explain select * from ((select 4 as a) union all (select 33 as a)) tmp order by a desc limit 1;
id	count	task	operator info
TopN_17	1.00	root	tmp.a:desc, offset:0, count:1
└─Union_21	2.00	root	
  ├─Projection_22	1.00	root	cast(a)
  │ └─Projection_23	1.00	root	4
  │   └─TableDual_24	1.00	root	rows:1
  └─Projection_26	1.00	root	33
    └─TableDual_27	1.00	root	rows:1
