@Override
  public IWritableMemChunk getWritableMemChunk(IDeviceID deviceId, String measurement) {
    IWritableMemChunkGroup memChunkGroup = memTableMap.get(deviceId);
    if (null == memChunkGroup) {
      return null;
    }
    return memChunkGroup.getWritableMemChunk(measurement);
  }
Create this MemChunk if it's not exist.

@param deviceId device id
@param schemaList measurement schemaList
@return this MemChunkGroup
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/storageengine/dataregion/memtable/AbstractMemTable.java