ExecProcNode() sees a NestedLoop node ...
   so dispatches to ExecNestedLoop() to get next tuple
   which invokes ExecProcNode() on its sub-plans
       in left sub-plan, ExecProcNode() sees an IndexScan node
            so dispatches to ExecIndexScan() to get next tuple
            if no more tuples, return END
            for this tuple, invoke ExecProcNode() on right sub-plan
                ExecProcNode() sees a SeqScan node
                    so dispatches to ExecSeqScan() to get next tuple
                    check for match and return joined tuples if found
                    continue scan until end
                reset right sub-plan iterator

Result: stream of result tuples returned via ExecutePlan()