Message-ID: <5193AF36.3040409@vmware.com>
Date: Wed, 15 May 2013 18:52:22 +0300
From: Heikki Linnakangas <hlinnakangas@vmware.com>
Organization: VMware
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To: sthomas@optionshouse.com
CC: "pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
Subject: Re: Thinking About Correlated Columns (again)
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On 15.05.2013 18:31, Shaun Thomas wrote:
> I've seen conversations on this since at least 2005. There were even
> proposed patches every once in a while, but never any consensus. Anyone
> care to comment?

Well, as you said, there has never been any consensus.

There are basically two pieces to the puzzle:

1. What metric do you use to represent correlation between columns?

2. How do use collect that statistic?

Based on the prior discussions, collecting the stats seems to be tricky. 
It's not clear for which combinations of columns it should be collected 
(all possible combinations? That explodes quickly...), or how it can be 
collected without scanning the whole table.

I think it would be pretty straightforward to use such a statistic, once 
we have it. So perhaps we should get started by allowing the DBA to set 
a correlation metric manually, and use that in the planner.

- Heikki


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