docs: public void addSortExpression(AbstractExpression sortExpr, SortDirectionType sortDir)
      {
          assert(sortExpr != null);
          // PlanNodes all need private deep copies of expressions
          // so that the resolveColumnIndexes results
          // don't get bashed by other nodes or subsequent planner runs
          m_sortExpressions.add(sortExpr.clone());
          m_sortDirections.add(sortDir);
      }