docs: public RelNode leafRel()
    {
      final Stage currentStage = stage();
  
      switch (currentStage) {
        case SORT_PROJECT:
          return sortProject;
        case SORT:
          return sort;
        case AGGREGATE_PROJECT:
          return aggregateProject;
        case HAVING_FILTER:
          return havingFilter;
        case AGGREGATE:
          return aggregate;
        case SELECT_SORT:
          return selectSort;
        case SELECT_PROJECT:
          return selectProject;
        case WHERE_FILTER:
          return whereFilter;
        case SCAN:
          return scan;
        default:
          throw new ISE("WTF?! Unknown stage: %s", currentStage);
      }
    }