Wrong result for greatest when args are unsigned bigint

[guo-shaoge]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

drop table t1;
create table t1(c1 bigint unsigned, c2 bigint unsigned);
insert into t1 values(9223372036854775808, 9223372036854775809);
select greatest(c1, c2) from t1;

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

mysql> select greatest(c1, c2) from t2;
+---------------------+
| greatest(c1, c2)    |
+---------------------+
| 9223372036854775808 |
+---------------------+
1 row in set (0.01 sec)

3. What did you see instead (Required)

mysql> select greatest(c1, c2) from t1;
+----------------------+
| greatest(c1, c2)     |
+----------------------+
| -9223372036854775807 |
+----------------------+
1 row in set (0.01 sec)

4. What is your TiDB version? (Required)

master

[sylzd]

/assign

[sylzd]

the expected result is

mysql> select greatest(c1, c2) from t2;
+---------------------+
| greatest(c1, c2)    |
+---------------------+
| 9223372036854775809 |
+---------------------+
1 row in set (0.01 sec)

[github-actions]

Please check whether the issue should be labeled with 'affects-x.y' or 'fixes-x.y.z', and then remove 'needs-more-info' label.