mysql 开发进阶篇系列 8 锁问题 (共享锁与排它锁演示)

1 .innodb 共享锁(lock in share mode)演示

　　SET autocommit=0是指：当前session禁用自动提交事务，需要显示 commit 才能提交事务。默认是autocommit=1 隐式提交事务。如果想控制何时提交或回滚，常用是START TRANSACTION（或BEGIN）语句挂起自动提交模式，最后用一条 COMMIT语句结束事务。如果发生错误，用一条ROLLBACK语句撤销事务。

　　lock in share mode 为IS锁
　　for update 为IX锁

会话1	会话2
SET autocommit=0; SELECT cityname FROM  city WHERE city_id=14; city_id      country_id        cityname CityCode 14     2       22     001	SET autocommit=0; SELECT cityname FROM  city WHERE city_id=14; city_id      country_id        cityname CityCode 14     2       22     001
-- 对 city_id=14加共享锁 SELECT * FROM  city WHERE city_id=14 LOCK IN SHARE MODE; city_id      country_id        cityname CityCode 14     2       22     001
	-- 也对 city_id=14加共享锁 SELECT * FROM  city WHERE city_id=14 LOCK IN SHARE MODE; city_id      country_id        cityname CityCode 14     2       22     001
-- 当前会话对锁定的记录进行更新操作，等待锁。 UPDATE city  SET cityname='深圳' WHERE city_id =14;  等待中....
	-- 会话2也对锁定的记录进行更新操作，则会导致死锁退出 UPDATE city  SET cityname='深圳' WHERE city_id =14; 错误代码： 1213 Deadlock found when trying to get lock; try restarting transaction
获得锁后，更新成功 查询：update city set cityname='深圳' where city_id =14 共 1 行受到影响

2.  innodb 排它锁(for update)演示

会话1	会话2
SET autocommit=0; SELECT * FROM  city WHERE city_id=14; city_id      country_id        cityname CityCode 14     2       22     001	SET autocommit=0; SELECT * FROM  city WHERE city_id=14; city_id      country_id        cityname CityCode 14     2       22     001
-- 对 city_id=14加for update 排它锁 SELECT cityname FROM  city WHERE city_id=14 FOR UPDATE ; cityname 11
	-- 可以查询 SELECT cityname FROM  city WHERE city_id=14 cityname 11 -- 但不能对 city_id=14加for update 排它锁 SELECT cityname FROM  city WHERE city_id=14 FOR UPDATE ; 等待中...
-- 更新后，commit 释放锁 UPDATE city  SET cityname='深圳' WHERE city_id =14; COMMIT;
	获取锁 for update共享锁,值还是11 cityname 11