After executing "modify column", its flag has changed

[zimulala]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

CREATE TABLE `t` (
  `a` varchar(123) DEFAULT NULL,
  `d` datetime DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin;
 
tidb> select d from t;
Field   1:  `d`
Catalog:    `def`
Database:   `test`
Table:      `t`
Org_table:  `t`
Type:       DATETIME
Collation:  binary (63)
Length:     19
Max_length: 0
Decimals:   0
Flags:      NOT_NULL BINARY

alter table t modify column d datetime

select d from t;

2. What did you expect to see?
I hope that after executing "modify column", its flag is the same as before.

3. What did you see instead?

tidb> alter table t modify column d datetime;
Query OK, 0 rows affected (0.01 sec)
tidb> select d from t;
Field 1: d
Catalog: def
Database: test
Table: t
Org_table: t
Type: DATETIME
Collation: binary (63)
Length: 19
Max_length: 0
Decimals: 0
Flags:

4. What version of TiDB are you using (`tidb-server -V` or run `select tidb_version();` on TiDB)?
["Release Version"=v4.0.0-alpha-119-g0ff5b50c1-dirty]
["Git Commit Hash"=0ff5b50c1509bed279a2c2ba7acdb38519b38c9c]

[zimulala]

Related to #11800

[ghost]

Confirming this issue still exists in master:

DROP TABLE IF EXISTS t;
CREATE TABLE `t` (
  `a` varchar(123) DEFAULT NULL,
  `d` datetime DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin;

select d from t;
alter table t modify column d datetime;
select d from t;


..

mysql> select d from t;
Field   1:  `d`
Catalog:    `def`
Database:   `test`
Table:      `t`
Org_table:  `t`
Type:       DATETIME
Collation:  binary (63)
Length:     19
Max_length: 0
Decimals:   0
Flags:      NOT_NULL BINARY 


0 rows in set (0.00 sec)

mysql> alter table t modify column d datetime;
Query OK, 0 rows affected (0.08 sec)

mysql> select d from t;
Field   1:  `d`
Catalog:    `def`
Database:   `test`
Table:      `t`
Org_table:  `t`
Type:       DATETIME
Collation:  binary (63)
Length:     19
Max_length: 0
Decimals:   0
Flags:      


0 rows in set (0.01 sec)

mysql> select tidb_version()\G
*************************** 1. row ***************************
tidb_version(): Release Version: v4.0.0-beta.2-807-g824df7675
Edition: Community
Git Commit Hash: 824df767559b8544af0cb71509135d322de1cd47
Git Branch: master
UTC Build Time: 2020-07-20 04:47:45
GoVersion: go1.13
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false
1 row in set (0.00 sec)

[wjhuang2016]

Duplicate with #20720