| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com> |
| Cc: | Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, Alexander Korotkov <a(dot)korotkov(at)postgrespro(dot)ru>, Mark Dilger <hornschnorter(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: improving GROUP BY estimation |
| Date: | 2016-03-31 20:59:41 |
| Message-ID: | 7900.1459457981@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)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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