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Re: More thoughts about planner's cost estimates

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Greg Stark <gsstark(at)mit(dot)edu>
Cc: josh(at)agliodbs(dot)com, David Fetter <david(at)fetter(dot)org>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: More thoughts about planner's cost estimates
Date: 2006-06-02 20:43:36
Message-ID: 10464.1149281016@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Greg Stark <gsstark(at)mit(dot)edu> writes:
> And a 5% sample is a pretty big. In fact my tests earlier showed the i/o from
> 5% block sampling took just as long as reading all the blocks. Even if we
> figure out what's causing that (IMHO surprising) result and improve matters I
> would only expect it to be 3-4x faster than a full scan.

One way to reduce the I/O pain from extensive sampling would be to turn
VACUUM ANALYZE into a genuine combined operation instead of a mere
notational shorthand for two separate scans.

I'd still be worried about the CPU pain though.  ANALYZE can afford to
expend a pretty fair number of cycles per sampled tuple, but with a
whole-table sample that's going to add up.

			regards, tom lane

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