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From: Pierre C <lists@peufeu.com>
To: Scott Marlowe <scott.marlowe@gmail.com>
To: Shaun Thomas <sthomas@peak6.com>
Cc: Craig Ringer <craig@postnewspapers.com.au>
Cc: Aren Cambre <aren@arencambre.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: Postgres refusing to use >1 core
Date: Wed, 11 May 2011 20:10:54 +0200
Message-ID: <op.vvbwsgdaeorkce@apollo13> (raw)
In-Reply-To: <4DCAB3A1.2090507@peak6.com>
References: <BANLkTimohiUa6rOYdxYTwqmvSUg2CRYMHg@mail.gmail.com>
	<4DC81A3F020000250003D433@gw.wicourts.gov>
	<BANLkTimNTLqQgQeyu=FPOn-5x6F4XnVBJA@mail.gmail.com>
	<4DC81D56020000250003D438@gw.wicourts.gov>
	<BANLkTi=FH4rX-7OtS9ozjuyXQTNTiRqCmg@mail.gmail.com>
	<BANLkTinwkj7syvKkYzU_O7DD14EMzk2Zmg@mail.gmail.com>
	<BANLkTi=BvGMvsBNq8DUt=_MPJXZjcC0TWg@mail.gmail.com>
	<4DC8B893.3060908@postnewspapers.com.au>
	<BANLkTimi+2nviLvWU4zKGtG688+Z84DF6w@mail.gmail.com>
	<4DC9E7C5.9080309@postnewspapers.com.au>
	<BANLkTi=PxROPQFDt1a6N68e_+5_AojagmA@mail.gmail.com>
	<4DCAB3A1.2090507@peak6.com>


> This is a problem I encounter constantly wherever I go. Programmer  
> selects millions of rows from giant table. Programmer loops through  
> results one by one doing some magic on them. Programmer submits queries  
> back to the database. Even in batches, that's going to take ages.

Reminds me of a recent question on stackoverflow :

http://stackoverflow.com/questions/5952020/how-to-optimize-painfully-slow-mysql-query-that-finds-cor...

And the answer :

http://stackoverflow.com/questions/5952020/how-to-optimize-painfully-slow-mysql-query-that-finds-cor...

OP was thinking "row-based", with subqueries in the role of "doing some  
magicm".
Using a set-based solution with cascading WITH CTEs (and using the  
previous CTE as a source in the next one for aggregation) => 100x speedup !



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