public static FillNode deserialize(ByteBuffer byteBuffer) {
    FillDescriptor fillDescriptor = FillDescriptor.deserialize(byteBuffer);
    Ordering scanOrder = Ordering.values()[ReadWriteIOUtils.readInt(byteBuffer)];
    PlanNodeId planNodeId = PlanNodeId.deserialize(byteBuffer);
    return new FillNode(planNodeId, fillDescriptor, scanOrder);
  }
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