mysql锁之死锁

    死锁是事务绕不开的话题，mysql当然也不例外，本文主要模拟一下mysql的死锁，以及应对措施。     首先看一个参数，默认innodb_print_all_deadlocks参数是关闭。开启后可以将死锁记录到error.log中。否则只能通过show engine innodb status查看。 mysql> SHOW VARIABLES LIKE 'INNODB_PRINT_ALL_DEADLOCKS'; +----------------------------+-------+ | Variable_name              | Value | +----------------------------+-------+ | innodb_print_all_deadlocks | OFF   | +----------------------------+-------+ 1 row in set (0.00 sec) 开启innodb_print_all_deadlocks,改参数是全局参数，可以动态调整。 mysql> SET GLOBAL innodb_print_all_deadlocks=1; Query OK, 0 rows affected (0.00 sec) 表test01上c1是主键，c2是唯一约束。 mysql> show create table test01\G *************************** 1. row ***************************        Table: test01 Create Table: CREATE TABLE `test01` (   `c1` bigint(20) NOT NULL AUTO_INCREMENT,   `c2` int(11) DEFAULT NULL,   PRIMARY KEY (`c1`),   UNIQUE KEY `uidx_test01_c2` (`c2`) ) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8 1 row in set (0.00 sec) mysql> select * from test01; +----+------+ | c1 | c2   | +----+------+ |  1 |    1 | |  2 |    2 | |  3 |    3 | +----+------+ 3 rows in set (0.00 sec) 会话A mysql> begin ; Query OK, 0 rows affected (0.00 sec) mysql> delete from test01 where c2=2; Query OK, 1 row affected (0.00 sec) 会话B mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> delete from test01 where c2=2; ERROR 1213 (40001): Deadlock found when trying to get lock; try restarting transaction 会话A mysql> insert into test01 select 2,2; Query OK, 1 row affected (0.00 sec) Records: 1  Duplicates: 0  Warnings: 0 会话A在（2,2）上加了X的行锁，会话B要删除相同的数据行，那么也要在该行上加X的行锁（lock_mode X locks rec but not gap），所以出现了等待（lock_mode X waiting）。后面会话A要插入一行（2,2），因为字段c2上有唯一索引，插入的时候要检查duplicate key的检查，这个过程需要申请S的锁，而在得到这个锁之前，它需要等会话B先得到会话A最开始执行的X锁。也就是说，会话B要等待会话A第一条语句释放X锁，会话A第二条语句又要等待会话B释放X锁，两个会话之间形成了等待的闭合回路，形成了死锁。出现死锁后，mysql会选择一个小事务进行回滚，以解决死锁。 show engine innodb status查看死锁信息，error.log中记录的死锁也类似下面： ------------------------ LATEST DETECTED DEADLOCK ------------------------ 2019-07-18 11:11:32 0x7fdc50298700 *** (1) TRANSACTION: TRANSACTION 713521, ACTIVE 122 sec starting index read mysql tables in use 1, locked 1 LOCK WAIT 2 lock struct(s), heap size 1136, 2 row lock(s) MySQL thread id 4, OS thread handle 140584214165248, query id 144 localhost root updating delete from test01 where c2=2 *** (1) WAITING FOR THIS LOCK TO BE GRANTED: RECORD LOCKS space id 25 page no 4 n bits 72 index uidx_test01_c2 of table `ming`.`test01` trx id 713521 lock_mode X waiting Record lock, heap no 3 PHYSICAL RECORD: n_fields 2; compact format; info bits 32  0: len 4; hex 80000002; asc     ;;  1: len 8; hex 8000000000000002; asc         ;; *** (2) TRANSACTION: TRANSACTION 713523, ACTIVE 16 sec inserting mysql tables in use 1, locked 1 4 lock struct(s), heap size 1136, 3 row lock(s), undo log entries 2 MySQL thread id 3, OS thread handle 140584214431488, query id 146 localhost root executing insert into test01 select 2,2 *** (2) HOLDS THE LOCK(S): RECORD LOCKS space id 25 page no 4 n bits 72 index uidx_test01_c2 of table `ming`.`test01` trx id 713523 lock_mode X locks rec but not gap Record lock, heap no 3 PHYSICAL RECORD: n_fields 2; compact format; info bits 32  0: len 4; hex 80000002; asc     ;;  1: len 8; hex 8000000000000002; asc         ;; *** (2) WAITING FOR THIS LOCK TO BE GRANTED: RECORD LOCKS space id 25 page no 4 n bits 72 index uidx_test01_c2 of table `ming`.`test01` trx id 713523 lock mode S waiting Record lock, heap no 3 PHYSICAL RECORD: n_fields 2; compact format; info bits 32  0: len 4; hex 80000002; asc     ;;  1: len 8; hex 8000000000000002; asc         ;; 针对死锁的举措： 1.如果死锁较多的话，那么建议开启innodb_print_all_deadlocks,因为show engine innodb status只显示最近一次的死锁信息。 2.保持事务小而短，并尽快提价 3.避免在一个事务里面修改几张表，或者是同一张表修改不同的结果集 4.可以尝试使用较低的隔离级别，比如RC。也可以使用锁定读 5.建立合适的索引 6.如果应用允许的话，串行化事务