Re: LIKE search and performance

Gregory Stark wrote:
> "Richard Huxton" <dev@archonet.com> writes:
>
>> Now you and I can look at a substring and probably make a good guess how common
>> it is (assuming we know the targets are British surnames or Japanese towns). PG
>> needs one number - or rather, it picks one number for each length of
>> search-string (afaik).
>
> I don't think that's true. Postgres calculates the lower and upper bound
> implied by the search pattern and then uses the histogram to estimate how
> selective that range is. It's sometimes surprisingly good but obviously it's
> not perfect.

Sorry - I'm obviously picking my words badly today.

I meant for the "contains" substring match. It gives different (goes
away and checks...yes) predictions based on string length. So it guesses
that LIKE '%aaa%' will match more than LIKE '%aaaa%'. Of course, if we
were matching surnames you and I could say that this is very unlikely,
but without some big statistics table I guess there's not much more PG
can do.

For a trailing wildcard LIKE 'aaa%' it can and does as you say convert
this into something along the lines of (>= 'aaa' AND < 'aab'). Although
IIRC that depends if your locale allows such (not sure, I don't really
use non-C/non-English locales enough).

--
   Richard Huxton
   Archonet Ltd