RR与RC隔离级别下索引和锁的测试脚本示例代码
基本概念
当前读与快照读
在MVCC中,读操作可以分成两类:快照读 (snapshot read)与当前读 (current read)。 快照读,读取的是记录的可见版本 (有可能是历史版本),不用加锁。当前读,读取的是记录的最新版本,并且对返回的记录,都会加上锁,保证在事务结束前,这条数据都是最新版本。
快照读:简单的select操作,属于快照读,不加锁(Serializable除外)。
select * from table where ?;
当前读:特殊的读操作,插入/更新/删除操作,属于当前读,需要加锁。
select * from table where ? lock in share mode; select * from table where ? for update; insert into table values (); update table set ? where ?; delete from table where ?;
隔离级别与加锁机制
- Read Uncommitted 会发生脏读,不考虑。
- Read Committed (RC) 针对当前读,RC隔离级别保证对读取到的记录加锁 (Gap Locking),存在幻读现象。
- Repeatable Read (RR) 针对当前读,RR隔离级别保证对读取到的记录加锁 (Record Locking),同时保证对读取的范围加锁,新的满足查询条件的记录不能够插入 (Gap Locking),不存在幻读现象。
- Serializable 所有的读操作均为退化为当前读,读写冲突,因此并发度急剧下降,不考虑。
测试脚本
-- 基本操作 -- -- 查询事务隔离级别,默认是RR show variables like '%isolation%'; -- 设置事务隔离级别为RC set session transaction isolation level read committed; -- 数据初始化 -- begin; drop table if exists user; CREATE TABLE `user` ( `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT, `email` varchar(64) NOT NULL, `age` int(11) NOT NULL, `address` varchar(64) NOT NULL, PRIMARY KEY (`id`), UNIQUE KEY `uniq_email` (`email`), KEY `idx_age` (`age`) ); insert into user (email, age, address) values ("[email protected]", 18, "address1"); insert into user (email, age, address) values ("[email protected]", 20, "address2"); insert into user (email, age, address) values ("[email protected]", 20, "address3"); commit; select * from user; -- 一、trx_id示例 begin; SELECT TRX_ID FROM INFORMATION_SCHEMA.INNODB_TRX WHERE TRX_MYSQL_THREAD_ID = CONNECTION_ID(); select * from user; SELECT TRX_ID FROM INFORMATION_SCHEMA.INNODB_TRX WHERE TRX_MYSQL_THREAD_ID = CONNECTION_ID(); SHOW ENGINE INNODB STATUS; update user set age = 22 where id = 3; -- 查询事务id SELECT TRX_ID FROM INFORMATION_SCHEMA.INNODB_TRX WHERE TRX_MYSQL_THREAD_ID = CONNECTION_ID(); -- INNODB 引擎状态 SHOW ENGINE INNODB STATUS; commit; -- 二、可重复读、不可重复读示例 -- session1 set session transaction isolation level read committed; begin; -- session2 set session transaction isolation level repeatable read; begin; -- session1 select * from user; -- session2 select * from user; -- session3 begin; insert into user (email, age, address) values ("[email protected]", 30, "address4"); commit; -- session1 这里因为是RC,所以可以读到trx3提交的新数据,这里如果是证明不可重复读的话应该使用update而不是insert select * from user; commit; -- session2 这里因为是RR,所以不会读到trx3提交的新数据 select * from user; commit; -- 三、快照读幻读示例 -- session1 set session transaction isolation level repeatable read; begin; -- 这里使用快照读 select * from user; -- session2 begin; insert into user (email, age, address) values ("[email protected]", 30, "address4"); commit; select * from user; -- session1 select * from user; -- 这里读不到test4@的数据,因为是RR -- 这里发生了幻读 insert into user (email, age, address) values ("[email protected]", 30, "address4"); -- 插入失败因为email唯一索引冲突 commit; -- 四、当前读幻读示例 -- RC -- session1 set session transaction isolation level read committed; begin; -- 这里会对所有满足条件的age=20的记录加锁,因为是RC,所以没有GAP锁 delete from user where age = 20; select * from user; -- session2 set session transaction isolation level read committed; begin; -- 因为trx1没有加GAP锁,所以之类可以插入age=20的记录 insert into user (email, age, address) values ("[email protected]", 20, "address4"); select * from user; -- 可以查到4条数据,可以读到trx1的删除数据,因为是RC,trx1未提交所以没影响trx2 commit; -- session1 select * from user; -- 可以读到trx2新插入的数据,虽然trx1是当前读,但是并未添加相应的next-key锁,没有阻止trx2的新数据插入 commit; --RR -- session1 set session transaction isolation level repeatable read; begin; delete from user where age = 20; select * from user; -- session2 begin; -- 这里会阻塞,因为trx1在age=20周围加了GAP锁 -- 非唯一索引,首先,通过索引定位到第一条满足查询条件的记录,加记录上的X锁,加GAP上的GAP锁,然后加主键聚簇索引上的记录X锁; -- 然后读取下一条,重复进行。