一、说明
本文将通过实验介绍mysql锁该如何排查，本实验的mysql版本为8.0.23，隔离级别为RC。
二、实验
场景一：会话a开启事务，删除全部数据。会话b开始事务，删除部分数据
会话A:
(root@localhost)[hello]> begin;
(root@localhost)[hello]> delete from x1;
(root@localhost)[hello]> delete from x2;
(root@localhost)[hello]> select * from x1;
(root@localhost)[hello]> select * from x2;
会话B:
(root@localhost)[hello]> begin;
(root@localhost)[hello]> delete from x1 limit 1; # 此处发生等待
再起一个窗口，查看会话列表
ID 34即会话A已执行完成，状态为Sleep。ID 37即会话B正在执行，状态为Query。
(root@localhost)[(none)]> select * from information_schema.processlist;
+----+------+-----------+-------+---------+------+-----------+----------------------------------------------+
| ID | USER | HOST | DB | COMMAND | TIME | STATE | INFO |
+----+------+-----------+-------+---------+------+-----------+----------------------------------------------+
| 34 | root | localhost | hello | Sleep | 49 | | NULL |
| 36 | root | localhost | NULL | Query | 0 | executing | select * from information_schema.processlist |
| 37 | root | localhost | hello | Query | 12 | updating | delete from x1 limit 1 |
+----+------+-----------+-------+---------+------+-----------+----------------------------------------------+
查看会话阻塞情况，waiting_pid、waiting_trx_id、blocking_pid、blocking_trx_id，分别表示被阻塞的事务和导致阻塞的事务。
结果显示事务23641阻塞了事务23654。
(root@localhost)[(none)]> select locked_table_schema, locked_table_name, waiting_query, waiting_pid, waiting_trx_id, blocking_pid, blocking_trx_id from sys.innodb_lock_waits;
+---------------------+-------------------+------------------------+-------------+----------------+--------------+-----------------+
| locked_table_schema | locked_table_name | waiting_query | waiting_pid | waiting_trx_id | blocking_pid | blocking_trx_id |
+---------------------+-------------------+------------------------+-------------+----------------+--------------+-----------------+
| hello | x1 | delete from x1 limit 1 | 37 | 23654 | 34 | 23641 |
+---------------------+-------------------+------------------------+-------------+----------------+--------------+-----------------+
查看锁的信息，engine_transaction_id对应事务编号，thread_id对应线程编号。
前面我们查到事务编号23641阻塞了事务编号23654，这里显示事务编号23641对应线程85，事务编号23654对应线程88。
(root@localhost)[(none)]> select distinct engine_transaction_id, thread_id, object_schema, object_name, lock_type, lock_mode, lock_status from performance_schema.data_locks;
+-----------------------+-----------+---------------+-------------+-----------+---------------+-------------+
| engine_transaction_id | thread_id | object_schema | object_name | lock_type | lock_mode | lock_status |
+-----------------------+-----------+---------------+-------------+-----------+---------------+-------------+
| 23654 | 88 | hello | x1 | TABLE | IX | GRANTED |
| 23654 | 88 | hello | x1 | RECORD | X,REC_NOT_GAP | WAITING |
| 23641 | 85 | hello | x1 | TABLE | IX | GRANTED |
| 23641 | 85 | hello | x1 | RECORD | X,REC_NOT_GAP | GRANTED |
| 23641 | 85 | hello | x2 | TABLE | IX | GRANTED |
| 23641 | 85 | hello | x2 | RECORD | X,REC_NOT_GAP | GRANTED |
+-----------------------+-----------+---------------+-------------+-----------+---------------+-------------+
查看造成阻塞的线程历史执行的语句，其中thread_id对应performance_schema.data_locks中的thread_id
结果显示线程85执行过“delete from x1”，这才是线程88执行“delete from x1 limit 1”等待的原因。
(root@localhost)[(none)]> select timer_start, thread_id, sql_text, message_text from performance_schema.events_statements_history where event_name like 'statement/sql%' and thread_id in (85) order by timer_start desc;
+-------------------+-----------+----------------------------------+--------------+
| timer_start | thread_id | sql_text | message_text |
+-------------------+-----------+----------------------------------+--------------+
| 22461001917916000 | 85 | select * from x2 | NULL |
| 22460129443671000 | 85 | select * from x1 | NULL |
| 22460050982468000 | 85 | delete from x2 | NULL |
| 22460020034606000 | 85 | delete from x1 | NULL |
| 22459990044910000 | 85 | begin | NULL |
| 22416295653612000 | 85 | select USER() | NULL |
| 22416294129231000 | 85 | select @@version_comment limit 1 | NULL |
+-------------------+-----------+----------------------------------+--------------+
将上面所有的语句进行合并，就可以得到锁的完整信息了
(root@localhost)[(none)]> select distinct a.wait_started, a.locked_table, a.waiting_query, a.waiting_pid, a.blocking_pid, c.sql_text
from sys.innodb_lock_waits a
join performance_schema.data_locks b on (a.blocking_trx_id = b.engine_transaction_id and a.locked_table_schema = b.object_schema and a.locked_table_name = b.object_name)
join performance_schema.events_statements_history c on (b.thread_id = c.thread_id and c.sql_text regexp b.object_name);
+---------------------+--------------+------------------------+-------------+--------------+------------------+
| wait_started | locked_table | waiting_query | waiting_pid | blocking_pid | sql_text |
+---------------------+--------------+------------------------+-------------+--------------+------------------+
| 2022-02-16 16:26:25 | `hello`.`x1` | delete from x1 limit 1 | 37 | 34 | delete from x1 |
| 2022-02-16 16:26:25 | `hello`.`x1` | delete from x1 limit 1 | 37 | 34 | select * from x1 |
+---------------------+--------------+------------------------+-------------+--------------+------------------+
场景二：会话a对表加元数据锁。会话b开始事务，删除部分数据
会话A:
(root@localhost)[hello]> lock table x1 write;
会话B:
(root@localhost)[hello]> delete from x1 limit 1; # 此处发生等待
查看会话列表
ID 39即会话A已执行完成，状态为Sleep。ID 40即会话B正在执行，状态为Query。
(root@localhost)[(none)]> select * from information_schema.processlist;
