为什么MySQL默认事务隔离级别是RR
曾多次听到“MySQL为什么选择RR为默认隔离级别”的问题，其实这是个历史遗留问题，当前以及解决，但是MySQL的各个版本沿用了原有习惯。历史版本中的问题是什么，本次就通过简单的测试来说明一下。
1、 准备工作
1.1 部署主从
部署一套主从架构的集群，创建过程较简单，可以参考历史文章部署 MySQL主从复制搭建 部署一主一从即可。
1.2 创建测试表及数据
在主库中创建表及测试数据
mysql> create table users(id int primary key auto_increment,user_name varchar(20),c_id tinyint(4),c_note varchar(50),key c_id(c_id)) engine=innodb;
Query OK, 0 rows affected (0.01 sec)
mysql> insert into users values(1,'刘备',2,null),(2,'曹操',1,null),(3,'孙权',3,null),(4,'关羽',2,null),(5,'司马懿',1,null);
Query OK, 5 rows affected (0.00 sec)
Records: 5 Duplicates: 0 Warnings: 0
mysql> create table class(c_id int primary key ,c_name varchar(1),c_note varchar(50)) engine=innodb;
Query OK, 0 rows affected (0.00 sec)
mysql> insert into class values(1,'魏',null),(2,'蜀',null),(3,'吴',null),(4,'晋','');
Query OK, 4 rows affected (0.00 sec)
Records: 4 Duplicates: 0 Warnings: 02、 RR隔离级别
MySQL默认的隔离级别为 RR（Repeatable Read），在此隔离级别下，对比binlog格式为ROW、STATEMENT是否会造成主从数据不一致
2.1 ROW格式
其实不用测试大家也应该对RR级别下ROW格式的binlog有信心，但是，万事皆需实践检验。
步骤说明如下：
- 步骤1 - 分别查看两个会话中的事务隔离级别及binlog格式（隔离级别均为RR，binlog为ROW格式）
- 步骤2 - SESSION A 开启事务，更新users 表中c_id字段存在于class表中的记录，结果为5条记录均更新，并将c_note内容更新为 t1
- 步骤3- SESSION B 开启事务，准备删除class表中 c_id等于2的记录，此时无法更新，处于阻塞状态，因为在RR级别下需要保证重复读。达到所等待超时时间后将会报错。
- 步骤4- SESSION A 提交事务（此步骤也可以在步骤3时操作，结果不一样，后续步骤中将采用此方式）
- 步骤5- SESSION B 重启事务，再次删除class表中 c_id等于2的记录，此时提交可以成功了，成功删除了一条记录
- 步骤6- SESSION A 开启事务，更新users 表中c_id字段存在于class表中的记录，结果为3条记录更新成功，并将c_note内容更新为 t2，有2条记录因为c_id不存在与class表中，因此不会更新
- 步骤7- 分别在SESSON A和SESSION B查看users表中的内容，结果一致
- 步骤8- 在从库查看users表中的内容，数据与主库一致
具体步骤如下：
步骤 | SESSION A | SESSION B |
|---|---|---|
1 | mysql>show variables like '%iso%'; +-----------------------+-----------------+ | Variable_name | Value | +-----------------------+-----------------+ | transaction_isolation | REPEATABLE-READ | | tx_isolation | REPEATABLE-READ | +-----------------------+-----------------+ 2 rows in set (0.00 sec) mysql>show variables like '%binlog_format%'; +---------------+-------+ | Variable_name | Value | +---------------+-------+ | binlog_format | ROW | +---------------+-------+ 1 row in set (0.00 sec) | mysql>show variables like '%iso%'; +-----------------------+-----------------+ | Variable_name | Value | +-----------------------+-----------------+ | transaction_isolation | REPEATABLE-READ | | tx_isolation | REPEATABLE-READ | +-----------------------+-----------------+ 2 rows in set (0.00 sec) mysql>show variables like '%binlog_format%'; +---------------+-------+ | Variable_name | Value | +---------------+-------+ | binlog_format | ROW | +---------------+-------+ 1 row in set (0.01 sec) |
