code: public WritableColumnVector reserveDictionaryIds(int capacity) {
      if (dictionaryIds == null) {
        dictionaryIds = reserveNewColumn(capacity, DataTypes.IntegerType);
      } else {
        dictionaryIds.reset();
        dictionaryIds.reserve(capacity);
      }
      return dictionaryIds;
    }
docs: Reserve a integer column for ids of dictionary.