The fact that an index exists adds a choice -- so by no means is the
index ignored.
But just because a Freeway exists across town doesn't make it faster
than the sideroads. It depends on the day of week, time of day, and
uncontrollable anomolies (accidents).
--
Rod Taylor
Your eyes are weary from staring at the CRT. You feel sleepy. Notice
how restful it is to watch the cursor blink. Close your eyes. The
opinions stated above are yours. You cannot imagine why you ever felt
otherwise.
----- Original Message -----
From: "mlw" <markw@mohawksoft.com>
To: "Thomas Lockhart" <thomas@fourpalms.org>
Cc: "Tom Lane" <tgl@sss.pgh.pa.us>; "Bruce Momjian"
<pgman@candle.pha.pa.us>; "Louis-David Mitterrand"
<vindex@apartia.org>; <pgsql-hackers@postgresql.org>
Sent: Wednesday, April 17, 2002 10:31 AM
Subject: Re: [HACKERS] Index Scans become Seq Scans after VACUUM
ANALYSE
> Thomas Lockhart wrote:
> > Systems which have optimizing planners can *never* be guaranteed
to
> > generate the actual lowest-cost query plan. Any impression that
Oracle,
> > for example, actually does do that may come from a lack of
visibility
> > into the process, and a lack of forum for discussing these edge
cases.
>
> And here in lies the crux of the problem. It isn't a purely
logical/numerical
> formula. It is a probability estimate, nothing more. Currently, the
statistics
> are used to calculate a probable best query, not a guaranteed best
query. The
> presence of an index should be factored into the probability of a
best query,
> should it not?
>
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