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From: Simon Riggs <simon@2ndQuadrant.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Gregory Stark <stark@enterprisedb.com>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: Kevin Grittner <Kevin.Grittner@wicourts.gov>
Cc: jd@commandprompt.com <jd@commandprompt.com>
Cc: Josh Berkus <josh@agliodbs.com>
Cc: Greg Smith <gsmith@gregsmith.com>
Cc: pgsql-hackers@postgresql.org
Subject: Re: benchmarking the query planner
Date: Fri, 12 Dec 2008 16:58:14 +0000
Message-ID: <1229101094.8673.44.camel@ebony.2ndQuadrant> (raw)
In-Reply-To: <6558.1229098576@sss.pgh.pa.us>
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	<603c8f070812092025k6325e0fcud001b205a507a634@mail.gmail.com>
	<4313.1228931932@sss.pgh.pa.us>
	<603c8f070812101828p1bd7df49yb8c916adedb49fe1@mail.gmail.com>
	<9565.1229018968@sss.pgh.pa.us>
	<1229033073.13078.179.camel@hp_dx2400_1>
	<87ej0etkfh.fsf@oxford.xeocode.com>
	<1229039044.13078.193.camel@hp_dx2400_1>
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	<603c8f070812120344m67ef2c1fs41806cfb4ff9e396@mail.gmail.com>
	<1229093975.13078.275.camel@ebony.2ndQuadrant>
	<87tz99s7bu.fsf@oxford.xeocode.com>
	<6558.1229098576@sss.pgh.pa.us>


On Fri, 2008-12-12 at 11:16 -0500, Tom Lane wrote:

> Perhaps a better plan is to try to de-emphasize use of ndistinct,
> though I concede I have no idea how to do that.

We don't actually care about the accuracy of the ndistinct much, just
the accuracy of our answer to the question "given work_mem = X, is it
better to use a hash plan".

So we just need to scan the table until we can answer that question
accurately enough. i.e. a variable sized sample.

Perhaps we could store a probability distribution for various values of
work_mem, rather than a single ndistinct value.

Anyway, definitely handwaving now to stimulate ideas.

-- 
 Simon Riggs           www.2ndQuadrant.com
 PostgreSQL Training, Services and Support




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