解决MySQL 5.7中定位DDL被阻塞的问题

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';

在配置文件中设置

[mysqld]performance-schema-instrument='wait/lock/metadata/sql/mdl=ON'

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: 0session2> 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)

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: NULLPROCESSLIST_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: NULLPROCESSLIST_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: 99072 rows in set (0.00 sec)

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_connectionFROM performance_schema.metadata_locks aJOIN performance_schema.metadata_locks b ON a.OBJECT_SCHEMA = b.OBJECT_SCHEMAAND a.OBJECT_NAME = b.OBJECT_NAMEAND a.lock_status = 'PENDING'AND b.lock_status = 'GRANTED'AND a.OWNER_THREAD_ID <> b.OWNER_THREAD_IDAND a.lock_type = 'EXCLUSIVE'JOIN performance_schema.threads c ON a.OWNER_THREAD_ID = c.THREAD_IDJOIN 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: NULLsql_kill_blocking_connection: KILL 21 row in set (0.00 sec)

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 2sql_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 4sql_kill_blocking_connection: KILL 42 rows in set (0.00 sec)

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_connectionFROM (  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 ) t2WHERE 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=1sql_kill_blocking_connection: KILL 21 row in set, 1 warning (0.00 sec)

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

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