@Override
  protected void serializeAttributes(ByteBuffer byteBuffer) {
    PlanNodeType.FILL.serialize(byteBuffer);
    fillDescriptor.serialize(byteBuffer);
    ReadWriteIOUtils.write(scanOrder.ordinal(), byteBuffer);
  }
FillNode is used to fill the empty field in one row.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/planner/plan/node/process/FillNode.java