Re: unusual performance for vac following 8.2 upgrade

For 8.1, we did have stats_block_level and stats_row_level on, so thats
not it either :-/ However, I did go on to an alternate database of ours
on the same machine, using the same install, same postmaster - that
holds primarily static relations, and not many of those (16 relations
total). The response of running a vac for a 1.3k static table was quick
(6 seconds - but it still did not set the last_vacuum field). Not sure
why we weren't seeing more probs with this on 8.1 for the full db, but
from the looks of things I think your theory on the primary problem with
our vacs is solid. I'm hoping we can fire up our old 8.1 dataset and run
some tests on there to confirm/reject the idea that it was doing any
better, but that will require quieter times on the machine than we've
got right now :)

We are going to try and upgrade to 8.2.1 as soon as we can, and if we
continue to see some of the other problems I mentioned as side-notes,
we'll build some information on those and pass it along...

Thanks so much!

Kim

Tom Lane wrote:

>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
>
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>TIP 2: Don't 'kill -9' the postmaster
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