> So, it would seem like my optimal plan should have hash joins with index
> scans.

No.  The thing you are looking at here is a nestloop join with inner
index scan, which has to be understood as a unit even though EXPLAIN
doesn't describe it that way.  The inner indexscan is repeated once
for each outer row, using a join key from the outer row as part of the
index lookup.  That's simply not relevant to the other kinds of joins,
because they expect the inner and outer relations to be scanned

                        regards, tom lane

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