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On 16/05/13 04:23, Craig James wrote:
On Wed, May 15, 2013 at 8:31 AM, Shaun
Thomas <sthomas@xxxxxxxxxxxxxxxx>
wrote:
[Inefficient plans for correlated columns] has been a pain
point for quite a while. While we've had several discussions
in the area, it always seems to just kinda trail off and
eventually vanish every time it comes up.
[...]
It's a very hard problem. There's no way you can keep
statistics about all possible correlations since the number of
possibilities is O(N^2) with the number of columns.
Actually far worse: N!/(N - K)!K! summed over K=1...N, assuming the
order of columns in the correlation is unimportant (otherwise it is
N factorial) - based on my hazy recollection of the relevant
maths...
[...]
Cheers,
Gavin
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