Dane Foster <studdu...@gmail.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


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