create table t1(a int, b int);
insert into t1 values(1,1),(2,2);

create table t2(a int, b int);
insert into t2 values(1,1),(2,2);

select sum(c) from (select t1.a as a, t1.a as c, length(t1.b) from t1  union select a, b, b from t2) t;

+--------------------------------+----------+------+-------------------------------------------------------------+
| id                             | count    | task | operator info                                               |
+--------------------------------+----------+------+-------------------------------------------------------------+
| StreamAgg_16                   | 1.00     | root | funcs:sum(t.c)                                              |
| └─HashAgg_30                   | 16000.00 | root | group by:length(t1.b), t1.a, t1.c, funcs:firstrow(t1.c)     |
|   └─Union_20                   | 20000.00 | root |                                                             |
|     ├─Projection_21            | 10000.00 | root | test.t1.a, test.t1.c, cast(length(t1.b))                    |
|     │ └─Projection_22          | 10000.00 | root | test.t1.a, test.t1.a, length(cast(test.t1.b))               |
|     │   └─TableReader_24       | 10000.00 | root | data:TableScan_23                                           |
|     │     └─TableScan_23       | 10000.00 | cop  | table:t1, range:[-inf,+inf], keep order:false, stats:pseudo |
|     └─Projection_25            | 10000.00 | root | test.t2.a, test.t2.b, cast(test.t2.b)                       |
|       └─TableReader_27         | 10000.00 | root | data:TableScan_26                                           |
|         └─TableScan_26         | 10000.00 | cop  | table:t2, range:[-inf,+inf], keep order:false, stats:pseudo |
+--------------------------------+----------+------+-------------------------------------------------------------+