在初学者从源码理解MySQL死锁问题中介绍了使用调试 MySQL 源码的方式来查看死锁的过程，这篇文章来讲讲一个常见的案例。
这次我们讲一段唯一索引 S 锁与 X 锁的爱恨情仇
我们来看一个简化过的例子
# 构造数据
CREATE TABLE `t1` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(10),
`level` int(11),
PRIMARY KEY (`id`),
UNIQUE KEY `uk_name` (`name`)
);
INSERT INTO `t1` (`name`, `level`) VALUES ('A',0);
# 出现问题的sql语句如下，并发情况下就会出现死锁
INSERT ignore INTO `t1` (`name`, `level`) VALUES ('A',0);
update t1 set level = 1 where name = "A";
我们用之前介绍过的源码分析方式，先来看下这两条语句分别加什么锁，然后分析死锁形成的过程。
第一条语句
INSERT ignore INTO t1 (name, level) VALUES ('A',0);在调试中得到的结果如下
可以看到这条语句对唯一键 uk_name 加共享锁（S锁），而且成功。
第二条语句
update t1 set level = 1 where name = "A";
通过唯一键更新数据库字段。
这种情况在之前的文章已经介绍过，会对唯一索引加 X 锁，然后对主键索引加 X 锁
这样就可以非常轻松的复现死锁的问题了，步骤如下
1.开启两个 session，分别 begin
2.session1 执行INSERT ignore INTO t1 (name, level) VALUES ('A',0);
3.session2 执行INSERT ignore INTO t1 (name, level) VALUES ('A',0);
4.session1 执行update t1 set level = 1 where name = "A"; 进入等待状态
5.session2 执行update t1 set level = 1 where name = "A";，死锁产生，被回滚，同时事务 1 执行成功
详细的锁状态变化如下
| t1 | t2 | 备注 |
|---|---|---|
| INSERT IGNORE INTO | - | t1成功获得uk的S锁 DB_SUCCESS |
| - | INSERT IGNORE INTO | t2成功获得uk的S锁 DB_SUCCESS |
| UPDATE | - | t1尝试获得uk的X锁，但没有成功，处于等待状态 DB_LOCK_WAIT |
| - | UPDATE | t2尝试获得uk的X锁，发现死锁产生 DB_DEADLOCK |
| - | Deadlock | t2释放S锁 |
| 成功 | - | - |
死锁日志如下：
LATEST DETECTED DEADLOCK ------------------------ 181208 23:00:52 *** (1) TRANSACTION: TRANSACTION 53A7, ACTIVE 162 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 12, OS thread handle 0x700010522000, query id 1424 localhost root Updating update t1 set level = 1 where name = "A" *** (1) WAITING FOR THIS LOCK TO BE GRANTED: RECORD LOCKS space id 89 page no 4 n bits 72 index `uk_name` of table `lock_demo2`.`t1` trx id 53A7 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 41; asc A;; 1: len 4; hex 80000001; asc ;; *** (2) TRANSACTION: TRANSACTION 53A8, ACTIVE 8 sec starting index read mysql tables in use 1, locked 1 3 lock struct(s), heap size 376, 2 row lock(s) MySQL thread id 96, OS thread handle 0x70001062e000, query id 1425 localhost root Updating update t1 set level = 1 where name = "A" *** (2) HOLDS THE LOCK(S): RECORD LOCKS space id 89 page no 4 n bits 72 index `uk_name` of table `lock_demo2`.`t1` trx id 53A8 lock mode S Record lock, heap no 2 PHYSICAL RECORD: n_fields 2; compact format; info bits 0 0: len 1; hex 41; asc A;; 1: len 4; hex 80000001; asc ;; *** (2) WAITING FOR THIS LOCK TO BE GRANTED: RECORD LOCKS space id 89 page no 4 n bits 72 index `uk_name` of table `lock_demo2`.`t1` trx id 53A8 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 41; asc A;; 1: len 4; hex 80000001; asc ;; *** WE ROLL BACK TRANSACTION (2)
来详细看一下这个死锁日志
*** (1) WAITING FOR THIS LOCK TO BE GRANTED: RECORD LOCKS space id 89 page no 4 n bits 72 index uk_name of table lock_demo2.t1 trx id 53A7 lock_mode X locks rec but not gap waiting
事务 1 想获取 uk_name 唯一索引上的 X 锁 （非 gap 锁的记录锁）
*** (2) HOLDS THE LOCK(S): RECORD LOCKS space id 89 page no 4 n bits 72 index uk_name of table lock_demo2.t1 trx id 53A8 lock mode S
事务 2 持有uk_name 唯一索引上的 S 锁（共享锁）
*** (2) WAITING FOR THIS LOCK TO BE GRANTED: RECORD LOCKS space id 89 page no 4 n bits 72 index uk_name of table lock_demo2.t1 trx id 53A8 lock_mode X locks rec but not gap waiting
事务 2 想获得 uk_name 唯一索引上的 X 锁（非 gap 锁的记录锁）
跟之前理论上推断的结论是一致的
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