ExecInitNode() sees a NestedLoop node ...
   so dispatches to ExecInitNestLoop() to set up iterator
   then invokes ExecInitNode() on left and right sub-plans
       in left subPlan, ExecInitNode() sees an IndexScan node
        so dispatches to ExecInitIndexScan() to set up iterator
       in right sub-plan, ExecInitNode() sees a SeqScan node
        so dispatches to ExecInitSeqScan() to set up iterator

Result: a plan state tree with same structure as plan tree.