Re: unusual performance for vac following 8.2 upgrade

Kim <kim(at)myemma(dot)com> writes:
> We were running on 8.1.1 previous to upgrading to 8.2, and yes, we
> definitely have a heafty pg_class. The inheritance model is heavily used
> in our schema (the results of the group by you wanted to see are down
> below). However, no significant problems were seen with vacs while we
> were on 8.1.

Odd, because the 8.1 code looks about the same, and it is perfectly
obvious in hindsight that its runtime is about O(N^2) in the number of
relations :-(. At least that'd be the case if the stats collector
output were fully populated. Did you have either stats_block_level or
stats_row_level turned on in 8.1? If not, maybe the reason for the
change is that in 8.2, that table *will* be pretty fully populated,
because now it's got a last-vacuum-time entry that gets made even if the
stats are otherwise turned off. Perhaps making that non-disablable
wasn't such a hot idea :-(.

What I think we need to do about this is

(1) fix pgstat_vacuum_tabstats to have non-O(N^2) behavior; I'm thinking
of using a hash table for the OIDs instead of a linear list. Should be
a pretty small change; I'll work on it today.

(2) Reconsider whether last-vacuum-time should be sent to the collector
unconditionally.

Comments from hackers?

regards, tom lane