From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andrea Arcangeli <andrea@cpushare.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: NOT LIKE much faster than LIKE?
Date: Mon, 09 Jan 2006 21:54:44 -0500
Message-ID: <24372.1136861684@sss.pgh.pa.us> (raw)
In-Reply-To: <20060110022303.GA20168@opteron.random>
References: <20060110014447.GB18474@opteron.random>
<24021.1136858688@sss.pgh.pa.us>
<20060110022303.GA20168@opteron.random>
Andrea Arcangeli <andrea@cpushare.com> writes:
> If you don't know the data, I think it's a bug that LIKE is assumed to
> have a selectivity above 50%.
Extrapolating from the observation that the heuristics don't work well
on your data to the conclusion that they don't work for anybody is not
good logic. Replacing that code with a flat 50% is not going to happen
(or if it does, I'll be sure to send the mob of unhappy users waving
torches and pitchforks to your door not mine ;-)).
I did just think of something we could improve though. The pattern
selectivity code doesn't make any use of the statistics about "most
common values". For a constant pattern, we could actually apply the
pattern test with each common value and derive answers that are exact
for the portion of the population represented by the most-common-values
list. If the MCV list covers a large fraction of the population then
this would be a big leg up in accuracy. Dunno if that applies to your
particular case or not, but it seems worth doing ...
regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, andrea@cpushare.com
Subject: Re: NOT LIKE much faster than LIKE?
In-Reply-To: <24372.1136861684@sss.pgh.pa.us>
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