MySQL数据库的一次死锁实例分析
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1、故事起因于2016年11月15日的一个生产bug。业务场景是：归档一个表里边的数据到历史表里边，同是删除主表记录。 2、背景场景简化如下（数据库引擎InnoDb，数据隔离级别RR[REPEATABLE]）
-- 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,
name varchar(10) NOT NULL,
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;
3、具体执行顺序
Mysql 官方解释 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。 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。 下面我们来具体的一步步分析下mysql的死锁 1、MySQL常用存储引擎的锁机制 MyISAM和MEMORY采用表级锁(table-level locking) BDB采用页面锁(page-level locking)或表级锁，默认为页面锁 InnoDB支持行级锁(row-level locking)和表级锁，默认为行级锁 2、各种锁特点 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 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 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 3、各种锁的适用场景 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 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 4、死锁 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。 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。 5、死锁举例分析 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。 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。 例如，一个表db。tab_test，结构如下： id：主键; state：状态; time：时间; 索引：idx_1(state，time) 出现死锁日志如下： #63;***(1) TRANSACTION: #63;TRANSACTION 0 677833455, ACTIVE 0 sec, process no 11393, OS thread id 278546 starting index read #63;mysql tables in use 1, locked 1 #63;LOCK WAIT 3 lock struct(s), heap size 320 #63;MySQL thread id 83, query id 162348740 dcnet03 dcnet Searching rows for update #63;update tab_test set state=1064,time=now() where state=1061 and time lt; date_sub(now(), INTERVAL 30 minute) (任务1的sql语句) #63;***(1) WAITING FOR THIS LOCK TO BE GRANTED: (任务1等待的索引记录) #63;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 #63;Record lock, heap no 92 PHYSICAL RECORD: n_fields 11; compact format; info bits 0 #63;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$;; #63;*** (2) TRANSACTION: #63;TRANSACTION 0 677833454, ACTIVE 0 sec, process no 11397, OS thread id 344086 updating or deleting, thread declared inside InnoDB 499 #63;mysql tables in use 1, locked 1 #63;3 lock struct(s), heap size 320, undo log entries 1 #63;MySQL thread id 84, query id 162348739 dcnet03 dcnet Updating update tab_test set state=1067,time=now () where id in (9921180) (任务2的sql语句) #63;*** (2) HOLDS THE LOCK(S): (任务2已获得的锁) #63;RECORD LOCKS space id 0 page no 849384 n bits 208 index `PRIMARY` of table `db/tab_test` trx id 0 677833454 lock_mode X locks rec but not gap #63;Record lock, heap no 92 PHYSICAL RECORD: n_fields 11; compact format; info bits 0 #63;0: len 8; hex 800000000097629c; asc b ;; 1: len 6; hex 00002866eaee; asc (f ;; 2: len 7; hex 00000d40040110; asc @ ;; 3: len 8; hex 80000000000050b2; asc P ;; 4: len 8; hex 800000000000502a; asc P*;; 5: len 8; hex 8000000000005426; asc T;; 6: len 8; hex 800012412c66d29c; asc A,f ;; 7: len 23; hex 75706c6f6164666972652e636f6d2f6 8616e642e706870; asc uploadfire.com/hand.php;; 8: len 8; hex 800000000000042b; asc +;; 9: len 4; hex 474bfa2b; asc GK +;; 10: len 8; hex 8000000000004e24; asc N$;; #63;*** (2) WAITING FOR THIS LOCK TO BE GRANTED: (任务2等待的锁) #63;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 #63;Record lock, heap no 395 PHYSICAL RECORD: n_fields 3; compact format; info bits 0 #63;0: len 8; hex 8000000000000425; asc %;; 1: len 8; hex 800012412c66d29c; asc A,f ;; 2: len 8; hex 800000000097629c; asc b ;; #63;*** WE ROLL BACK TRANSACTION (1) #63;(回滚了任务1，以解除死锁) 原因分析： （编辑：淮北站长网） 【声明】本站内容均来自网络，其相关言论仅代表作者个人观点，不代表本站立场。若无意侵犯到您的权利，请及时与联系站长删除相关内容! |
