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
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会话1 |
会话2 |
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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 |
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-- 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 |
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-- 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 |
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-- 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.... |
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-- 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 |
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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 |
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2. innodb 排它锁(for update)演示
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会话1 |
会话2 |
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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 |
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-- 对 city_id=14加for update 排它锁 SELECT cityname FROM city WHERE city_id=14 FOR UPDATE ; cityname 11 |
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-- 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... |
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-- 更新后，commit 释放锁 UPDATE city SET cityname='深圳' WHERE city_id =14; COMMIT; |
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获取锁 for update共享锁,值还是11 cityname 11 |
