entire contents of the subplan into the tuplestore before we can return
any tuples.  Instead, the tuplestore holds what we've already read, and
we fetch additional rows from the subplan as needed.  Random access to
the previously-read rows works with the tuplestore, and doesn't affect
the state of the partially-read subplan.  This is a step towards fixing
the problems with cursors over complex queries --- we don't want to
stick in Materialize nodes if they'll prevent quick startup for a cursor.