Re: Thinking About Correlated Columns (again)

От: Gavin Flower
Тема: Re: Thinking About Correlated Columns (again)
Дата: ,
Msg-id: 5193ECC6.4080901@archidevsys.co.nz
(см: обсуждение, исходный текст)
Ответ на: Re: Thinking About Correlated Columns (again)  (Craig James)
Ответы: Re: Thinking About Correlated Columns (again)  (Craig James)
Список: pgsql-performance

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Thinking About Correlated Columns (again)  (Shaun Thomas, )
 Re: Thinking About Correlated Columns (again)  (Heikki Linnakangas, )
  Re: Thinking About Correlated Columns (again)  (Shaun Thomas, )
  Re: Thinking About Correlated Columns (again)  (Nikolas Everett, )
   Re: Thinking About Correlated Columns (again)  (eggyknap, )
  Re: Thinking About Correlated Columns (again)  (Gavin Flower, )
 Re: Thinking About Correlated Columns (again)  (Craig James, )
  Re: Thinking About Correlated Columns (again)  (Andrew Dunstan, )
  Re: Thinking About Correlated Columns (again)  (Gavin Flower, )
   Re: Thinking About Correlated Columns (again)  (Craig James, )
 Re: Thinking About Correlated Columns (again)  (Thomas Kellerer, )
  Re: Thinking About Correlated Columns (again)  (Shaun Thomas, )

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