Dane Foster <studdugie(at)gmail(dot)com> writes:
>> If the sort is the inner input to a merge join, this could reflect
>> mark-and-restore rescanning of the sort's output. Are there a
>> whole lot of duplicate keys on the merge's other side?
> Yes. The course_id column's values repeat a LOT on the merge side.
Hmm. The planner should avoid using a merge join if it knows that
to be true. Maybe analyze'ing that table would prompt it to use
some other join method?
regards, tom lane