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Re: I/O on select count(*)

From: Jan de Visser <jdevisser(at)digitalfairway(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: "Greg Smith" <gsmith(at)gregsmith(dot)com>, "Alvaro Herrera" <alvherre(at)commandprompt(dot)com>, pgsql-performance(at)postgresql(dot)org
Subject: Re: I/O on select count(*)
Date: 2008-05-15 13:15:40
Message-ID: 200805150915.40793.jdevisser@digitalfairway.com (view raw or flat)
Thread:
Lists: pgsql-performance
On Thursday 15 May 2008 03:02:19 Tom Lane wrote:
> "Jan de Visser" <jdevisser(at)digitalfairway(dot)com> writes:
> > Obviously, this issue is tied to the slow count(*) one, as I found out
> > the hard way. Consider the following scenario:
> > * Insert row
> > * Update that row a couple of times
> > * Rinse and repeat many times
> >
> > Now somewhere during that cycle, do a select count(*) just to see
> > where you are. You will be appalled by how slow that is, due to not
> > only the usual 'slow count(*)' reasons. This whole hint bit business
> > makes it even worse, as demonstrated by the fact that running a vacuum
> > before the count(*) makes the latter noticably faster.
>
> Uh, well, you can't blame that entirely on hint-bit updates.  The vacuum
> has simply *removed* two-thirds of the rows in the system, resulting in
> a large drop in the number of rows that the select even has to look at.
>
> It's certainly true that hint-bit updates cost something, but
> quantifying how much isn't easy.  The off-the-cuff answer is to do the
> select count(*) twice and see how much cheaper the second one is.  But
> there are two big holes in that answer: the first is the possible cache
> effects from having already read in the pages, and the second is that
> the follow-up scan gets to avoid the visits to pg_clog that the first
> scan had to make (which after all is the point of the hint bits).
>
> I don't know any easy way to disambiguate the three effects that are at
> work here.  But blaming it all on the costs of writing out hint-bit
> updates is wrong.
>
> 			regards, tom lane

True. But it still contributes to the fact that queries sometimes behave in a 
non-deterministic way, which IMHO is the major annoyance when starting to 
work with pgsql. And contrary to other causes (vacuum, checkpoints) this is 
woefully underdocumented.

jan

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