From: Tom Lane <tgl@sss.pgh.pa.us>
To: Zeugswetter Andreas SB SD <ZeugswetterA@spardat.at>
Cc: Daniel Kalchev <daniel@digsys.bg>
Cc: pgsql-hackers@postgresql.org
Subject: Re: again on index usage
Date: Thu, 10 Jan 2002 10:07:11 -0500
Message-ID: <20478.1010675231@sss.pgh.pa.us> (raw)
In-Reply-To: <46C15C39FEB2C44BA555E356FBCD6FA41EB499@m0114.s-mxs.net>
References: <46C15C39FEB2C44BA555E356FBCD6FA41EB499@m0114.s-mxs.net>
"Zeugswetter Andreas SB SD" <ZeugswetterA@spardat.at> writes:
> I cannot really see how 284 rows can have an estimated index cost of 100506 ?
The estimated number of indexscanned rows is more like 50k. The number
you are looking at includes the estimated selectivity of the
non-indexable WHERE clauses, too.
> 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.
regards, tom lane
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To: pgsql-hackers@postgresql.org
Cc: tgl@sss.pgh.pa.us, ZeugswetterA@spardat.at, daniel@digsys.bg
Subject: Re: again on index usage
In-Reply-To: <20478.1010675231@sss.pgh.pa.us>
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