code: protected void fillHeaders(JSONObject eachRow, List<String> joinFields, ActionType action) {
          if (action == ActionType.FIRST_CUT) {
              baseFieldNames.add("timestamp");
          } 
          if (eachRow != null) {
              if (isJoinType(action)) {
                  Set<String> keysCopy = new LinkedHashSet<>(eachRow.keySet());
                  keysCopy.removeAll(joinFields);
                  baseFieldNames.addAll(keysCopy);
              } else {
                  baseFieldNames.addAll(eachRow.keySet());
              }
          }
      }
docs: TODO: Make this like tryRefillHeaders() method in BaseMapper because some items in the json resultset may not have all the fields.