From: | "Matthew T(dot) O'Connor" <matthew(at)zeut(dot)net> |
---|---|
To: | Alvaro Herrera <alvherre(at)commandprompt(dot)com> |
Cc: | "Jim C(dot) Nasby" <decibel(at)decibel(dot)org>, Michael Paesold <mpaesold(at)gmx(dot)at>, "Joshua D(dot) Drake" <jd(at)commandprompt(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: Still recommending daily vacuum... |
Date: | 2007-07-06 20:11:39 |
Message-ID: | 468EA1FB.70500@zeut.net |
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Lists: | pgsql-hackers pgsql-patches |
Alvaro Herrera wrote:
> Matthew T. O'Connor wrote:
> Well, if a table has 10 rows, and we keep the current threshold of 1000
> rows, then this table must have 1002 dead tuples (99% dead tuples, 1002
> dead + 10 live) before being vacuumed. This seems wasteful because
> there are 500 dead tuples on it and only 10 live tuples. So each scan
> must wade through all the dead tuples.
>
> Another small table with 100 tuples will be vacuumed on every iteration
> as well, even if there are just two dead tuples. So you are right --
> maybe dropping it all the way to 0 is too much. But a small value of 10
> is reasonable? That will make the 10 tuple table be vacuumed when there
> are 10 dead tuples (50% of dead tuples), and the 100 tuple table when
> there are 11 (11% of dead tuples). It decreases quickly to the scale
> factor (2%, or do we want to decrease it to 1%?)
I think it's probably fine. I think, that the optimal number for the
base_threhold is probably dependant on the width of the row, for a very
narrow row where you might have many on the same page, 20 or 50 might be
right, but for a very wide table a smaller number might be optimal,
however I think it probably doesn't matter much anyway.
Reducing the default to 10 seems fine, and perhaps even removing it as a
tuning knob. I think there are too many autovacuum knobs and it
confuses people. Is it too late to possibly remove this GUC altogether?
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