CREATE TABLE `t3` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`a` varchar(5) COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT '',
`b` varchar(5) COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT '',
PRIMARY KEY (`id`),UNIQUE KEY `uk_a` (`a`), KEY `idx_b` (`b`)
)
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sql语句如下
t1
update t3 set b = '' where a = "1";
t2
update t3 set b = '' where b = "2";
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我们先用之前debug的方式来看一下,这里两条语句分别加了哪些锁
第一条语句(通过唯一索引去更新记录)
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
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,写个代码去跑一下
马上就出现了
------------------------
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)
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跟我们线上的死锁日志一模一样