code: public String getIndexFields() {
          String orderFieldName = getOrderFieldName();
          if (orderFieldName == null) return getKeyFieldName();
          else return String.join(",", getKeyFieldName(), orderFieldName);
      }
docs: Gets index fields for the spec table. WARNING: this MUST be called on a spec table (i.e., one of the constant
  tables defined in this class). Otherwise, it could return fields that should not be indexed.
  @return