new-only-full-group-by mode can not tell special union all case

[AilinKid]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

mysql> create table t(a int,b int,c int);
Query OK, 0 rows affected (0.11 sec)

mysql> insert into t values(1,2,3);
Query OK, 1 row affected (0.01 sec)

mysql> select a,max(b) as max_b from (select * from t union all select * from t) x ;
ERROR 1140 (42000): In aggregated query without GROUP BY, expression #1 of SELECT list contains nonaggregated column 'x.a'; this is incompatible with sql_mode=only_full_group_by
mysql> select a,max(b) as max_b from (select * from t union all select 1,2,3 ) x ;
ERROR 1140 (42000): In aggregated query without GROUP BY, expression #1 of SELECT list contains nonaggregated column 'x.a'; this is incompatible with sql_mode=only_full_group_by
mysql> select version();
+-----------+
| version() |
+-----------+
| 8.3.0     |
+-----------+
1 row in set (0.01 sec)

tidb with new-only-full-group= on
tidb> select a,max(b) as max_b from (select * from t union all select * from t) x ;
ERROR 8123 (HY000): In aggregated query without GROUP BY, expression #1 of SELECT list contains nonaggregated column 'x.a'; this is incompatible with sql_mode=only_full_group_by
tidb> select a,max(b) as max_b from (select * from t union all select 1,2,3 ) x ;
+------+-------+
| a    | max_b |
+------+-------+
|    1 |     2 |
+------+-------+
1 row in set (0.01 sec)

2. What did you expect to see? (Required)

both two queries should both error

3. What did you see instead (Required)

the 2nd can run success?

4. What is your TiDB version? (Required)

master

[AilinKid]

in the place marked with the red line, we found that since the union all have multiple children, while they share the same project column schema which is desirable, projecting column#8 as a constant is only right in the behavior of what the 2nd child does, not the 1st child. So for union-all case, maybe the functional dependency merge is not always right, we should discard fds of both child.

[AilinKid]

we didn't implement a specific fds deriving logic for union all operator which is default as merging