由不同的索引更新解决MySQL死锁套路

前几篇文章介绍了用源码的方式来调试锁相关的信息，这里同样用这个工具来解决一个线上实际的死锁案例，也是我们介绍的第一个两条 SQL 就造成死锁的情况。因为线上的表结构比较复杂，做了一些简化以后如下

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CREATE TABLE `t3` (   `id` int (11) NOT NULL AUTO_INCREMENT,   `a` varchar (5),   `b` varchar (5),   PRIMARY KEY (`id`),   UNIQUE KEY `uk_a` (`a`),   KEY `idx_b` (`b`) ) INSERT INTO `t3` (`id`, `a`, `b`) VALUES   (1, '1' , '2' ); # sql语句如下   # 事务1：t1 update t3 set b = '' where a = "1" ;   # 事务2：t2 update t3 set b = '' where b = "2" ;

两条语句造成死锁的情况用手动的方式比较难复现，我们先来分析一下加锁的过程

第一条语句（通过唯一索引去更新记录）

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1	update t3 set b = '' where a = "1" ;

 

整理一下，加了3个X锁，顺序分别是

序号	索引	锁类型
1	uk_a	X
2	PRIMARY	X
3	idx_b	X

第二条语句

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1	update t3 set b = '' where b = "2" ;

整理一下，加了 3 个 X 锁，顺序分别是

序号	索引	锁类型
1	idx_b	X
2	PRIMARY	X
3	idx_b	X

两条语句从加锁顺序看起来就已经有构成死锁的条件了

手动是比较难模拟的，写个代码并发的去同时执行那两条 SQL 语句，马上就出现死锁了

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------------------------ LATEST DETECTED DEADLOCK ------------------------ 181102 12:45:05 *** (1) TRANSACTION : TRANSACTION 50AF, ACTIVE 0 sec starting index read mysql tables in use 1, locked 1 LOCK WAIT 3 lock struct(s), heap size 376, 2 row lock(s) MySQL thread id 34, OS thread handle 0x70000d842000, query id 549 localhost 127.0.0.1 root Searching rows for update update t3 set b = '' where b = "2" *** (1) WAITING FOR THIS LOCK TO BE GRANTED: RECORD LOCKS space id 67 page no 3 n bits 72 index ` PRIMARY ` of table `d1`.`t3` trx id 50AF lock_mode X locks rec but not gap waiting Record lock, heap no 2 PHYSICAL RECORD: n_fields 5; compact format; info bits 0   0: len 4; hex 80000001; asc ;;   1: len 6; hex 0000000050ae; asc P ;;   2: len 7; hex 03000001341003; asc 4 ;;   3: len 1; hex 31; asc 1;;   4: len 0; hex ; asc ;;   *** (2) TRANSACTION : TRANSACTION 50AE, ACTIVE 0 sec updating or deleting mysql tables in use 1, locked 1 4 lock struct(s), heap size 1248, 3 row lock(s), undo log entries 1 MySQL thread id 35, OS thread handle 0x70000d885000, query id 548 localhost 127.0.0.1 root Updating update t3 set b = '' where a = "1" *** (2) HOLDS THE LOCK(S): RECORD LOCKS space id 67 page no 3 n bits 72 index ` PRIMARY ` of table `d1`.`t3` trx id 50AE lock_mode X locks rec but not gap Record lock, heap no 2 PHYSICAL RECORD: n_fields 5; compact format; info bits 0   0: len 4; hex 80000001; asc ;;   1: len 6; hex 0000000050ae; asc P ;;   2: len 7; hex 03000001341003; asc 4 ;;   3: len 1; hex 31; asc 1;;   4: len 0; hex ; asc ;;   *** (2) WAITING FOR THIS LOCK TO BE GRANTED: RECORD LOCKS space id 67 page no 5 n bits 72 index `idx_b` of table `d1`.`t3` trx id 50AE lock_mode X locks rec but not gap waiting Record lock, heap no 2 PHYSICAL RECORD: n_fields 2; compact format; info bits 0   0: len 1; hex 32; asc 2;;   1: len 4; hex 80000001; asc ;;   *** WE ROLL BACK TRANSACTION (1)

分析一下死锁日志

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1 2	*** (1) WAITING FOR THIS LOCK TO BE GRANTED: RECORD LOCKS space id 67 page no 3 n bits 72 index PRIMARY of table d1.t3 trx id 50AF lock_mode X locks rec but not gap waiting

事务2：想获取主键索引的 X 锁

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1 2	*** (2) HOLDS THE LOCK(S): RECORD LOCKS space id 67 page no 3 n bits 72 index PRIMARY of table d1.t3 trx id 50AE lock_mode X locks rec but not gap

事务1：持有主键索引的 X 锁

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1 2	*** (2) WAITING FOR THIS LOCK TO BE GRANTED: RECORD LOCKS space id 67 page no 5 n bits 72 index idx_b of table d1.t3 trx id 50AE lock_mode X locks rec but not gap waiting

事务1：想获取普通索引 idx_b 的 X 锁

与我们分析的完全一致，也与线上的死锁日志一模一样

以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持服务器之家。

原文链接：https://juejin.im/post/5ce3cfa46fb9a07ece67a4a9