CastStringAsDecimal return wrong result of negative number.

[LittleFall]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create table t(a char(5));
alter table t set tiflash replica 1;
insert into t values ('-1');
set tidb_enforce_mpp=1; -- force cast push down to tiflash

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

mysql> SELECT /*+ READ_FROM_STORAGE(tikv[t]) */ cast(a AS DECIMAL) FROM t;
+--------------------+
| cast(a AS DECIMAL) |
+--------------------+
|                 -1 |
+--------------------+
1 row in set (0.00 sec)

3. What did you see instead (Required)

SELECT /*+ READ_FROM_STORAGE(tiflash[t]) */ cast(a AS DECIMAL) FROM t;
+--------------------+
| cast(a AS DECIMAL) |
+--------------------+
|                  1 |
+--------------------+

4. What is your TiFlash version? (Required)

4.0+

[LittleFall]

come from https://asktug.com/t/topic/273768/19 and pingcap/tidb#30542