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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Josh Berkus <josh@agliodbs.com>
Cc: pgsql-performance@postgresql.org, Shaun Thomas <sthomas@peak6.com>
Subject: Re: Postgres refusing to use >1 core
Date: Thu, 12 May 2011 10:51:42 -0400
Message-ID: <15686.1305211902@sss.pgh.pa.us> (raw)
In-Reply-To: <4DCB346E.8070608@agliodbs.com>
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	<4DC81A3F020000250003D433@gw.wicourts.gov>
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	<4DC81D56020000250003D438@gw.wicourts.gov>
	<BANLkTi=FH4rX-7OtS9ozjuyXQTNTiRqCmg@mail.gmail.com>
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	<4DC8B893.3060908@postnewspapers.com.au>
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	<BANLkTi=PxROPQFDt1a6N68e_+5_AojagmA@mail.gmail.com>
	<4DCAB3A1.2090507@peak6.com>
	<201105111953.060147@ms14.lnh.mail.rcn.net>
	<4DCB0802.9060505@peak6.com>
	<4DCB346E.8070608@agliodbs.com>

Josh Berkus <josh@agliodbs.com> writes:
> On 5/11/11 3:04 PM, Shaun Thomas wrote:
>> The original query, with our very large tables, ran for over *two hours*
>> thanks to a nested loop iterating over the subquery. My replacement ran
>> in roughly 30 seconds. If we were using a newer version of PG, we could
>> have used a CTE. But do you get what I mean? Temp tables are a fairly
>> common technique, but how would a coder know about CTEs? They're pretty
>> new, even to *us*.

> For that matter, it would be even better if PostgreSQL realized that a
> materialize of the subquery was a better execution plan, and just did it
> for you.

It does.  I was a bit surprised that Shaun apparently got a plan that
didn't include a materialize step, because when I test a similar query
here, I get:
1. a hash join, until I turn off enable_hashjoin; then
2. a merge join, until I turn off enable_mergejoin; then
3. a nestloop with materialize on the subquery scan.
In 9.0 and up I can get a nestloop without materialize by also turning
off enable_material, but pre-9.0 there's no such option ...

			regards, tom lane



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