozen" map, we could mark pages that are completely frozen and thus do
> not need any vacuuming; but we don't (I don't recall the reasons for
> this. Maybe it's just that no one has gotten around to it, or maybe
> there's something else).
Offhand I think the reason is that you'd have to trust the frozen bit
to be 100% correct (or at least never set to 1 in error). Currently,
both the FSM and visibility forks are just hints, and we won't suffer
data corruption if they're wrong; so we don't get too tense about WAL
logging or fsync'ing updates. I believe Heikki is looking into what
it'd take to make the visibility map 100% reliable, in connection with
the desire for index-only scans. If we get that and the overhead isn't
too terrible maybe we could build a frozen-status map the same way.
regards, tom lane