From: | "Zeugswetter Andreas SB SD" <ZeugswetterA(at)spardat(dot)at> |
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To: | "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | "Daniel Kalchev" <daniel(at)digsys(dot)bg>, <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: again on index usage |
Date: | 2002-01-10 16:04:29 |
Message-ID: | 46C15C39FEB2C44BA555E356FBCD6FA41EB49F@m0114.s-mxs.net |
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Lists: | pgsql-hackers |
> > What is actually estimated wrong here seems to be the estimated
> > effective cache size, and thus the cache ratio of page fetches.
>
> Good point, but I think the estimates are only marginally sensitive
> to estimated cache size (if they're not, we have a problem, considering
> how poorly we can estimate the kernel's disk buffer size). It would
> be interesting for Daniel to try a few different settings of
> effective_cache_size and see how much the EXPLAIN costs change.
Well, the number I told him (29370) should clearly prefer the index.
The estimate is very sensitive to this value :-(
With 29370 (=229 Mb) the index cost is 1,364 instead of 3,887 with the
default of 1000 pages ==> index scan.
229 Mb file cache with 512Mb Ram is a reasonable value, I have
a lot more here:
Memory Real Virtual
free 0 MB 218 MB
procs 95 MB 293 MB
files 159 MB
total 256 MB 512 MB
Andreas
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