group_concat() does not consider collatioin resulting errors

[fzhedu]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

drop table if exists test.t
create table test.t(id int, value varchar(20) charset utf8mb4 collate utf8mb4_general_ci, value1 varchar(20) charset utf8mb4 collate utf8mb4_bin)
alter table test.t set tiflash replica 1
insert into test.t values (1, 'abc', 'abc '),(4, 'Abc', 'abc'),(3,'def', 'def ');

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

mysql> select /*+ read_from_storage(tiflash[t]) */ upper(group_concat(distinct value order by 1)) from test.t;
+------------------------------------------------+
| upper(group_concat(distinct value order by 1)) |
+------------------------------------------------+
| ABC,DEF                                        |
+------------------------------------------------+
1 row in set (0.01 sec)

3. What did you see instead (Required)

mysql> select /*+ read_from_storage(tikv[t]) */ upper(group_concat(distinct value order by 1)) from test.t;
+------------------------------------------------+
| upper(group_concat(distinct value order by 1)) |
+------------------------------------------------+
| ABC,ABC,DEF                                    |
+------------------------------------------------+
1 row in set (0.00 sec)

4. What is your TiDB version? (Required)

master

[fzhedu]

what about other agg functions, like count(distinct x)?

[ti-srebot]

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