Mysql之死锁案例

删除后插入死锁问题

环境准备

1. 数据库版本

   $ mysql -V
   mysql  Ver 14.14 Distrib 5.6.35, for macos10.12 (x86_64) using  EditLine wrapper
   或者
   mysql> select version();
   +-----------+
   | version() |
   +-----------+
   | 5.6.35    |
   +-----------+
   

2. sql语句

   DROP TABLE IF EXISTS `t_lock`;
   CREATE TABLE `t_lock` (
     `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
     `a` varchar(10) DEFAULT NULL,
     PRIMARY KEY (`id`),
     UNIQUE KEY `unique_a` (`a`)
   ) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;
   

开启监控

1. 开启监控

   //第一种方式:基于系统参数
   set GLOBAL innodb_status_output=ON  开启
   set GLOBAL innodb_status_output=OFF 关闭
   //第二种方式:创建innodb_monitor的数据表
   CREATE TABLE innodb_monitor (a INT) ENGINE=INNODB;
   DROP TABLE innodb_monitor;
   

步骤

1. 数据库信息相关操作

   # mysql -u root -p
   # use lock
   //从innodb status获取更多的锁信息
   # set GLOBAL innodb_status_output_locks=ON;
   

   查看已有数据

   mysql> select * from t_lock;
   +----+------+
   | id | a    |
   +----+------+
   |  1 | a    |
   |  2 | b    |
   +----+------+
   2 rows in set (0.00 sec)  
   

   查看mysql事务隔离级别

   //查看mysql系统的事务隔离级别
    mysql> SELECT @@global.tx_isolation;
    +-----------------------+
    | @@global.tx_isolation |
    +-----------------------+
    | REPEATABLE-READ       |
    +-----------------------+
    1 row in set (0.00 sec)
       
   

   查看mysql会话的事务隔离级别

   mysql> SELECT @@tx_isolation;
   +-----------------+
   | @@tx_isolation  |
   +-----------------+
   | REPEATABLE-READ |
   +-----------------+
   1 row in set (0.00 sec)
    
   mysql> SELECT @@session.tx_isolation;
   +------------------------+
   | @@session.tx_isolation |
   +------------------------+
   | REPEATABLE-READ        |
   +------------------------+
   1 row in set (0.00 sec)
   

   查看是否自动提交事务(默认是自动提交)

   mysql> select @@autocommit;
   +--------------+
   | @@autocommit |
   +--------------+
   |            1 |
   +--------------+
   1 row in set (0.00 sec)
   

   设置不自动提交事务

   mysql> set autocommit = 0;
   Query OK, 0 rows affected (0.00 sec)
   

2. 事务执行

   session1	session2
   mysql> begin; @1
   	mysql> begin; @11
   mysql> delete from t_lock where a = ‘aa’;  @2
   	mysql> delete from t_lock where a = ‘aa’; @12
   	mysql> insert into t_lock(a) values (‘aa’); @13 (等待锁)
   insert into t_lock(a) values ( ‘aa’);   @3 死锁

       @13:此处执行时，命令行处于等待状态。
       当@3执行完毕时，出现`ERROR 1213 (40001): Deadlock found when trying to get lock; try restarting transaction`异常时
       @13此时打印`Query OK, 1 row affected (4.19 sec)`
   

3. 查看死锁日志

       ------------------------
       LATEST DETECTED DEADLOCK
       ------------------------
       2019-02-20 14:38:27 700009ce6000
       *** (1) TRANSACTION:
       //事务编号6719766，活跃11s   
       TRANSACTION 6719766, ACTIVE 11 sec inserting
       mysql tables in use 1, locked 1
       ///LOCK WAIT表示正在等待锁，获得3个锁结构
       LOCK WAIT 3 lock struct(s), heap size 360, 2 row lock(s), undo log entries 1
       MySQL thread id 3, OS thread handle 0x700009c62000, query id 95 localhost root update
       insert into t_lock(a) values ('aa')
       *** (1) WAITING FOR THIS LOCK TO BE GRANTED:
       //等待insert intention(插入意向锁) waiting
       RECORD LOCKS space id 4921 page no 4 n bits 72 index `unique_a` of table `lock`.`t_lock` trx id 6719766 lock_mode X locks gap before rec insert intention waiting
       Record lock, heap no 3 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
        0: len 1; hex 62; asc b;;
        1: len 8; hex 0000000000000002; asc         ;;
       
