bug: union all get wrong result

[zhyass]

mysql> create table t1 (a int, b int);
Query OK, 0 rows affected (0.09 sec)

mysql> insert into t1 values (1, 2), (2, 3);
+-------------------------+
| number of rows inserted |
+-------------------------+
|                       2 |
+-------------------------+
1 row in set (0.21 sec)
Read 2 rows, 18.00 B in 0.123 sec., 16.28 rows/sec., 146.50 B/sec.

mysql> create table t2 (a int, b int);
Query OK, 0 rows affected (0.12 sec)

mysql> insert into t2 values (1, 2), (2, 3) ;
+-------------------------+
| number of rows inserted |
+-------------------------+
|                       2 |
+-------------------------+
1 row in set (0.19 sec)
Read 2 rows, 18.00 B in 0.104 sec., 19.17 rows/sec., 172.49 B/sec.

mysql> select a, b from (select * from t1 where a>1 union all select * from t2 where b>2);
+------+------+
| a    | b    |
+------+------+
|    2 |    3 |
|    2 |    3 |
+------+------+
2 rows in set (0.12 sec)
Read 4 rows, 36.00 B in 0.048 sec., 82.94 rows/sec., 746.45 B/sec.

mysql> select b,a from (select * from t1 where a>1 union all select * from t2 where b>2);
+------+------+
| b    | a    |
+------+------+
|    2 |    3 |
|    2 |    3 |
+------+------+
2 rows in set (0.15 sec)
Read 4 rows, 36.00 B in 0.060 sec., 66.63 rows/sec., 599.66 B/sec.

[zhyass]

I tried to fix it but it doesn't look easy so PTAL @sundy-li