我要评论在上篇文章《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 name='c' 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。
session3> select * from performance_schema.threads where thread_id in (27,29)\g
*************************** 1. row ***************************
thread_id: 27
name: thread/sql/one_connection
type: foreground
processlist_id: 2
processlist_user: root
processlist_host: localhost
processlist_db: null
processlist_command: sleep
processlist_time: 214
processlist_state: null
processlist_info: null
parent_thread_id: 1
role: null
instrumented: yes
history: yes
connection_type: socket
thread_os_id: 9800
*************************** 2. row ***************************
thread_id: 29
name: thread/sql/one_connection
type: foreground
processlist_id: 4
processlist_user: root
processlist_host: localhost
processlist_db: null
processlist_command: query
processlist_time: 172
processlist_state: waiting for table metadata lock
processlist_info: alter table slowtech.t1 add c1 int
parent_thread_id: 1
role: null
instrumented: yes
history: yes
connection_type: socket
thread_os_id: 9907
2 rows in set (0.00 sec)
将这两张表结合，借鉴sys.innodb_lock _waits的输出，实际上我们也可以直观地呈现mdl的等待关系。
select
a.object_schema as locked_schema,
a.object_name as locked_table,
"metadata lock" as locked_type,
c.processlist_id as waiting_processlist_id,
c.processlist_time as waiting_age,
c.processlist_info as waiting_query,
c.processlist_state as waiting_state,
d.processlist_id as blocking_processlist_id,
d.processlist_time as blocking_age,
d.processlist_info as blocking_query,
concat('kill ', d.processlist_id) as sql_kill_blocking_connection
from
performance_schema.metadata_locks a
join performance_schema.metadata_locks b on a.object_schema = b.object_schema
and a.object_name = b.object_name
and a.lock_status = 'pending'
and b.lock_status = 'granted'
and a.owner_thread_id <> b.owner_thread_id
and a.lock_type = 'exclusive'
join performance_schema.threads c on a.owner_thread_id = c.thread_id
join performance_schema.threads d on b.owner_thread_id = d.thread_id\g
*************************** 1. row ***************************
locked_schema: slowtech
locked_table: t1
locked_type: metadata lock
waiting_processlist_id: 4
waiting_age: 259
waiting_query: alter table slowtech.t1 add c1 int
waiting_state: waiting for table metadata lock
blocking_processlist_id: 2
blocking_age: 301
blocking_query: null
sql_kill_blocking_connection: kill 2
1 row in set (0.00 sec)
输出一目了然，ddl操作如果要获得mdl，执行kill 2即可。
官方的sys.schematablelock_waits
实际上，mysql 5.7在sys库中也集成了类似功能，同样的场景，其输出如下，
mysql> select * from sys.schema_table_lock_waits\g
*************************** 1. row ***************************
object_schema: slowtech
object_name: t1
waiting_thread_id: 29
waiting_pid: 4
waiting_account: root@localhost
waiting_lock_type: exclusive
waiting_lock_duration: transaction
waiting_query: alter table slowtech.t1 add c1 int
waiting_query_secs: 446
waiting_query_rows_affected: 0
waiting_query_rows_examined: 0
blocking_thread_id: 27
blocking_pid: 2
blocking_account: root@localhost
blocking_lock_type: shared_read
blocking_lock_duration: transaction
sql_kill_blocking_query: kill query 2
sql_kill_blocking_connection: kill 2
*************************** 2. row ***************************
object_schema: slowtech
object_name: t1
waiting_thread_id: 29
waiting_pid: 4
waiting_account: root@localhost
