Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> writes:
> What I am more concerned about is a join that uses the most common
> value. We do an index scan in that case.
No, we do whichever plan looks cheapest. Again, it's all about
statistics.
Right now, eqjoinsel() is just a stub that returns a constant
selectivity estimate. It might be useful to compute some more
sophisticated value based on pg_statistic entries for the two
columns, but right now I doubt you could tell much. Should keep
the join case in mind when we extend the statistics...
regards, tom lane