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Re: More thoughts about planner's cost estimates

From: Kenneth Marshall <ktm(at)it(dot)is(dot)rice(dot)edu>
To: Josh Berkus <josh(at)agliodbs(dot)com>
Cc: Greg Stark <gsstark(at)mit(dot)edu>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>,pgsql-hackers(at)postgresql(dot)org
Subject: Re: More thoughts about planner's cost estimates
Date: 2006-06-01 21:33:09
Message-ID: 20060601213309.GL15172@it.is.rice.edu (view raw or flat)
Thread:
Lists: pgsql-hackers
Josh, Greg, and Tom,

I do not know how sensitive the plans will be to the correlation,
but one thought might be to map the histogram X histogram correlation
to a square grid of values. Then you can map them to an integer which
would give you 8 x 8 with binary values, a 5 x 5 with 4 values per
point, or a 4 x 4 with 8 values per point. If close is good enough,
that would be a compact way to store some histogram cross correlation
information.

Ken

On Thu, Jun 01, 2006 at 01:50:26PM -0700, Josh Berkus wrote:
> Greg, Tom,
> 
> ...
> > 2) It isn't even clear what data you're exactly looking for. Certainly
> >    "correlation" is just shorthand here and isn't what you're actually
> > looking for. 
> 
> Actually, I'd think that a correlation number estimate (0 = complete 
> uncorrelated, 1 = completely correlated) would be sufficient to improve 
> row count estimation significantly, without incurring the vast overhead of 
> histogramXhistorgram manipulation.
> ...

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