On 16/05/13 04:23, Craig James wrote:
On Wed, May 15, 2013 at 8:31 AM, Shaun Thomas
<stho...@optionshouse.com <mailto:stho...@optionshouse.com>> 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
