@Override
  public long size() {
    long sum = 0;
    for (IWritableMemChunkGroup writableMemChunkGroup : memTableMap.values()) {
      sum += writableMemChunkGroup.count();
    }
    return sum;
  }
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