code: public boolean isOutputOrdered (List<AbstractExpression> sortExpressions, List<SortDirectionType> sortDirections) {
          assert(sortExpressions.size() == sortDirections.size());
          if (m_children.size() == 1) {
              return m_children.get(0).isOutputOrdered(sortExpressions, sortDirections);
          }
          return false;
      }
docs: Does the plan guarantee a result sorted according to the required sort order.
  The default implementation delegates the question to its child if there is only one child.
  
  @param sortExpressions list of ordering columns expressions
  @param sortDirections list of corresponding sort orders
  
  @return TRUE if the node's output table is ordered. FALSE otherwise