| 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 |
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| 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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