死鎖案例二
MySQL5.6.33,隔離級別是RR。表結構及數據:
Create table t1(id int not null primary key auto_increment,c1 int,c2 int,c3 int, unique key(c1),unique key(c2)); insert into t1(c1,c2,c3) values(1,3,4),(6,6,10),(9,9,14);
2、測試用例
session1 | session2 | session3 |
begin; | begin; | begin; |
insert into t1 (c1,c2,c3) values(4,4,1); | ||
insert into t1 (c1,c2,c3) values(4,4,2); | ||
insert into t1 (c1,c2,c3) values(4,4,3); | ||
commit; | ||
Update t1 set c3=5 where c1=4; | ||
update t1 set c3=5 where c1=4; |
3、死鎖日誌
------------------------ LATEST DETECTED DEADLOCK ------------------------ 2018-07-07 06:27:15 a347bb90 *** (1) TRANSACTION: TRANSACTION 7973, ACTIVE 43 sec starting index read mysql tables in use 1, locked 1 LOCK WAIT 3 lock struct(s), heap size 320, 2 row lock(s) MySQL thread id 4, OS thread handle 0xa34acb90, query id 75 localhost root updating Update t1 set c3=5 where c1=4 *** (1) WAITING FOR THIS LOCK TO BE GRANTED: RECORD LOCKS space id 18 page no 4 n bits 72 index `c1` of table `yzs`.`t1` trx id 7973 lock_mode X locks rec but not gap waiting Record lock, heap no 5 PHYSICAL RECORD: n_fields 2; compact format; info bits 0 0: len 4; hex 80000004; asc ;; 1: len 4; hex 80000006; asc ;; *** (2) TRANSACTION: TRANSACTION 7974, ACTIVE 33 sec starting index read mysql tables in use 1, locked 1 3 lock struct(s), heap size 320, 2 row lock(s) MySQL thread id 5, OS thread handle 0xa347bb90, query id 76 localhost root updating Update t1 set c3=5 where c1=4 *** (2) HOLDS THE LOCK(S): RECORD LOCKS space id 18 page no 4 n bits 72 index `c1` of table `yzs`.`t1` trx id 7974 lock mode S Record lock, heap no 5 PHYSICAL RECORD: n_fields 2; compact format; info bits 0 0: len 4; hex 80000004; asc ;; 1: len 4; hex 80000006; asc ;; *** (2) WAITING FOR THIS LOCK TO BE GRANTED: RECORD LOCKS space id 18 page no 4 n bits 72 index `c1` of table `yzs`.`t1` trx id 7974 lock_mode X locks rec but not gap waiting Record lock, heap no 5 PHYSICAL RECORD: n_fields 2; compact format; info bits 0 0: len 4; hex 80000004; asc ;; 1: len 4; hex 80000006; asc ;; *** WE ROLL BACK TRANSACTION (2)
4、分析死鎖日誌
從死鎖日誌上可以看到:
TRANSACTION 7973:
Update t1 set c3=5 where c1=4語句在等待二級索引c1上(4,6)上的X類型的記錄鎖(lock_mode X locks rec but not gap)
TRANSACTION 7974:
擁有二級索引c1上(4,6)上S類型的next key鎖(lock mode S),等待申請(4,6)上的記錄鎖(lock_mode X locks rec but not gap)
但從死鎖日誌上只看到兩個update語句互相等待,不知道業務邏輯場景的話,很難找到原因。
註:這裏留下疑問,為什麽主鍵是6呢,不是4?這個和自增鍵有關,關於自增值這裏不做過多考慮,感興趣的自行測試分析。
5、加鎖原理
1)關於insert唯一鍵加鎖時重復鍵判斷加S類型next-key鎖的加鎖原理見之前博客:
https://blog.csdn.net/yanzongshuai/article/details/79326637
以及https://blog.csdn.net/yanzongshuai/article/details/79301868
註意,這裏發生重復鍵加S 類型next key鎖時,不論是什麽隔離級別,都會加這樣的鎖。
2)關於隱式鎖轉換顯式鎖流程見之前博客:
https://blog.csdn.net/yanzongshuai/article/details/79306514
https://blog.csdn.net/yanzongshuai/article/details/79254031
https://blog.csdn.net/yanzongshuai/article/details/79252679
3)關於update加鎖原理見之前博客:
https://blog.csdn.net/yanzongshuai/article/details/80870949
https://blog.csdn.net/yanzongshuai/article/details/80870957
https://blog.csdn.net/yanzongshuai/article/details/80872095
6、解析
1)session1執行insert into t1 (c1,c2,c3) values(4,4,1);實際上是沒有加任何鎖的。
2)session2執行insert into t1 (c1,c2,c3) values(4,4,2);二級索引(4)和session1的發生沖突,使session1的隱式鎖轉換成顯式鎖;發生唯一沖突,則對(4)加S類型的next key鎖,此時session1已經加了X鎖,發生等待;
3)session3執行insert into t1 (c1,c2,c3) values(4,4,3);同理,等待session1釋放二級索引c1(4)上的X鎖,申請S類型的next key鎖。
4)session1執行commit後,session2和session3報錯:ERROR 1062 (23000): Duplicate entry '4' for key 'c1',同時會申請到S類型的next key鎖。
5)session2執行Update t1 set c3=5 where c1=4;從之前博客
https://blog.csdn.net/yanzongshuai/article/details/80872095
可知在search階段會對二級索引記錄(4)申請X類型的記錄鎖。session3已擁有S類型next key鎖,所以發生等待;
6)session3再執行Update t1 set c3=5 where c1=4;同理,會申請X類型的記錄鎖,等待session2釋放其S類型next key鎖。此時發生死鎖。
7、解決方法
楊奇龍老師解釋可以使用使用insert on duplicate key語句來代替原來的insert語句。這2個語句的加鎖不一樣,感興趣的可以研究下。
8、參考
https://mp.weixin.qq.com/s/96CDhpgu5uUQ7qKYhKgt3w
死鎖案例二