From: Rod Taylor <pg@rbt.ca>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-hackers@postgresql.org
In-Reply-To: <11319.1149287800@sss.pgh.pa.us>
References: <14821.1149104747@sss.pgh.pa.us> <11319.1149287800@sss.pgh.pa.us>
Date: Fri, 02 Jun 2006 19:05:15 -0400
Message-Id: <1149289515.6071.211.camel@home>
Mime-Version: 1.0
Subject: Re: More thoughts about planner's cost estimates
Content-Type: text/plain
Content-Transfer-Encoding: 7bit

> 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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