Date: Thu, 24 May 2007 17:54:51 -0400
From: mark@mark.mielke.cc
To: Mark Lewis <mark.lewis@mir3.com>
Cc: 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: <20070524215450.GA3858@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>
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On Thu, May 24, 2007 at 02:02:40PM -0700, Mark Lewis wrote:
> PG could scan the index looking for matches first and only load the
> actual rows if it found a match, but that could only be a possible win
> if there were very few matches, because the difference in cost between a
> full index scan and a sequential scan would need to be greater than the
> cost of randomly fetching all of the matching data rows from the table
> to look up the visibility information.  

> So yes it would be possible, but the odds of it being faster than a
> sequential scan are small enough to make it not very useful.

> 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.

What real life examples, that could not be implemented a better way,
would behave poorly if like/ilike looked at the index first to filter?

I don't understand... :-)

Cheers,
mark

-- 
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