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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