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. ---------------------------(end of broadcast)--------------------------- TIP 2: Don't 'kill -9' the postmaster
