Re: Multi-column distinctness.

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Multi-column distinctness.
Date: 2015-09-05 19:46:42
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On Fri, Aug 28, 2015 at 05:33:34PM +0900, Kyotaro HORIGUCHI wrote:
> Hello, this patch enables planner to be couscious of inter-column
> correlation.
> Sometimes two or more columns in a table has some correlation
> which brings underestimate, which leads to wrong join method and
> ends with slow execution.
> Tomas Vondra is now working on heavily-equipped multivariate
> statistics for OLAP usage. In contrast, this is a lightly
> implemented solution which calculates only the ratio between a
> rows estimated by current method and a actual row number. I think
> this doesn't conflict with his work except the grammar part.

I am very glad multi-variate statistics are being addressed. I think
this is our most common cause of optimizer failures.

Bruce Momjian <bruce(at)momjian(dot)us>

+ Everyone has their own god. +

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