Re: Multi-column distinctness.

On Fri, Aug 28, 2015 at 05:33:34PM +0900, Kyotaro HORIGUCHI wrote:
> Hello, this patch enables planner to be couscious of inter-column
> correlation.
>
> Sometimes two or more columns in a table has some correlation
> which brings underestimate, which leads to wrong join method and
> ends with slow execution.
>
> Tomas Vondra is now working on heavily-equipped multivariate
> statistics for OLAP usage. In contrast, this is a lightly
> implemented solution which calculates only the ratio between a
> rows estimated by current method and a actual row number. I think
> this doesn't conflict with his work except the grammar part.

I am very glad multi-variate statistics are being addressed.  I think
this is our most common cause of optimizer failures.