On Wed, May 15, 2013 at 1:15 PM, Gavin Flower <gavinflo...@archidevsys.co.nz > wrote:
> On 16/05/13 04:23, Craig James wrote: > > On Wed, May 15, 2013 at 8:31 AM, Shaun Thomas <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... > Right ... I was only thinking of combinations for two columns. Craig > > [...] > > Cheers, > Gavin > >
