在上篇文章《MySQL表结构变更，不可不知的Metadata Lock》中，我们介绍了MDL引入的背景，及基本概念，从“道”的层面知道了什么是MDL。下面就从“术”的层面看看如何定位MDL的相关问题。
在MySQL 5.7中，针对MDL，引入了一张新表performance_schema.metadata_locks，该表可对外展示MDL的相关信息，包括其作用对象，类型及持有等待情况。
开启MDL的instrument
但是相关instrument并没有开启（MySQL 8.0是默认开启的），其可通过如下两种方式开启，
临时生效
修改performance_schema.setup_instrume nts表，但实例重启后，又会恢复为默认值。
UPDATE performance_schema.setup_instruments SET ENABLED = 'YES', TIMED = 'YES'
WHERE NAME = 'wait/lock/metadata/sql/mdl';
永久生效
在配置文件中设置
[mysqld]
performance-schema-instrument='wait/lock/metadata/sql/mdl=ON'
测试场景
下面结合一个简单的Demo，来看看在MySQL 5.7中如何定位DDL操作的阻塞问题。
session1> begin;
Query OK, 0 rows affected (0.00 sec)
session1> delete from slowtech.t1 where id=2;
Query OK, 1 row affected (0.00 sec)
session1> select * from slowtech.t1;
+------+------+
| id | name |
+------+------+
| 1 | a |
+------+------+
1 row in set (0.00 sec)
session1> update slowtech.t1 set where id=1;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
session2> alter table slowtech.t1 add c1 int; ##被阻塞
session3> show processlist;
+----+------+-----------+------+---------+------+---------------------------------+------------------------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+------+-----------+------+---------+------+---------------------------------+------------------------------------+
| 2 | root | localhost | NULL | Sleep | 51 | | NULL |
| 3 | root | localhost | NULL | Query | 0 | starting | show processlist |
| 4 | root | localhost | NULL | Query | 9 | Waiting for table metadata lock | alter table slowtech.t1 add c1 int |
+----+------+-----------+------+---------+------+---------------------------------+------------------------------------+
3 rows in set (0.00 sec)
session3> select object_type,object_schema,object_name,lock_type,lock_duration,lock_status,owner_thread_id from performance_schema.metadata_locks;
+-------------+--------------------+----------------+---------------------+---------------+-------------+-----------------+
| object_type | object_schema | object_name | lock_type | lock_duration | lock_status | owner_thread_id |
+-------------+--------------------+----------------+---------------------+---------------+-------------+-----------------+
| TABLE | slowtech | t1 | SHARED_WRITE | TRANSACTION | GRANTED | 27 |
| GLOBAL | NULL | NULL | INTENTION_EXCLUSIVE | STATEMENT | GRANTED | 29 |
| SCHEMA | slowtech | NULL | INTENTION_EXCLUSIVE | TRANSACTION | GRANTED | 29 |
| TABLE | slowtech | t1 | SHARED_UPGRADABLE | TRANSACTION | GRANTED | 29 |
| TABLE | slowtech | t1 | EXCLUSIVE | TRANSACTION | PENDING | 29 |
| TABLE | performance_schema | metadata_locks | SHARED_READ | TRANSACTION | GRANTED | 28 |
+-------------+--------------------+----------------+---------------------+---------------+-------------+-----------------+
6 rows in set (0.00 sec)
这里，重点关注lock_status，"PENDING"代表线程在等待MDL，而"GRANTED"则代表线程持有MDL。
如何找出引起阻塞的会话
结合owner_thread_id，可以可到，是29号线程在等待27号线程的MDL，此时，可kill掉52号线程。
但需要注意的是，owner_thread_id给出的只是线程ID，并不是show processlist中的ID。如果要查找线程对应的processlist id，需查询performance_schema.threads表。