code: private static List<JoinNode> generateOuterJoinOrdersForTree(JoinNode subTree) {
          List<JoinNode> treePermutations = new ArrayList<>();
          treePermutations.add(subTree);
          return treePermutations;
      }
docs: Helper method to generate join orders for an OUTER join tree.
  At the moment, permutations for LEFT Joins are not supported yet
  
  @param subTree join tree
  @return list of valid join orders