Re: LIKE search and performance

mark(at)mark(dot)mielke(dot)cc wrote:
> On Fri, May 25, 2007 at 09:13:25AM +0100, Richard Huxton wrote:
>> mark(at)mark(dot)mielke(dot)cc wrote:
>>>> And since it's basically impossible to know the selectivity of this kind
>>>> of where condition, I doubt the planner would ever realistically want to
>>>> choose that plan anyway because of its poor worst-case behavior.
>>> What is a real life example where an intelligent and researched
>>> database application would issue a like or ilike query as their
>>> primary condition in a situation where they expected very high
>>> selectivity?
>>> Avoiding a poor worst-case behaviour for a worst-case behaviour that
>>> won't happen doesn't seem practical.
>> But if you are also filtering on e.g. date, and that has an index with
>> good selectivity, you're never going to use the text index anyway are
>> you? If you've only got a dozen rows to check against, might as well
>> just read them in.
>> The only time it's worth considering the behaviour at all is *if* the
>> worst-case is possible.
>
> I notice you did not provide a real life example as requested. :-)

OK - any application that allows user-built queries: <choose column:
foo> <choose filter: contains> <choose target: "bar">

Want another? Any application that has a "search by name" box - users
can (and do) put one letter in and hit enter.

Unfortunately you don't always have control over the selectivity of
queries issued.

> This seems like an ivory tower restriction. Not allowing best performance
> in a common situation vs not allowing worst performance in a not-so-common
> situation.

What best performance plan are you thinking of? I'm assuming we're
talking about trailing-wildcard matches here, rather than "contains"
style matches.

--
Richard Huxton
Archonet Ltd