cast int to decimal got wrong result when exceeds range

[guo-shaoge]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

drop table if exists t1;
create table t1(c1 int);
insert into t1 values(9999);
alter table t1 set tiflash replica 1;
set tidb_enforce_mpp = 1;                                                                                                                                                                                                                                                                          set tidb_isolation_read_engines = 'tiflash';
select cast(c1 as decimal(4, 1)) from t1;

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

MySQL [test]> select cast(c1 as decimal(4, 1)) from t1;
+---------------------------+
| cast(c1 as decimal(4, 1)) |
+---------------------------+
|                     999.9 |
+---------------------------+
1 row in set, 1 warning (0.00 sec)

3. What did you see instead (Required)

MySQL [test]> select cast(c1 as decimal(4, 1)) from t1;
+---------------------------+
| cast(c1 as decimal(4, 1)) |
+---------------------------+
|                    9999.0 |
+---------------------------+
1 row in set (0.02 sec)

4. What is your TiFlash version? (Required)

master

[fuzhe1989]

In a word: when cast int as decimal, the overflow check ignores the scale and will accept some illegal data if scale > 0.