Field length of binary arithmetic between integer and decimal is different from MySQL

[riteme]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Connect to MySQL/TiDB with --column-type-info in command line.

create table test.t (a int, b decimal(1, 0));
select a % b from test.t;

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

In MySQL 5.7/8.0:

Field   1:  `a % b`
Catalog:    `def`
Database:   ``
Table:      ``
Org_table:  ``
Type:       NEWDECIMAL
Collation:  binary (63)
Length:     11
Max_length: 0
Decimals:   0
Flags:      BINARY NUM 

3. What did you see instead (Required)

In TiDB:

Field   1:  `a % b`
Catalog:    `def`
Database:   ``
Table:      ``
Org_table:  ``
Type:       NEWDECIMAL
Collation:  binary (63)
Length:     12
Max_length: 0
Decimals:   0
Flags:      NOT_NULL BINARY NUM 

4. What is your TiDB version? (Required)

mysql> select tidb_version();
+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tidb_version()                                                                                                                                                                                                                                                                                                                                |
+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Release Version: v5.2.0-alpha-211-ga8d23fda4-dirty
Edition: Community
Git Commit Hash: a8d23fda419a1d9a40f2b8ba85b6f22a6884b737
Git Branch: master
UTC Build Time: 2021-07-20 10:34:06
GoVersion: go1.16.6
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false |
+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.001 sec)