compatibility: “show full columns” prints wrong Collation for date, datatime, timestamp columns

[tshqin]

create table t1 (a date,b datetime,c timestamp);
show full columns from t1;

TiDB shows a different collation value ("binary") than mysql(null), this is not a big deal in most cases. It's reported that a CDC software called "Attunity" check the "collation" output of "show full columns" of the tables that managed by itself.
the tool can't pass the db usability check ,pls take a look.

MySQL 5.7.20

+-------+-----------+-----------+------+-----+---------+-------+---------------------------------+---------+
| Field | Type | Collation | Null | Key | Default | Extra | Privileges | Comment |
+-------+-----------+-----------+------+-----+---------+-------+---------------------------------+---------+
| a | date | NULL | YES | | NULL | | select,insert,update,references | |
| b | datetime | NULL | YES | | NULL | | select,insert,update,references | |
| c | timestamp | NULL | YES | | NULL | | select,insert,update,references | |
+-------+-----------+-----------+------+-----+---------+-------+---------------------------------+---------+
3 rows in set (0.00 sec)

TiDB 2.1.6:

+-------+-----------+-----------+------+------+---------+-------+---------------------------------+---------+
| Field | Type | Collation | Null | Key | Default | Extra | Privileges | Comment |
+-------+-----------+-----------+------+------+---------+-------+---------------------------------+---------+
| a | date | binary | YES | | NULL | | select,insert,update,references | |
| b | datetime | binary | YES | | NULL | | select,insert,update,references | |
| c | timestamp | binary | YES | | NULL | | select,insert,update,references | |
+-------+-----------+-----------+------+------+---------+-------+---------------------------------+---------+
3 rows in set (0.01 sec)

Release Version: v2.1.6
Git Commit Hash: 66e639e
Git Branch: HEAD
UTC Build Time: 2019-03-15 09:09:07
GoVersion: go version go1.11.2 linux/amd64
Race Enabled: false
TiKV Min Version: 2.1.0-alpha.1-ff3dd160846b7d1aed9079c389fc188f7f5ea13e
Check Table Before Drop: false

[tshqin]

pls reopen the issue, MySQL prints the nothing 'null', not the character 'NULL'. TiDB prints character 'NULL'.
Can't pass the check of "Attunity" by the same reason.

MySQL 5.7.20

mysql> show full columns from t1 where Collation is null;
+-------+-----------+-----------+------+-----+---------+-------+---------------------------------+---------+
| Field | Type | Collation | Null | Key | Default | Extra | Privileges | Comment |
+-------+-----------+-----------+------+-----+---------+-------+---------------------------------+---------+
| a | date | NULL | YES | | NULL | | select,insert,update,references | |
| b | datetime | NULL | YES | | NULL | | select,insert,update,references | |
| c | timestamp | NULL | YES | | NULL | | select,insert,update,references | |
+-------+-----------+-----------+------+-----+---------+-------+---------------------------------+---------+
3 rows in set (0.00 sec)

mysql> show full columns from t1 where Collation ='NULL';
Empty set (0.00 sec)