Date: Fri, 25 May 2007 10:16:30 -0400
From: mark@mark.mielke.cc
To: Richard Huxton <dev@archonet.com>
Cc: Mark Lewis <mark.lewis@mir3.com>,
	James Mansion <james@mansionfamily.plus.com>,
	Magnus Hagander <magnus@hagander.net>,
	Alexander Staubo <alex@purefiction.net>, Andy <frum@ar-sd.net>,
	pgsql-performance@postgresql.org
Subject: Re: LIKE search and performance
Message-ID: <20070525141630.GA12162@mark.mielke.cc>
References: <009501c79d4c$445428f0$0b00a8c0@mpsro.dom>
	<88daf38c0705230905u1a981102k691b47bd36011a80@mail.gmail.com>
	<4655DE75.4080506@mansionfamily.plus.com>
	<4655E64B.5050503@hagander.net>
	<4655FB8A.1040707@mansionfamily.plus.com>
	<1180040560.31471.262.camel@archimedes>
	<20070524215450.GA3858@mark.mielke.cc>
	<46569AA5.10205@archonet.com>
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On Fri, May 25, 2007 at 09:13:25AM +0100, Richard Huxton wrote:
> mark@mark.mielke.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. :-)

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.

Cheers,
mark

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