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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Dean Rasheed <dean.a.rasheed@gmail.com>
Cc: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: Alexander Korotkov <a.korotkov@postgrespro.ru>
Cc: Mark Dilger <hornschnorter@gmail.com>
Cc: PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: improving GROUP BY estimation
Date: Thu, 31 Mar 2016 16:59:41 -0400
Message-ID: <7900.1459457981@sss.pgh.pa.us> (raw)
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Dean Rasheed <dean.a.rasheed@gmail.com> writes:
> Yeah, that makes sense. In fact, if we only apply the adjustment when
> reldistinct > 0 and rel->rows < rel->tuples, and rewrite the first
> argument to pow() as (rel->tuples - rel->rows) / rel->tuples, then it
> is guaranteed to be non-negative. If rel->rows >= rel->tuples (not
> sure if it can be greater), then we just want the original
> reldistinct.

Works for me.

			regards, tom lane


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