Re: Thinking About Correlated Columns (again)

On Wed, May 15, 2013 at 1:15 PM, Gavin Flower <GavinFlower(at)archidevsys(dot)co(dot)nz
> wrote:

> On 16/05/13 04:23, Craig James wrote:
>
> On Wed, May 15, 2013 at 8:31 AM, Shaun Thomas <sthomas(at)optionshouse(dot)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
>
>