ddl blocked when the tidb_enable_concurrent_ddl is set to off

[ChenPeng2013]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

set global tidb_enable_concurrent_ddl=off;
create database if not exists test2;

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

3. What did you see instead (Required)

MySQL [test]> show processlist;
+---------------+------+------------------+------+---------+------+------------+-------------------------------------+
| Id            | User | Host             | db   | Command | Time | State      | Info                                |
+---------------+------+------------------+------+---------+------+------------+-------------------------------------+
| 2199023255955 | root | 172.17.0.1:38336 | test | Query   |  228 | autocommit | create database if not exists test2 |
| 2199023255957 | root | 172.17.0.1:38534 | test | Query   |    0 | autocommit | show processlist                    |
+---------------+------+------------------+------+---------+------+------------+-------------------------------------+
2 rows in set (0.01 sec)

4. What is your TiDB version? (Required)

Release Version: v6.5.0-alpha-55-gb0c338139
Edition: Community
Git Commit Hash: b0c338139c344df751e6738dea9d6fb983e108a0
Git Branch: master
UTC Build Time: 2022-11-14 07:08:30
GoVersion: go1.19.1
Race Enabled: false
TiKV Min Version: 6.2.0-alpha
Check Table Before Drop: false
Store: unistore

[wjhuang2016]

It's the expected behavior, we should return an error if users want to disable concurrent DDL while MDL is enabled.

[Defined2014]

Maybe we could remove original ddl framework to reduce development complexity. @wjhuang2016

[wjhuang2016]

We need to keep it for a while(at least for the next LTS version)

[Defined2014]

We need to keep it for a while(at least for the next LTS version)

Maybe we could do it next year :)