直至进行到第一条不满足条件的记录,此时,不需要加记录X锁,但是仍旧需要加GAP锁,最后返回结束。 insert into user (email, age, address) values ("[email protected]", 20, "address4"); -- 直到超时,ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction -- 此时如果查询可以看到3条记录 commit; -- session1 -- 此时只能看到1条记录,另外两条被删除了 select * from user; commit; -- 唯一索引+RC -- session1 set session transaction isolation level read committed; begin; delete from user where email = "[email protected]"; -- session2 begin; -- 可以读到,因为trx1是RC select * from user where email = "[email protected]"; -- 尝试更新这个记录的age,会阻塞直到超时,因为email是唯一索引已经被trx1锁住了,同时也会在对应的主键索引上加锁 -- 注意这里操作的id=3就是trx1中操作的email的同一行记录 update user set age = 40 where id = 3; -- session1 commit; -- session2 commit; -- 无索引+RC -- session1 set session transaction isolation level read committed; begin; -- 由于address字段无索引,所以Innodb会对所有行进行加锁,由MySQL server进行判断并释放锁 delete from user where address = "address3"; -- session2 set session transaction isolation level read committed; begin; -- 这一行会成功,因为这一行没有加锁(先加了后释放了) update user set age = 10 where address = "address2"; -- 这一行同样会被阻塞,原因是它已经被trx1的语句加了锁了,全部符合条件的都加锁了 update user set age = 10 where address = "address3"; -- session1 commit; -- session2 commit; -- 非唯一索引+RR -- session1 set session transaction isolation level repeatable read; begin; delete from user where age = 20; -- session2 set session transaction isolation level repeatable read; begin; -- 这里会阻塞,因为trx1中已经锁住了age=20的记录以及加上了GAP锁,所以这里18已经落入锁区间 insert into user (email, age, address) values ("[email protected]", 18, "address4"); -- session1 commit; -- session2 commit; -- 无索引RR -- session1 set session transaction isolation level repeatable read; begin; -- 没有索引,那么会锁上表中的所有记录,同时会锁上主键索引上的所有GAP,杜绝所有的并发更新操作 delete from user where address = "address3"; -- session2 set session transaction isolation level repeatable read; begin; -- 这里会阻塞,原因是主键已经被加上了GAP锁,所以新的插入不能执行成功 insert into user (email, age, address) values ("[email protected]", 18, "address4"); -- session1 commit; -- session2 commit; -- 死锁 简单示例 -- session1 begin; delete from user where id = 1; -- session2 begin; delete from user where id = 3; -- session1 delete from user where id = 3; -- seession2 -- 这里MySQL判断发生了死锁,中断了一个trx -- ERROR 1213 (40001): Deadlock found when trying to get lock; try restarting transaction delete from user where id = 1; -- session1 rollback; -- session2; rollback; -- 五、死锁 insert示例 drop table if exists t1; begin; create table t1 ( `id` bigint not null auto_increment, primary key (`id`) ); insert into t1 values(1); insert into t1 values(5); commit; select * from t1; -- session1 begin; insert into t1 values (2); -- sessioin2 begin; -- 这里会阻塞 insert into t1 values (2); -- session3 begin; -- 这里会阻塞 insert into t1 values (2); -- session1; -- 此时回滚,trx2和trx3收到通知,MySQL自动中断一个trx,因为发生了死锁 -- ERROR 1213 (40001): Deadlock found when trying to get lock; try restarting transaction rollback; --session2; rollback; --session3; rollback;
总结
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