> Bruce Momjian <pgman@candle.pha.pa.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.
OK, I think I have the proper value for FIXED_CHAR_SEL. It is currently
0.04 or 1/26, meaning the letters are random, though this is not usually
the case.
If we assume our optimizer buckets have given us a reasonable value for
the first character, suppose it is an 'F', there are only a few valid
characters after that, at least in English. There are vowels, and a few
consonants, and given that character, there are only a few characters
that can be valid after that. To my thinking, it is two characters that
represent the same distribution as one random character, leaving 0.20 as
the proper value for FIXED_CHAR_SEL because 0.20 * 0.20 is the same as
0.04.
Added to TODO:
* Change FIXED_CHAR_SEL to 0.20 from 0.04 to give better selectivity (Bruce)
If people think there is a better value for this, please chime in.
--
Bruce Momjian | http://candle.pha.pa.us
pgman@candle.pha.pa.us | (610) 853-3000
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