set tidb_cost_model_version=1;
explain format = 'brief' select * from ((select 4 as a) union all (select 33 as a)) tmp order by a desc limit 1;
id	estRows	task	access object	operator info
TopN	1.00	root		Column#3:desc, offset:0, count:1
└─Union	2.00	root		
  ├─Projection	1.00	root		4->Column#3
  │ └─TableDual	1.00	root		rows:1
  └─Projection	1.00	root		33->Column#3
    └─TableDual	1.00	root		rows:1