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.
