@Override
  public long ramBytesUsed() {
    return INSTANCE_SIZE
        + MemoryEstimationHelper.getEstimatedSizeOfAccountableObject(child)
        + MemoryEstimationHelper.getEstimatedSizeOfAccountableObject(operatorContext);
  }
Used for table previous fill with group.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/execution/operator/process/PreviousFillWithGroupOperator.java