Msql-数据库死锁

实验案例

CREATE TABLE `t1_deadlock` (
`id` int(11) NOT NULL,
`name` varchar(100) DEFAULT NULL,
`age` int(11) NOT NULL,
`address` varchar(255) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `idx_age` (`age`) USING BTREE,
KEY `idx_name` (`name`) USING BTREE
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
Insert into t1_deadlock(id,name,age,address) values (1,'刘备',18,'蜀国');
Insert into t1_deadlock(id,name,age,address) values (2,'关羽',17,'蜀国');
Insert into t1_deadlock(id,name,age,address) values (3,'张飞',16,'蜀国');
Insert into t1_deadlock(id,name,age,address) values (4,'关羽',16,'蜀国');
Insert into t1_deadlock(id,name,age,address) values (5,'诸葛亮',35,'蜀国');
Insert into t1_deadlock(id,name,age,address) values (6,'曹孟德',32,'魏国');

案例1

Time	Transaction1	Transaction2
T1	begin;	begin;
T2	select * from t1_deadlock where id=1 for update;
T3		delete from t1_deadlock where id=5;
T4	update t1_deadlock set name=‘qqq’ where id=5;
T5		delete from t1_deadlock where id=1;
T6	死锁	死锁

-- Session01
begin;
select * from t1_deadlock where id=1 for update;
update t1_deadlock set name='qqq' where id=5;
commit;
-- Session02
begin;
delete from t1_deadlock where id=5;
delete from t1_deadlock where id=1;
-- 死锁
commit;

在这里插入图片描述
上图紫色表示当前事务持有的锁，红色表示需要等待花获取的锁。我们按照表中是执行顺序，首先事务1给1这条记录加了记录锁，然后事务2给5这条记录加了记录锁，这个时候事务1又想取更新记录5就需要等待事务2提交释放5的记录锁，同样事务2想要删除记录为1的数据也需要等待事务1提交释放1记录的锁。这样事务1等待事务2，事务2等待事务1就造成了死锁。

SHOW ENGINE INNODB STATUS;

在这里插入图片描述

案例2

Time	Transaction1	Transaction2
T1	begin;	begin;
T2	delete from t2_deadlock where cnt=‘abc-130-sz’;
T3		delete from t2_deadlock where cnt=‘abc-130-sz’;
T4	insert into t2_deadlock (cnt) values (‘abc-130-sz’);
T5	死锁	死锁

CREATE TABLE `t2_deadlock` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`cnt` varchar(32) DEFAULT NULL,PRIMARY KEY (`id`),
UNIQUE index `idx_cnt` (`cnt`) USING BTREE
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
insert into t1_deadlock03(id,cnt) values (1,'abc-130-sz');
``
```sql
-- Session01
begin;
delete from t2_deadlock where cnt='abc-130-sz';
insert into t2_deadlock(cnt) values ('abc-130-sz');
commit;
-- Session02
begin;
delete from t2_deadlock where cnt='abc-130-sz';
commit;