<simon@2ndquadrant.com> writes:
> Unfortunately, yes thats true - thats is for correctness, not an
> optimization decision. Outer joins constrain you on both join order AND
> on join type. Nested loops and hash joins avoid touching all rows in
> the right hand table, which is exactly what you don't want when you
> have a right outer join to perform, since you wish to include rows in
> that table when there is no match. Thus, we MUST choose a merge join
> even when (if it wasn't an outer join) we would have chosen a nested
> loops or hash.
The alternative of course is to flip it around to be a left outer join
so that we can use those plan types. But depending on the relative
sizes of the two tables this may be a loser.
If you are using a FULL join then it is indeed true that mergejoin is
the only supported plan type. I don't think that was at issue here
though.
regards, tom lane