waiting_lock_type: exclusive
waiting_lock_duration: transaction
waiting_query: alter table slowtech.t1 add c1 int
waiting_query_secs: 446
waiting_query_rows_affected: 0
waiting_query_rows_examined: 0
blocking_thread_id: 29
blocking_pid: 4
blocking_account: root@localhost
blocking_lock_type: shared_upgradable
blocking_lock_duration: transaction
sql_kill_blocking_query: kill query 4
sql_kill_blocking_connection: kill 4
2 rows in set (0.00 sec)
具体分析下官方的输出，
只有一个alter table操作，却产生了两条记录，而且两条记录的kill对象竟然还不一样，对表结构不熟悉及不仔细看记录内容的话，难免会kill错对象。
不仅如此，如果有n个查询被ddl操作堵塞，则会产生n*2条记录。在阻塞操作较多的情况下，这n*2条记录完全是个噪音。
而之前的sql，无论有多少操作被阻塞，一个alter table操作，就只会输出一条记录。
如何查看阻塞会话已经执行过的操作
但上面这个sql也有遗憾，其blocking_query为null，而在会话1中，其明明已经执行了三个sql。
这个与performance_schema.threads（类似于show processlist）有关，其只会输出当前正在运行的sql，对于已经执行过的，实际上是没办法看到。
但在线上，kill是一个需要谨慎的操作，毕竟你很难知道kill的是不是业务关键操作？又或者，是个批量update操作？那么，有没有办法抓到该事务之前的操作呢？
答案，有。
即performance schema中记录statement event（操作事件）的表，具体包括
events_statements_current,events_statements_history,events_statements_history_long,prepared_statements_instances。
常用的是前面三个。
三者的表结构完全一致，其中，events_statements_history又包含了events_statements_current的操作，所以我们这里会使用events_statements_history。
终极sql如下，
select
locked_schema,
locked_table,
locked_type,
waiting_processlist_id,
waiting_age,
waiting_query,
waiting_state,
blocking_processlist_id,
blocking_age,
substring_index(sql_text,"transaction_begin;" ,-1) as blocking_query,
sql_kill_blocking_connection
from
(
select
b.owner_thread_id as granted_thread_id,
a.object_schema as locked_schema,
a.object_name as locked_table,
"metadata lock" as locked_type,
c.processlist_id as waiting_processlist_id,
c.processlist_time as waiting_age,
c.processlist_info as waiting_query,
c.processlist_state as waiting_state,
d.processlist_id as blocking_processlist_id,
d.processlist_time as blocking_age,
d.processlist_info as blocking_query,
concat('kill ', d.processlist_id) as sql_kill_blocking_connection
from
performance_schema.metadata_locks a
join performance_schema.metadata_locks b on a.object_schema = b.object_schema
and a.object_name = b.object_name
and a.lock_status = 'pending'
and b.lock_status = 'granted'
and a.owner_thread_id <> b.owner_thread_id
and a.lock_type = 'exclusive'
join performance_schema.threads c on a.owner_thread_id = c.thread_id
join performance_schema.threads d on b.owner_thread_id = d.thread_id
) t1,
(
select
thread_id,
group_concat( case when event_name = 'statement/sql/begin' then "transaction_begin" else sql_text end order by event_id separator ";" ) as sql_text
from
performance_schema.events_statements_history
group by thread_id
) t2
where
t1.granted_thread_id = t2.thread_id \g
*************************** 1. row ***************************
locked_schema: slowtech
locked_table: t1
locked_type: metadata lock
waiting_processlist_id: 4
waiting_age: 294
waiting_query: alter table slowtech.t1 add c1 int
waiting_state: waiting for table metadata lock
blocking_processlist_id: 2
blocking_age: 336
blocking_query: delete from slowtech.t1 where id=2;select * from slowtech.t1;update slowtech.t1 set name='c' where id=1
sql_kill_blocking_connection: kill 2
1 row in set, 1 warning (0.00 sec)
从上面的输出可以看到，blocking_query中包含了会话1中当前事务的所有操作，按执行的先后顺序输出。
需要注意的是，默认情况下，events_statements_history只会保留每个线程最近的10个操作，如果事务中进行的操作较多，实际上也是没办法抓全的。
总结
以上所述是小编给大家介绍的解决mysql 5.7中如何定位ddl被阻塞的问题，希望对大家有所帮助
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