length(time(2)) is not compatible with mysql

[ChenPeng2013]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

use test;
drop table if exists t;
create table t(a time(2));
insert into t values("2020-02-02 22:22:22.94");
select length(a),a from t;

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

release-5.1 & master & mysql 8.0.21

mysql> select length(a),a from t;
+-----------+-------------+
| length(a) | a           |
+-----------+-------------+
|        11 | 22:22:22.94 |
+-----------+-------------+
1 row in set (0.00 sec)

3. What did you see instead (Required)

release-4.0 & release-5.0

mysql> select length(a),a from t;
+-----------+-------------+
| length(a) | a           |
+-----------+-------------+
|         8 | 22:22:22.94 |
+-----------+-------------+
1 row in set (0.00 sec)

4. What is your TiDB version? (Required)

Release Version: v4.0.14
Edition: Community
Git Commit Hash: 4f919b07f8f013e48521894c70cb69004f43c622
Git Branch: release-4.0
UTC Build Time: 2021-08-02 02:01:11
GoVersion: go1.16.5
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false

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