       *** (2) TRANSACTION:
       TRANSACTION 6719765, ACTIVE 18 sec inserting
       mysql tables in use 1, locked 1
       3 lock struct(s), heap size 360, 2 row lock(s), undo log entries 1
       MySQL thread id 9, OS thread handle 0x700009ce6000, query id 96 localhost root update
       insert into t_lock(a) values ( 'aa')
       *** (2) HOLDS THE LOCK(S):
       //locks gap before rec  表示为gap锁
       RECORD LOCKS space id 4921 page no 4 n bits 72 index `unique_a` of table `lock`.`t_lock` trx id 6719765 lock_mode X locks gap before rec
       Record lock, heap no 3 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
        0: len 1; hex 62; asc b;;
        1: len 8; hex 0000000000000002; asc         ;;
       
       *** (2) WAITING FOR THIS LOCK TO BE GRANTED:
       // 等待insert intention(插入意向锁) waiting
       RECORD LOCKS space id 4921 page no 4 n bits 72 index `unique_a` of table `lock`.`t_lock` trx id 6719765 lock_mode X locks gap before rec insert intention waiting
       Record lock, heap no 3 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
        0: len 1; hex 62; asc b;;
        1: len 8; hex 0000000000000002; asc         ;;
        //回滚事务2
        *** WE ROLL BACK TRANSACTION (2)
     
   

   当session1执行删除语句时，由于条件a = 'aa'的记录不存在。session1 获得2个锁结构，分别是排他锁（行级锁）、gap锁（行级锁）。session2执行删除时，由于条件a = 'aa'的记录不存在，session2也获得2个锁结构。session1, session2 都在等待插入意向锁， 插入意向锁与gap锁冲突，双方都没有释放gap锁，又都在等待插入意向锁，死锁发生。

   session1	session2
   begin;
   	begin;
   delete from t_lock where a = ‘aa’;
   	delete from t_lock where a = ‘aa’; 等待
   insert into t_lock(a) values ( ‘aa’);
   	死锁错误

解决方案

1. 在不修改隔离级别的情况下(RR),修改业务代码逻辑，删除记录之前，先select，确认该记录存在，再执行delete删除该记录。

插入更新导致的死锁

1. 表结构

   DROP TABLE IF EXISTS `t_test`;
   CREATE TABLE `t_test` (
     `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
     `a` varchar(10) CHARACTER SET utf8 NOT NULL,
     `b` varchar(10) CHARACTER SET utf8 NOT NULL,
     PRIMARY KEY (`id`)
   ) ENGINE=InnoDB  DEFAULT CHARSET=utf8 COLLATE=utf8_bin;
   
     BEGIN;
   INSERT INTO `t_test` VALUES (1, 'a1', 'b1');
   INSERT INTO `t_test` VALUES (2, 'a2', 'b2');
   INSERT INTO `t_test` VALUES (3, 'a3', 'b3');
   INSERT INTO `t_test` VALUES (4, 'a4', 'b4');
   COMMIT;
   
   

2. 第一种情况，因为没有索引，所以会锁表

   时间序	Session1	Session2
   1	BEGIN;
   2		BEGIN;
   3	insert into t_test(a,b) select a,b from t_test where a = “a1”;
   4		update t_test set b = “b11” where a = “a1”; 阻塞
   5	update t_test set b = “b11” where a = “a2”;
   6		Deadlock found when trying to get lock; try restarting transaction。出现死锁

3. 第二种情况,如果给a列加一列索引，此时复现死锁

   时间序	Session1	Session2
   1	BEGIN;
   2		BEGIN;
   3	insert into t_test(a,b) select a,b from t_test where a = “a1”;
   4		update t_test set b = “b11” where a = “a1”; 阻塞
   5	update t_test set b = “b11” where a = “a1”;
   6		Deadlock found when trying to get lock; try restarting transaction。出现死锁