| From: | Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> |
|---|---|
| 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 19:55:30 |
| Message-ID: | 200112041955.fB4JtUA26642@candle.pha.pa.us |
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| Lists: | pgsql-general pgsql-hackers |
> Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> writes:
> > 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?
>
> Yeah, I don't think that the estimates are bad for one or two
> characters. But the estimate gets real small real fast as you
> increase the number of match characters in the LIKE pattern.
> We need to slow that down some.
Yea, maybe a log base 2 decrease:
1 char 1x
2 char 2x
4 char 3x
8 char 4x
16 char 5x
--
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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