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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 16:43:01 +0000
Message-ID: <1229100181.8673.32.camel@ebony.2ndQuadrant> (raw)
In-Reply-To: <6558.1229098576@sss.pgh.pa.us>
References: <603c8f070812061019r5aca3aajeaa5493b11c5feff@mail.gmail.com>
	<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>
	<4136ffa0812111823u645b6ec9wdca60b3da4b00499@mail.gmail.com>
	<1229072675.13078.196.camel@hp_dx2400_1>
	<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:

> If you want ANALYZE to be cheap then you simply don't get to have 
> a trustworthy value of ndistinct.

Understood, but a cheap ANALYZE isn't always a higher priority than all
other considerations.

Solutions can also include

* allowing user to note that we would actually like to scan the whole
table (stats_target = -2?)

* manual mechanism for setting ndistinct that doesn't keep getting
overwritten by subsequent ANALYZEs

* have the executor do non-transactional update of the value of
ndistinct if it ever builds a hash table that is larger than expected
(simple learning optimizer action)

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




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