MySQL事务隔离级别-案例驱动

CREATE TABLE `traning`.`account` (
  `id` INT NOT NULL AUTO_INCREMENT COMMENT  ‘主键id‘,
  `uid` INT NOT NULL COMMENT ‘用户uid‘,
  `amount` INT NOT NULL COMMENT ‘金额‘,
  PRIMARY KEY (`id`));

INSERT INTO `traning`.`account` (`uid`, `amount`) VALUES (‘1‘, ‘1000‘);
INSERT INTO `traning`.`account` (`uid`, `amount`) VALUES (‘2‘, ‘2000‘);

step2 修改隔离级别

场景1 读未提交导致脏读

Client1：SET GLOBAL TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;
Client2：SET GLOBAL TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;
关闭会话重新进入MySQL客户端。
Client2：show variables like ‘transaction_isolation‘;
Client1: start TRANSACTION;
Client2: start TRANSACTION;
Client2: select amount from account where uid=1; （1000）
update account set amount=amount-500 where uid=1;
Client1: select amount from account where uid=1; (读到别人没提交事务:500)
-- update account set amount=(amount+100) where uid=1;（数据被锁，不能更新）
Client2: rollback;(Client1再次读到的是1000，但是没有读)
-- Client1: select amount from account where uid=1; (500)
Client1: update account set amount=(500+100) where uid=1; （读到旧数据去进行更新操作，导致账户丢失500）

场景2 读已提交解决脏读问题

Client1：SET GLOBAL TRANSACTION ISOLATION LEVEL READ COMMITTED;
Client2：SET GLOBAL TRANSACTION ISOLATION LEVEL READ COMMITTED;
关闭会话重启。
Client1：start TRANSACTION;
Client2：start TRANSACTION;
Client2: select amount from account where uid=1; （1000）
update account set amount=amount-500 where uid=1; (500)
Client1：select amount from account where uid=1 （读取不到别人没提交的事务：1000);
update account set amount=1000-500 where uid=1;（数据被锁不能更新）
Client2: rollback;
Client1: update account set amount=(1000+100) where uid=1;
Client1: commit;

场景3 读已提交导致不可重复读问题

Client1：SET GLOBAL TRANSACTION ISOLATION LEVEL READ COMMITTED;
Client2：SET GLOBAL TRANSACTION ISOLATION LEVEL READ COMMITTED;
关闭会话重启。
Client1：start TRANSACTION;
Client2：start TRANSACTION;
Client1：select amount from account where uid=1; （1000);
Client2：update account set amount=amount-100 where uid=1;
Client2: commit;
Client1: select amount from account where uid=1(amount=900,发现自己钱少了100);

场景4 可重复读解决不可重复读问题

Client1：SET GLOBAL TRANSACTION ISOLATION LEVEL REPEATABLE READ;
Client2：SET GLOBAL TRANSACTION ISOLATION LEVEL REPEATABLE READ;
Client1：start TRANSACTION;
Client2：start TRANSACTION;
Client2: select amount from account where uid=1;（1000）
Client1：select amount from account where uid=1; （1000);
Client2：commit;
Client1：select amount from account where uid=1;（amount=1000);

场景5 可重复读导致幻读问题

Client1：SET GLOBAL TRANSACTION ISOLATION LEVEL REPEATABLE READ;
Client2：SET GLOBAL TRANSACTION ISOLATION LEVEL REPEATABLE READ;
Client1：start TRANSACTION;
Client2：start TRANSACTION;
Client2: select amount from account ; （2行数据）
Client1：select amount from account ; (2行数据);
Client2：INSERT INTO traning.account (uid, amount) VALUES (‘3‘, ‘3000‘);
Client2：commit;
Client1：select amount from account ;（3条数据);
Clinet1 :update account set amount=1 where id=3(Client读不到但是可以更新);
Clinet1 :select amount from account ;（4条数据-phantom read here!!!!影响了client1的判断和数据);
Clinet1 :commit;

场景6 可序列化解决幻读问题

Client1：SET GLOBAL TRANSACTION ISOLATION LEVEL SERIALIZABLE;
Client2：SET GLOBAL TRANSACTION ISOLATION LEVEL SERIALIZABLE;
Client1：start TRANSACTION;
Client2：start TRANSACTION;
Client2: select amount from account ; （2行数据）
Client1：select amount from account ; (2行数据);
Client2：INSERT INTO traning.account (uid, amount) VALUES (‘3‘, ‘3000‘); (插入不进去，数据被锁)
Lock wait timeout exceeded; try restarting transaction

以下是整理的图：

当然还有第一类丢失更新(脏写)和第二类丢失更新，第一类丢失更新永远不会发生，第二类丢失更新会在RU和RC情况下发生。

这是完整的图
包含了更多的隔离级别和可能发生数据异常情况：

MySQL事务隔离级别-案例驱动

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