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Date: Fri, 15 Dec 2006 13:30:52 +0100
From: Martijn van Oosterhout <kleptog@svana.org>
To: Simon Riggs <simon@2ndquadrant.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Kelly Burkhart <kelly.burkhart@gmail.com>,
	Evgeny Gridasov <eugrid@fpm.kubsu.ru>,
	pgsql-performance@postgresql.org, pgsql-hackers@postgresql.org
Subject: Re: [HACKERS] EXPLAIN ANALYZE on 8.2
Message-ID: <20061215123052.GJ958@svana.org>
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On Fri, Dec 15, 2006 at 12:20:46PM +0000, Simon Riggs wrote:
> > I
> > wrote a patch that tried statistical sampling, but the figures were too
> > far off for people's liking.
>=20
> Well, I like your ideas, so if you have any more...
>=20
> Maybe sampling every 10 rows will bring things down to an acceptable
> level (after the first N). You tried less than 10 didn't you?

Yeah, it reduced the number of calls as the count got larger. It broke
somewhere, though I don't quite remember why.

> Maybe we can count how many real I/Os were required to perform each
> particular row, so we can adjust the time per row based upon I/Os. ISTM
> that sampling at too low a rate means we can't spot the effects of cache
> and I/O which can often be low frequency but high impact.

One idea is to sample at fixed interval. Where the I/O cost is high,
there'll be a lot of sampling points. The issue being that the final
result are not totally accurate anymore.

> > I think the best option is setitimer(), but it's not POSIX so
> > platform support is going to be patchy.
>=20
> Don't understand that. I thought that was to do with alarms and signals.

On my system the manpage say setitimer() conforms to: SVr4, 4.4BSD. I'm
unsure how many of the supported platforms fall under that
catagorisation.

Have a nice day,
--=20
Martijn van Oosterhout   <kleptog@svana.org>   http://svana.org/kleptog/
> From each according to his ability. To each according to his ability to l=
itigate.

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