Re: Autovacuum and OldestXmin

Added to TODO:

o Prevent autovacuum from running if an old transaction is still
running from the last vacuum

http://archives.postgresql.org/pgsql-hackers/2007-11/msg00899.php

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Christopher Browne wrote:
> The world rejoiced as alvherre(at)alvh(dot)no-ip(dot)org (Alvaro Herrera) wrote:
> > Simon Riggs wrote:
> >> I notice that slony records the oldestxmin that was running when it last
> >> ran a VACUUM on its tables. This allows slony to avoid running a VACUUM
> >> when it would be clearly pointless to do so.
> >>
> >> AFAICS autovacuum does not do this, or did I miss that?
> >
> > Hmm, I think it's just because nobody suggested it and I didn't came up
> > with the idea.
> >
> > Whether it's a useful thing to do is a different matter. Why store it
> > per table and not more widely? Perhaps per database would be just as
> > useful; and maybe it would allow us to skip running autovac workers
> > when there is no point in doing so.
>
> I think I need to take blame for that feature in Slony-I ;-).
>
> I imagine it might be useful to add it to autovac, too. I thought it
> was pretty neat that this could be successfully handled by comparison
> with a single value (e.g. - eldest xmin), and I expect that using a
> single quasi-global value should be good enough for autovac.
>
> If there is some elderly, long-running transaction that isn't a
> VACUUM, that will indeed inhibit VACUUM from doing any good, globally,
> across the cluster, until such time as that transaction ends.
>
> To, at that point, "inhibit" autovac from bothering to run VACUUM,
> would seem like a good move. There is still value to running ANALYZE
> on tables, so it doesn't warrant stopping autovac altogether, but this
> scenario suggests a case for suppressing futile vacuuming, at least...
> --
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Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
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