private List<Object> explainInputs(List<RelNode> inputs) {
   final List<Object> list = jsonBuilder.list();
   for (RelNode input : inputs) {
     String id = relIdMap.get(input);
     if (id == null) {
       input.explain(this);
       id = previousId;
     }
     list.add(id);
   }
   return list;
 }