@Override
  public long calculateRetainedSizeAfterCallingNext() {
    // we may need to cache the last tsblock in groupKeyComparator
    return child.calculateRetainedSizeAfterCallingNext() + child.calculateMaxReturnSize();
  }
Used for table previous fill with group.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/execution/operator/process/PreviousFillWithGroupOperator.java