Re: Stats for multi-column indexes

On Tue, 2007-03-20 at 18:12, Josh Berkus wrote:
> Tom,
> 
> > Actually, I think you don't particularly need stats for that in most
> > cases --- if the planner simply took note that the FK relationship
> > exists, it would know that each row of the FK side joins to exactly
> > one row of the PK side, which in typical cases is sufficient.
> 
> Is it?  What about the other direction?  Currently, doesn't the planner 
> assume that the rowcount relationship is 1 to ( child total rows / 
> parent total rows) ?  That's ok for tables with relatively even 
> distribution, but not for skewed ones.

Wouldn't that be improved if the MCVs/histogram of the FK column are
taken into account ? Considering that the FK part is unique, the
skewness in the relationship is completely determined by the FK parts
histogram. That would give at least a lower/upper bound and MCVs to the
relationship.

Cheers,
Csaba.