explain 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_17 1.00 root Column#3:desc, offset:0, count:1 └─Union_21 2.00 root ├─Projection_22 1.00 root 4->Column#3 │ └─TableDual_23 1.00 root rows:1 └─Projection_24 1.00 root 33->Column#3 └─TableDual_25 1.00 root rows:1