MySQL中找出谁持有MDL锁
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一、MDL锁的介绍
mysql 5.7版本之前并没有提供一个方便的途径来查看MDL锁，github上有一名为mysql-plugin-mdl-info的项目，通过插件的方式来查看，于是在MySQL 5.7中的performance_schea库下新增了一张表metadata_locks，用其来查看MDL锁那是相当的方便:
不过默认PS并没有打开此功能，需要手工将wait/lock/metadata/sql/mdl监控给打开:
UPDATE performance_schema.setup_consumers SET ENABLED = \'YES\' WHERE NAME =\'global_instrumentation\';
UPDATE performance_schema.setup_instruments SET ENABLED = \'YES\' WHERE NAME =\'wait/lock/metadata/sql/mdl\';
目前MDL有如下锁模式：
锁模式 对应SQL
MDL_INTENTION_EXCLUSIVE GLOBAL对象、SCHEMA对象操作会加此锁
MDL_SHARED FLUSH TABLES with READ LOCK
MDL_SHARED_HIGH_PRIO 仅对MyISAM存储引擎有效
MDL_SHARED_READ SELECT查询
MDL_SHARED_WRITE DML语句
MDL_SHARED_WRITE_LOW_PRIO 仅对MyISAM存储引擎有效
MDL_SHARED_UPGRADABLE ALTER TABLE
MDL_SHARED_READ_ONLY LOCK xxx READ
MDL_SHARED_NO_WRITE FLUSH TABLES xxx,yyy,zzz READ
MDL_SHARED_NO_READ_WRITE FLUSH TABLE xxx WRITE
MDL_EXCLUSIVE ALTER TABLE xxx PARTITION BY …
root@tidb06 08:33: [test001]> select * from performance_schema.setup_instruments WHERE NAME =\'wait/lock/metadata/sql/mdl\';
+----------------------------+---------+-------+
| NAME | ENABLED | TIMED |
+----------------------------+---------+-------+
| wait/lock/metadata/sql/mdl | YES | YES |
+----------------------------+---------+-------+
1 row in set (0.00 sec)
root@tidb06 08:44: [test001]> select * from performance_schema.setup_consumers where NAME =\'global_instrumentation\';
+------------------------+---------+
| NAME | ENABLED |
+------------------------+---------+
| global_instrumentation | YES |
+------------------------+---------+
1 row in set (0.00 sec)
二、模拟MDL锁
演示环境:mysql5.7.22
为了演示 下面参数设置时间如下:
innodb_lock_wait_timeout=600
interactive_timeout = 600
wait_timeout =600
创建测试表和插入测试数据:
CREATE TABLE `test001` (
`id` int(8) NOT NULL AUTO_INCREMENT,
`username` varchar(20) COLLATE utf8mb4_unicode_ci NOT NULL,
`password` varchar(20) COLLATE utf8mb4_unicode_ci NOT NULL,
`create_time` varchar(20) COLLATE utf8mb4_unicode_ci NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=0 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci
insert into test001(username,password,create_time)value(\'小花\',\'abc123\',now());
insert into test001(username,password,create_time)value(\'王五\',\'ccc123\',now());
会话1 开启事务更新1条sql:
\'tidb03\' root@localhost 09:04:56 test001>begin;
Query OK, 0 rows affected (0.00 sec)
\'tidb03\' root@localhost 09:05:03 test001>update test001 set username=\'zhangsan\' where id=2;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
会话2 更新同一个记录: 产生行锁等待
\'tidb03\' root@localhost 12:40:02 test001>update test001 set username=\'王五\' where id=2;
会话3 给test001表添加索引 : 产生DML表锁等待
\'tidb03\' root@localhost 12:42:36 test001>alter table test001 add index idx_username(username);
会话4 给test001表删除id=2的记录数:同样产生MDL锁等待
\'tidb03\' root@localhost 12:42:42 (none)>show full processlist;
+--------+------+-----------------+---------+---------+------+---------------------------------+------------------------------------------------------+
| Id | User | Host | db | Command | Time | State | Info |
+--------+------+-----------------+---------+---------+------+---------------------------------+------------------------------------------------------+
| 134004 | root | localhost:40418 | test001 | Sleep | 111 | | NULL |
| 134122 | root | localhost:40806 | test001 | Query | 89 | updating | update test001 set username=\'王五\' where id=2 |
| 134336 | root | localhost:41542 | test001 | Query | 14 | Waiting for table metadata lock | alter table test001 add index idx_username(username) |
| 134384 | root | localhostMySQL找出锁等待