Re: Autovacuum and OldestXmin

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.

> Why isn't VACUUM optimised the same way HOT is?
> Why doesn't VACUUM continue onto the next block when !PageIsPrunable().
> Nothing is documented though it seems "obvious" that it should.
>
> Perhaps an integration oversight?

Yeah.

> [Also there is a comment saying "this is a bug" in autovacuum.c
> Are we thinking to go production with that phrase in the code?]

Yeah, well, it's only a comment ;-) The problem is that a worker can
decide that a table needs to be vacuumed, if another worker has finished
vacuuming it in the last 500 ms. I proposed a mechanism to close the
hole but it was too much of a hassle.

Maybe we could remove the comment for the final release? :-)

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Alvaro Herrera Valdivia, Chile ICBM: S 39º 49' 18.1", W 73º 13' 56.4"
Management by consensus: I have decided; you concede.
(Leonard Liu)