1.查看DDL阻塞

查看DDL操作对应的状态

show processlist；

2.(MySQL5.6)定位哪些会话阻塞了DDL操作

select * from information_schema.innodb_trx;

information_schema.innodb_trx.trx_mysql_thread_id是线程id,再结合information_schema.processlist进一步缩小范围，定位出执行时间早于DDL的事务，生成kill会话语句

SELECT concat('kill ', i.trx_mysql_thread_id, ';')
FROM information_schema.innodb_trx i, (
    SELECT MAX(time) AS max_time
    FROM information_schema.processlist
    WHERE state = 'Waiting for table metadata lock'
      AND (info LIKE 'alter%'
      OR info LIKE 'create%'
      OR info LIKE 'drop%'
      OR info LIKE 'truncate%'
      OR info LIKE 'rename%'
  )) p
WHERE timestampdiff(second, i.trx_started, now()) > p.max_time;

3.(MySQL5.7)定位哪些会话阻塞了DDL操作

select * from sys.schema_table_lock_waits；

重点关注列：blocking_lock_type

定位出需要Kill的会话。

SELECT
    sql_kill_blocking_connection 
FROM
    sys.schema_table_lock_waits 
WHERE
    blocking_lock_type <> 'SHARED_UPGRADABLE' 
    AND waiting_query = 'alter table xxx';

不过，sys.schema_table_lock_waits表是MySQL 5.7引入的，会显示MDL的相关信息，包括作用对象、锁的类型及锁的状态等。必须首先开启MDL相关的instrument。但在MySQL 5.7中，默认关闭；MySQL 8.0才默认开启。 查看是否开启：

mysql> select * from performance_schema.setup_instruments where name='wait/lock/metadata/sql/mdl';

临时开启：

UPDATE performance_schema.setup_instruments SET ENABLED = 'YES', TIMED = 'YES'
WHERE NAME = 'wait/lock/metadata/sql/mdl';

永久生效：

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

https://mp.weixin.qq.com/s/0KbenLqQKHR5Aj2kFS_Dcg