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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Simon Riggs <simon@2ndquadrant.com>
Cc: theo@flame.co.za
Cc: pgsql-performance@postgresql.org
Subject: Re: Optimizer : query rewrite and execution plan ?
Date: Mon, 11 Feb 2008 11:44:42 -0500
Message-ID: <2685.1202748282@sss.pgh.pa.us> (raw)
In-Reply-To: <1202723087.4247.150.camel@ebony.site>
References: <60F4687513E90748AE8933DEA5D054E79ACBDE@SLAM0018.match-supermarket.com>
	<1202298826.4252.972.camel@ebony.site>
	<1202299962.4274.22.camel@localhost6.localdomain6>
	<9924.1202313626@sss.pgh.pa.us>
	<1202723087.4247.150.camel@ebony.site>

Simon Riggs <simon@2ndquadrant.com> writes:
> Say if cost of best plan >= N then recheck query for strangeness. If
> anything found, re-plan query.

Whatever makes you think that would be useful?

The usual result of undetected duplicate WHERE clauses is an
*underestimate* of runtime, not an overestimate (because it thinks
too few tuples will be selected).

			regards, tom lane



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