MYSQL教程MySQL数据库的一次死锁实例分析

Mysql 官方解释MYSQL入门

Deadlock occurs here because client A needs an X lock to delete the row. However,that lock request cannot be granted because client B already has a request for an X lock and is waiting for client A to release its S lock. Nor can the S lock held by A be upgraded to an X lock because of the prior request by B for an X lock. As a result,InnoDBgenerates an error for one of the clients and releases its locks. The client returns this error.
MYSQL入门

实际场景和mysql文档有些区别,文档里边要获取的是X锁.具体事例里边要获取的是S锁.MYSQL入门

下面我们来具体的一步步分析下mysql的死锁MYSQL入门

1、MySQL常用存储引擎的锁机制MYSQL入门

　　MyISAM和MEMORY采用表级锁(table-level locking)MYSQL入门

　　BDB采用页面锁(page-level locking)或表级锁,默认为页面锁MYSQL入门

　　InnoDB支持行级锁(row-level locking)和表级锁,默认为行级锁MYSQL入门

2、各种锁特点MYSQL入门

　　表级锁：开销小,加锁快;不会出现死锁;锁定粒度大,发生锁冲突的概率最高,并发度最低MYSQL入门

　　行级锁：开销大,加锁慢;会出现死锁;锁定粒度最小,发生锁冲突的概率最低,并发度也最高MYSQL入门

　　页面锁：开销和加锁时间界于表锁和行锁之间;会出现死锁;锁定粒度界于表锁和行锁之间,并发度一般MYSQL入门

3、各种锁的适用场景MYSQL入门

　　表级锁更适合于以查询为主,只有少量按索引条件更新数据的应用,如Web应用MYSQL入门

　　行级锁则更适合于有大量按索引条件并发更新数据,同时又有并发查询的应用,如一些在线事务处理系统MYSQL入门

4、死锁MYSQL入门

　　是指两个或两个以上的进程在执行过程中,因争夺资源而造成的一种互相等待的现象,若无外力作用,它们都将无法推进下去.MYSQL入门

　　表级锁不会产生死锁.所以解决死锁主要还是针对于最常用的InnoDB.MYSQL入门

5、死锁举例分析MYSQL入门

　　在MySQL中,行级锁并不是直接锁记录,而是锁索引.索引分为主键索引和非主键索引两种,如果一条sql语句操作了主键索引,MySQL就会锁定这条主键索引;如果一条语句操作了非主键索引,MySQL会先锁定该非主键索引,再锁定相关的主键索引.MYSQL入门

　　在UPDATE、DELETE操作时,MySQL不仅锁定WHERE条件扫描过的所有索引记录,而且会锁定相邻的键值,即所谓的next-key locking.MYSQL入门

