## Problem summary
When we want to push the filter through the union. We should check whether the union's children are `OneRowRelation` or not. If there are some `OneRowRelation`, we shouldn't push down the filter to that part
Before this PR
```
mysql> select * from (select 1 as a, 2 as b union all select 3, 3) t where a = 1;
+------+------+
| a | b |
+------+------+
| 1 | 2 |
| 3 | 3 |
+------+------+
2 rows in set (0.01 sec)
```
After this PR
```
mysql> select * from (select 1 as a, 2 as b union all select 3, 3) t where a = 1;
+------+------+
| a | b |
+------+------+
| 1 | 2 |
+------+------+
1 row in set (0.38 sec)
```
a855253543