+----+------+-----------+-------+---------+------+---------------------------------+----------------------------------------------+
| ID | USER | HOST | DB | COMMAND | TIME | STATE | INFO |
+----+------+-----------+-------+---------+------+---------------------------------+----------------------------------------------+
| 49 | root | localhost | hello | Sleep | 81 | | NULL |
| 51 | root | localhost | NULL | Query | 0 | executing | select * from information_schema.processlist |
| 52 | root | localhost | hello | Query | 9 | Waiting for table metadata lock | delete from x1 limit 1 |
+----+------+-----------+-------+---------+------+---------------------------------+----------------------------------------------+
查看innodb_lock_waits和data_locks，都没有数据。
(root@localhost)[(none)]> select * from sys.innodb_lock_waits;
Empty set (0.00 sec)
(root@localhost)[(none)]> select * from performance_schema.data_locks;
Empty set (0.00 sec)
查看当前数据库执行过的sql，能找到锁表的语句，注意这里的thread_id并不对应processlist表中的id。
(root@localhost)[(none)]> select thread_id, event_name, sql_text from performance_schema.events_statements_current;
+-----------+---------------------------+------------------------------------------------------------------------------------------+
| thread_id | event_name | sql_text |
+-----------+---------------------------+------------------------------------------------------------------------------------------+
| 100 | statement/sql/lock_tables | lock table x1 write |
| 102 | statement/sql/select | select thread_id, event_name, sql_text from performance_schema.events_statements_current |
| 103 | statement/sql/delete | delete from x1 limit 1 |
+-----------+---------------------------+------------------------------------------------------------------------------------------+
查看threads表，这里的thread_id对应events_statements_current表中thread_id，processlist_id对应processlist表中id。
(root@localhost)[(none)]> select thread_id, processlist_id, processlist_user, processlist_db, processlist_command, processlist_time, processlist_state, processlist_info from performance_schema.threads where type = 'FOREGROUND';
+-----------+----------------+------------------+----------------+---------------------+------------------+---------------------------------+------------------------+
| thread_id | processlist_id | processlist_user | processlist_db | processlist_command | processlist_time | processlist_state | processlist_info |
+-----------+----------------+------------------+----------------+---------------------+------------------+---------------------------------+------------------------+
| 48 | 6 | NULL | NULL | Daemon | 31219 | Suspending | NULL |
| 100 | 49 | root | hello | Sleep | 172 | NULL | NULL |
| 103 | 52 | root | hello | Query | 100 | Waiting for table metadata lock | delete from x1 limit 1 |
+-----------+----------------+------------------+----------------+---------------------+------------------+---------------------------------+------------------------+
将以上sql进行合并，从sql_text中找锁表的语句，杀掉对应的会话即可。
(root@localhost)[(none)]> select b.processlist_id, b.processlist_user, b.processlist_db, b.processlist_command, b.processlist_time, b.processlist_state, b.processlist_info, a.event_name, a.sql_text
from performance_schema.events_statements_current a
join performance_schema.threads b on (a.thread_id = b.thread_id);
+----------------+------------------+----------------+---------------------+------------------+---------------------------------+-------------------------+---------------------------+------------------------+
| processlist_id | processlist_user | processlist_db | processlist_command | processlist_time | processlist_state | processlist_info | event_name | sql_text |
+----------------+------------------+----------------+---------------------+------------------+---------------------------------+-------------------------+---------------------------+------------------------+
| 49 | root | hello | Sleep | 289 | NULL | NULL | statement/sql/lock_tables | lock table x1 write |
| 52 | root | hello | Query | 217 | Waiting for table metadata lock | delete from x1 limit 1 | statement/sql/delete | delete from x1 limit 1 |
+----------------+------------------+----------------+---------------------+------------------+---------------------------------+-------------------------+---------------------------+------------------------+
三、监控
查看事务信息
(root@localhost)[(none)]> select trx_id, trx_state, trx_started, trx_mysql_thread_id, trx_query from information_schema.innodb_trx;
+--------+-----------+---------------------+---------------------+------------------------+
| trx_id | trx_state | trx_started | trx_mysql_thread_id | trx_query |
+--------+-----------+---------------------+---------------------+------------------------+
| 23654 | LOCK WAIT | 2022-02-16 16:26:25 | 37 | delete from x1 limit 1 |
| 23641 | RUNNING | 2022-02-16 16:25:47 | 34 | NULL |
+--------+-----------+---------------------+---------------------+------------------------+
因此可以通过下面的语句查出当前最长的事务。
(root@localhost)[(none)]> select max(time_to_sec(timediff(now(), trx_started))) max_trx_time from information_schema.innodb_trx;
+--------------+
| max_trx_time |
+--------------+
| 571 |
+--------------+
四、总结
- 等待行锁最大的等待时长参数为innodb_lock_wait_timeout，默认50s。等待元数据锁最大的等待时长参数为lock_wait_timeout，默认31536000s（一年）。建议对lock_wait_timeout进行调整。
- 通过information_schema.innodb_trx对数据库进行长事务监控。
- 通过sys.innodb_lock_waits查看被阻塞的sql
- 元数据锁通过performance_schema.events_statements_current，performance_schema.threads关联起来看