这篇文章主要为大家展示了“怎么解决MySQL 5.7中定位DDL被阻塞的问题”，内容简而易懂，条理清晰，希望能够帮助大家解决疑惑，下面让小编带领大家一起研究并学习一下“怎么解决MySQL 5.7中定位DDL被阻塞的问题”这篇文章吧。
创新互联，为您提供成都网站建设公司、成都网站制作、网站营销推广、网站开发设计，对服务成都报废汽车回收等多个行业拥有丰富的网站建设及推广经验。创新互联网站建设公司成立于2013年,提供专业网站制作报价服务,我们深知市场的竞争激烈，认真对待每位客户，为客户提供赏心悦目的作品。 与客户共同发展进步，是我们永远的责任！
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。
以上是“怎么解决MySQL 5.7中定位DDL被阻塞的问题”这篇文章的所有内容，感谢各位的阅读！相信大家都有了一定的了解，希望分享的内容对大家有所帮助，如果还想学习更多知识，欢迎关注创新互联行业资讯频道！
