From: | Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> |
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To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | drheart(at)wanadoo(dot)es, Lista PostgreSql <pgsql-general(at)postgresql(dot)org>, pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: Problem (bug?) with like |
Date: | 2001-12-04 19:00:32 |
Message-ID: | 200112041900.fB4J0W421336@candle.pha.pa.us |
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Lists: | pgsql-general pgsql-hackers |
> I am thinking that the rules for selectivity of LIKE patterns probably
> need to be modified. Presently the code assumes that a long constant
> string has probability of occurrence proportional to the product of the
> probabilities of the individual letters. That might be true in a random
> world, but people don't search for random strings. I think we need to
> back off the selectivity estimate by some large factor to account for
> the fact that the pattern being searched for is probably not random.
> Anyone have ideas how to do that?
But what about '%A%' vs. '%AC%'. Seems the second is reasonably
different from the first the our optimizer may be fine with that. Is it
only when the strings get longer that we lose specificity?
--
Bruce Momjian | http://candle.pha.pa.us
pgman(at)candle(dot)pha(dot)pa(dot)us | (610) 853-3000
+ If your life is a hard drive, | 830 Blythe Avenue
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