在数据库中，除传统的计算资源(CPU、RAM、IO)的争用以外，数据也是一种供许多用户共享的资源。如何保证数据并发访问的一致性、有效性是所有数据库必须解决的一个问题，锁冲突也是影响数据库并发访问性能的一个重要因素。从这个角度来说，锁对教据库而言显得尤其重要，也更加复杂。
本文将通过实验介绍MySQL8.0版锁该如何排查，以及找到阻塞的sql语句，实验的MySQL版本为8.0.26，隔离级别为RR。
information_schema.innodb_trx ##正在运行的事务信息。
sys.innodb_lock_waits ##处于锁等待的关联事务信息。
performance_schema.threads ##SQL线程及线程号、进程号、OS线程号等信息
# 确认有没有锁等待:
show status like 'innodb_row_lock%';
select * from information_schema.innodb_trx;
# 查询锁等待详细信息
select * from sys.innodb_lock_waits; ----> blocking_pid(锁源的连接线程)
# 通过连接线程ID找SQL线程语句
select * from performance_schema.threads;
# 通过SQL线程找到SQL语句
select * from performance_schema.events_statements_history;
mysql> use world;
mysql> show tables;
+-----------------+
| Tables_in_world |
+-----------------+
| city |
| country |
| countrylanguage |
+-----------------+
3 rows in set (0.00 sec)
s1:
# 加排他锁
mysql> begin;
mysql> select * from world.city where id=1 for update;
s2:
# 加排他锁
mysql> begin;
mysql> update city set name='girl' where id=1;
执行完处于夯住状态，默认50秒会超时回滚。
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> set innodb_lock_wait_timeout=5000; ##锁等待超时参数,这里设置为5000便于测试.
mysql> update city set name='girl' where id=1;
mysql> use information_schema;
mysql> select trx_id,trx_state,trx_started,trx_tables_locked,trx_rows_locked from innodb_trx\G
*************************** 1. row ***************************
trx_id: 8995 ##刚刚运行的第二个语句事务ID
trx_state: LOCK WAIT ##处于锁等待状态
trx_started: 2022-12-23 16:00:42
trx_tables_locked: 1 ##锁了1张表
trx_rows_locked: 2 ##锁了2行
*************************** 2. row ***************************
trx_id: 8994 ##刚刚运行的第一个语句事务ID
trx_state: RUNNING ##获得锁的状态
trx_started: 2022-12-23 15:59:41
trx_tables_locked: 1
trx_rows_locked: 1
2 rows in set (0.00 sec)
mysql> select * from sys.innodb_lock_waits\G
*************************** 1. row ***************************
wait_started: 2022-12-23 16:01:57
wait_age: 00:00:52
wait_age_secs: 52
locked_table: `world`.`city`
locked_table_schema: world
locked_table_name: city
locked_table_partition: NULL
locked_table_subpartition: NULL
locked_index: PRIMARY
locked_type: RECORD
waiting_trx_id: 8995
waiting_trx_started: 2022-12-23 16:00:42
waiting_trx_age: 00:02:07
waiting_trx_rows_locked: 2
waiting_trx_rows_modified: 0
waiting_pid: 33
waiting_query: update city set name='girl' where id=1
waiting_lock_id: 140190433225944:16:6:2:140190349859736
waiting_lock_mode: X,REC_NOT_GAP
blocking_trx_id: 8994 ##阻塞者事务ID
blocking_pid: 32 ##阻塞者进程ID, show processlist可查;
blocking_query: NULL
blocking_lock_id: 140190433226752:16:6:2:140190349865536
blocking_lock_mode: X,REC_NOT_GAP
blocking_trx_started: 2022-12-23 15:59:41
blocking_trx_age: 00:03:08
blocking_trx_rows_locked: 1
blocking_trx_rows_modified: 0
sql_kill_blocking_query: KILL QUERY 32
sql_kill_blocking_connection: KILL 32 ##解锁方法
1 row in set (0.00 sec)
mysql> show processlist;
+----+-----------------+-----------------+--------------------+---------+-------+------------------------+-----------------------------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+-----------------+-----------------+--------------------+---------+-------+------------------------+-----------------------------------------+
| 5 | event_scheduler | localhost | NULL | Daemon | 27235 | Waiting on empty queue | NULL |
| 29 | root | localhost:43074 | information_schema | Query | 0 | init | show processlist |
| 32 | root | localhost:43080 | world | Sleep | 248 | | NULL |
| 33 | root | localhost:43082 | world | Query | 112 | updating | update city set name='girl' where id=1 |
+----+-----------------+-----------------+--------------------+---------+-------+------------------------+-----------------------------------------+
4 rows in set (0.00 sec)
mysql> select thread_id,processlist_id from performance_schema.threads where processlist_id=32;
+-----------+----------------+
| thread_id | processlist_id |
+-----------+----------------+
| 75 | 32 |
+-----------+----------------+
1 row in set (0.00 sec)
mysql> select thread_id,sql_text from performance_schema.events_statements_history where thread_id=75\G
*************************** 1. row ***************************
thread_id: 75
sql_text: NULL
*************************** 2. row ***************************
thread_id: 75
sql_text: NULL
*************************** 3. row ***************************
thread_id: 75
sql_text: NULL
*************************** 4. row ***************************
thread_id: 75
sql_text: show tables
*************************** 5. row ***************************
thread_id: 75
sql_text: set autocommit=0
*************************** 6. row ***************************
thread_id: 75
sql_text: begin
*************************** 7. row ***************************
thread_id: 75
sql_text: select * from world.city where id=1 for update
*************************** 8. row ***************************
thread_id: 75
sql_text: NULL
*************************** 9. row ***************************
thread_id: 75
sql_text: show databases
*************************** 10. row ***************************
thread_id: 75
sql_text: show tables
10 rows in set (0.00 sec)
找到select * from world.city where id=1 for update语句，确认后如果没问题可以kill掉。
kill 32;
set innodb_lock_wait_timeout=500; #设置回滚申请锁的事务的时间。
innodb_lock_wait_timeout指的是事务等待获取资源等待的最长时间，超过这个时间还未分配到资源则会返回应用失败；
参数的时间单位是秒，最小可设置为1s，最大可设置1073741824秒（34年）,默认安装时这个值是50s.
当锁等待超过设置时间的时候，就会报如下的错误；
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
参数支持范围为session和global，且支持动态修改，所以可以通过两种方法修改；
- 通过语句修改
set innodb_lock_wait_timeout=50;
set global innodb_lock_wait_timeout=50;
注意global的修改对当前线程是不生效的，只有建立新的连接才生效
- 修改参数文件/etc/my.cnf
innodb_lock_wait_timeout = 50
