Re: join selectivity

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: "Mark Cave-Ayland" <m(dot)cave-ayland(at)webbased(dot)co(dot)uk>
Cc: strk(at)refractions(dot)net, pgsql-hackers(at)postgresql(dot)org, postgis-devel(at)postgis(dot)refractions(dot)net
Subject: Re: join selectivity
Date: 2004-12-16 18:56:29
Message-ID: 26930.1103223389@sss.pgh.pa.us
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"Mark Cave-Ayland" <m(dot)cave-ayland(at)webbased(dot)co(dot)uk> writes:
> ... But in the case of <column> <op>
> <unknown constant>, if we're estimating the number of rows to return then
> that becomes harder

I didn't say it was easy ;-). The existing selectivity functions can't
do better than a rough guess in such cases, and I don't expect you can
either.

regards, tom lane

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