Create Table: CREATE TABLE `t1` (
`id` int(11) NOT NULL,
`b` int(11) DEFAULT NULL,
`c` int(11) DEFAULT NULL,
`d` int(11) DEFAULT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `b` (`b`,`c`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
mysql> insert into t1 select 1,1,2,3;
mysql> insert into t1 select 5,2,2,3;
mysql> select * from t1;
+----+------+------+------+
| id | b | c | d |
+----+------+------+------+
| 1 | 1 | 2 | 3 |
| 5 | 2 | 2 | 3 |
+----+------+------+------+
session A:begin; select * from t1 where b=1 and c=2 for update;
session B:begin; select * from t1 where b=1 and c=2 for update;
sessionA :insert into t1 select 3,1,2,3;
報錯順序是會話B先報錯，會話A再報錯，下面會講。
會話B報錯：ERROR 1213 (40001): Deadlock found when trying to get lock; try restarting transaction
會話A報錯：ERROR 1062 (23000): Duplicate entry '1-2' for key 'b'
輸出下死鎖信息：show engine innodb status\G
------------------------
LATEST DETECTED DEADLOCK
------------------------
2020-01-21 16:25:00 0x7fd5deeb2700
*** (1) TRANSACTION:
TRANSACTION 15117731, ACTIVE 28 sec starting index read
mysql tables in use 1, locked 1
LOCK WAIT 2 lock struct(s), heap size 1136, 1 row lock(s)
MySQL thread id 233174, OS thread handle 140556482070272, query id 5674894 106.1
1.34.12 root statistics
select * from t1 where b=1 and c=2 for update
*** (1) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 21542 page no 4 n bits 72 index b of table `ceshi`.`t1` tr
x id 15117731 lock_mode X locks rec but not gap waiting
Record lock, heap no 2 PHYSICAL RECORD: n_fields 3; compact format; info bits 0
0: len 4; hex 80000001; asc ;;
1: len 4; hex 80000002; asc ;;
2: len 4; hex 80000001; asc ;;
*** (2) TRANSACTION:
TRANSACTION 15117718, ACTIVE 49 sec inserting
mysql tables in use 1, locked 1
4 lock struct(s), heap size 1136, 3 row lock(s), undo log entries 1
MySQL thread id 208385, OS thread handle 140556544714496, query id 5675592 106.1
1.34.12 root executing
insert into t1 select 3,1,2,3
*** (2) HOLDS THE LOCK(S):
RECORD LOCKS space id 21542 page no 4 n bits 72 index b of table `ceshi`.`t1` tr
x id 15117718 lock_mode X locks rec but not gap
Record lock, heap no 2 PHYSICAL RECORD: n_fields 3; compact format; info bits 0
0: len 4; hex 80000001; asc ;;
1: len 4; hex 80000002; asc ;;
2: len 4; hex 80000001; asc ;;
*** (2) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 21542 page no 4 n bits 72 index b of table `ceshi`.`t1` tr
x id 15117718 lock mode S waiting
Record lock, heap no 2 PHYSICAL RECORD: n_fields 3; compact format; info bits 0
0: len 4; hex 80000001; asc ;;
1: len 4; hex 80000002; asc ;;
2: len 4; hex 80000001; asc ;;
*** WE ROLL BACK TRANSACTION (1)
下面分析下加鎖流程：
會話A：對唯一索引（1,2）這條記錄加X鎖。
會話B：對唯一索引（1,2）這條記錄加X鎖，會話A持有鎖，會話B只能等待。
會話A：寫入一條（3,1,2,3）記錄，可以看到1,2會造成唯一鍵沖突，
數據庫會在主鍵沖突的記錄上加S鎖，這時就會有環路出現，
B------->A：B在等A的X鎖。
A------->B：A要拿記錄的S鎖，但要等B的X鎖。
1、數據庫檢測到有死鎖，會自動KILL掉其中一個事務，根據日志信息，可以看到KILL掉了B會話，
2、B會話KILL掉之后，死鎖消失，此時A會繼續執行insert into t1 select 3,1,2,3;這條寫入記錄，會報唯一鍵沖突報錯。