　　例如,一个表db.tab_test,结构如下：MYSQL入门

　　id：主键;MYSQL入门

　　state：状态;MYSQL入门

　　time：时间;MYSQL入门

　　索引：idx_1(state,time)MYSQL入门

　　出现死锁日志如下：MYSQL入门

　　?***(1) TRANSACTION:
　　?TRANSACTION 0 677833455,ACTIVE 0 sec,process no 11393,OS thread id 278546 starting index read
　　?mysql tables in use 1,locked 1
　　?LOCK WAIT 3 lock struct(s),heap size 320
　　?MySQL thread id 83,query id 162348740 dcnet03 dcnet Searching rows for update
　　?update tab_test set state=1064,time=now() where state=1061 and time < date_sub(now(),INTERVAL 30 minute) (任务1的sql语句)
　　?***(1) WAITING FOR THIS LOCK TO BE GRANTED: (任务1等待的索引记录)
　　?RECORD LOCKS space id 0 page no 849384 n bits 208 index `PRIMARY` of table `db/tab_test` trx id 0 677833455 _mode X locks rec but not gap waiting
　　?Record lock,heap no 92 PHYSICAL RECORD: n_fields 11; compact format; info bits 0
　　?0: len 8; hex 800000000097629c; asc b ;; 1: len 6; hex 00002866eaee; asc (f ;; 2: len 7; hex 00000d40040110; asc @ ;; 3: len 8; hex 80000000000050b2; asc P ;; 4: len 8; hex 800000000000502a; asc P*;; 5: len 8; hex 8000000000005426; asc T&;; 6: len 8; hex 800012412c66d29c; asc A,f ;; 7: len 23; hex 75706c6f6164666972652e636f6d2f6 8616e642e706870; asc xxx.com/;; 8: len 8; hex 800000000000042b; asc +;; 9: len 4; hex 474bfa2b; asc GK +;; 10: len 8; hex 8000000000004e24; asc N$;;
　　?*** (2) TRANSACTION:
　　?TRANSACTION 0 677833454,process no 11397,OS thread id 344086 updating or deleting,thread declared inside InnoDB 499
　　?mysql tables in use 1,locked 1
　　?3 lock struct(s),heap size 320,undo log entries 1
　　?MySQL thread id 84,query id 162348739 dcnet03 dcnet Updating update tab_test set state=1067,time=now () where id in (9921180) (任务2的sql语句)
　　?*** (2) HOLDS THE LOCK(S): (任务2已获得的锁)
　　?RECORD LOCKS space id 0 page no 849384 n bits 208 index `PRIMARY` of table `db/tab_test` trx id 0 677833454 lock_mode X locks rec but not gap
　　?Record lock,f ;; 7: len 23; hex 75706c6f6164666972652e636f6d2f6 8616e642e706870; asc uploadfire.com/hand.php;; 8: len 8; hex 800000000000042b; asc +;; 9: len 4; hex 474bfa2b; asc GK +;; 10: len 8; hex 8000000000004e24; asc N$;;
　　?*** (2) WAITING FOR THIS LOCK TO BE GRANTED: (任务2等待的锁)
　　?RECORD LOCKS space id 0 page no 843102 n bits 600 index `idx_1` of table `db/tab_test` trx id 0 677833454 lock_mode X locks rec but not gap waiting
　　?Record lock,heap no 395 PHYSICAL RECORD: n_fields 3; compact format; info bits 0
　　?0: len 8; hex 8000000000000425; asc %;; 1: len 8; hex 800012412c66d29c; asc A,f ;; 2: len 8; hex 800000000097629c; asc b ;;
　　?*** WE ROLL BACK TRANSACTION (1)
　　?(回滚了任务1,以解除死锁)

　　原因分析：MYSQL入门

　　当“update tab_test set state=1064,INTERVAL 30 minute)”执行时,MySQL会使用idx_1索引,因此首先锁定相关的索引记录,因为idx_1是非主键索引,为执行该语句,MySQL还会锁定主键索引.MYSQL入门

　　假设“update tab_test set state=1067,time=now () where id in (9921180)”几乎同时执行时,本语句首先锁定主键索引,由于需要更新state的值,所以还需要锁定idx_1的某些索引记录.MYSQL入门

　　这样第一条语句锁定了idx_1的记录,等待主键索引,而第二条语句则锁定了主键索引记录,而等待idx_1的记录,这样死锁就产生了.MYSQL入门

　　6、解决办法MYSQL入门

　　拆分第一条sql,先查出符合条件的主键值,再按照主键更新记录：MYSQL入门

　　select id from tab_test where state=1061 and time < date_sub(now(),INTERVAL 30 minute);
　　update tab_test state=1064,time=now() where id in(......);

　　关于MySQL死锁问题的实例分析及解决方法就介绍到这里了,希望本次的介绍能够对您有所收获!MYSQL入门

Mysql 官方文档：http://dev.mysql.com/doc/refman/5.7/en/innodb-deadlock-example.htmlMYSQL入门

（编辑：李大同）

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-- 创建表test1 
CREATE TABLE test1 ( 
id int(11) NOT NULL AUTO_INCREMENT,name varchar(10) NOT NULL,PRIMARY KEY (id) 
); 
insert into test1 values('hello'); 
 
-- 创建表test2 
CREATE TABLE test2 ( 
id int(11) NOT NULL AUTO_INCREMENT,PRIMARY KEY (id) 
); 
 
-- Transcation 1 
begin; 
insert into test2 select * from test1 where id = 1; 
delete from test1 where id = 1; 
 
-- Transcation 2 
begin; 
insert into test2 select * from test1 where id = 1;