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From: Richard Huxton <dev@archonet.com>
To: PFC <lists@peufeu.com>
Cc: mark@mark.mielke.cc, Mark Lewis <mark.lewis@mir3.com>
Cc: James Mansion <james@mansionfamily.plus.com>
Cc: Magnus Hagander <magnus@hagander.net>
Cc: Alexander Staubo <alex@purefiction.net>
Cc: Andy <frum@ar-sd.net>
Cc: pgsql-performance@postgresql.org
Subject: Re: LIKE search and performance
Date: Fri, 25 May 2007 18:32:39 +0100
Message-ID: <46571DB7.2040607@archonet.com> (raw)
In-Reply-To: <op.tsv8aefscigqcu@apollo13>
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>
	<20070525141630.GA12162@mark.mielke.cc>
	<4657023A.20209@archonet.com>
	<op.tsv6fevgcigqcu@apollo13>
	<46571825.30707@archonet.com>
	<op.tsv8aefscigqcu@apollo13>

PFC wrote:
>> None of which address the question of what plan PG should produce for: 
>> SELECT * FROM bigtable WHERE foo LIKE 's%'
> 
>     Ah, this one already uses the btree since the '%' is at the end.
>     My point is that a search like this will yield too many results to 
> be useful to the user anyway, so optimizing its performance is a kind of 
> red herring.

At the *application level* yes.
At the *query planner* level no.

At the query planner level I just want it to come up with the best plan 
it can. The original argument was that PG's estimate of the number of 
matching rows was too optimistic (or pessimistic) in the case where we 
are doing a contains substring-search.

-- 
   Richard Huxton
   Archonet Ltd



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