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Re: More thoughts about planner's cost estimates

From: Rod Taylor <pg(at)rbt(dot)ca>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: More thoughts about planner's cost estimates
Date: 2006-06-02 23:05:15
Message-ID: 1149289515.6071.211.camel@home (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
> One objection to this is that after moving "off the gold standard" of
> 1.0 = one page fetch, there is no longer any clear meaning to the
> cost estimate units; you're faced with the fact that they're just an
> arbitrary scale.  I'm not sure that's such a bad thing, though.  For
> instance, some people might want to try to tune their settings so that
> the estimates are actually comparable to milliseconds of real time.

Any chance that the correspondence to time could be made a part of the
design on purpose and generally advise people to follow that rule? If we
could tell people to run *benchmark* and use those numbers directly as a
first approximation tuning, it could help quite a bit for people new to
PostgreSQL experiencing poor performance.

effective_cache_size then becomes essentially the last hand-set variable
for medium sized installations.

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