public HashMap<TsPrimitiveType, Long> get(long index) {
    HashMap<TsPrimitiveType, Long> result = array.get(index);
    if (result == null) {
      result = new HashMap<>();
      array.set(index, result);
    }
    return result;
  }
Returns the element of this big array at specified index.

@param index a position in this big array.
@return the element of this big array at the specified position.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/execution/operator/source/relational/aggregation/grouped/array/MapBigArray.java