2 | mysql>set autocommit=0; mysql>update users set c_note='t1' where c_id in (select c_id from class); Query OK, 5 rows affected (0.00 sec) Rows matched: 5 Changed: 5 Warnings: 0 | |
3 | mysql>set autocommit=0; Query OK, 0 rows affected (0.00 sec) mysql>delete from class where c_id=2; ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction | |
4 | mysql>commit; Query OK, 0 rows affected (0.00 sec) | |
5 | mysql>set autocommit=0; Query OK, 0 rows affected (0.00 sec) mysql>delete from class where c_id=2; Query OK, 1 row affected (0.00 sec) mysql>commit; Query OK, 0 rows affected (0.00 sec) | |
6 | mysql>update users set c_note='t2' where c_id in (select c_id from class); Query OK, 3 rows affected (0.00 sec) Rows matched: 3 Changed: 3 Warnings: 0 mysql>commit; Query OK, 0 rows affected (0.00 sec) | |
7 | mysql>select * from users; +----+-----------+------+--------+ | id | user_name | c_id | c_note | +----+-----------+------+--------+ | 1 | 刘备 | 2 | t1 | | 2 | 曹操 | 1 | t2 | | 3 | 孙权 | 3 | t2 | | 4 | 关羽 | 2 | t1 | | 5 | 司马懿 | 1 | t2 | +----+-----------+------+--------+ 5 rows in set (0.00 sec) | mysql>select * from users; +----+-----------+------+--------+ | id | user_name | c_id | c_note | +----+-----------+------+--------+ | 1 | 刘备 | 2 | t1 | | 2 | 曹操 | 1 | t2 | | 3 | 孙权 | 3 | t2 | | 4 | 关羽 | 2 | t1 | | 5 | 司马懿 | 1 | t2 | +----+-----------+------+--------+ 5 rows in set (0.00 sec) |
8 | 在从库查看数据 root@testdb:3307 12:02:20>select * from users; +----+-----------+------+--------+ | id | user_name | c_id | c_note | +----+-----------+------+--------+ | 1 | 刘备 | 2 | t1 | | 2 | 曹操 | 1 | t2 | | 3 | 孙权 | 3 | t2 | | 4 | 关羽 | 2 | t1 | | 5 | 司马懿 | 1 | t2 | +----+-----------+------+--------+ 5 rows in set (0.00 sec) | |
2.2 STATEMENT格式
为了和之前的步骤一致，先初始化数据
root@testdb:3306 12:14:27>truncate table users;
Query OK, 0 rows affected (0.08 sec)
root@testdb:3306 12:14:29>truncate table class;
Query OK, 0 rows affected (0.04 sec)
root@testdb:3306 12:14:50>insert into users values(1,'刘备',2,null),(2,'曹操',1,null),(3,'孙 权',3,null),(4,'关羽',2,null),(5,'司马懿',1,null);
Query OK, 5 rows affected (0.00 sec)
Records: 5 Duplicates: 0 Warnings: 0
root@testdb:3306 12:15:10>insert into class values(1,'魏',null),(2,'蜀',null),(3,'吴',null),(4,'晋','');
Query OK, 4 rows affected (0.00 sec)
Records: 4 Duplicates: 0 Warnings: 0再将binlog日志格式改为STATAMENT格式（全局及会话级都改一下，或者修改全局变量后重新登录也行，当然 只改会话级别的也可以测试），然后 再次进行测试。
步骤说明如下：
- 步骤1 - 分别查看两个会话中的事务隔离级别及binlog格式（隔离级别均为RR，binlog为STATENENT格式）
- 步骤2 - SESSION A 开启事务，更新users 表中c_id字段存在于class表中的记录，结果为5条记录均更新，并将c_note内容更新为 t1
- 步骤3- SESSION B 开启事务，准备删除class表中 c_id等于2的记录，此时无法更新，处于阻塞状态，立即进行步骤4
- 步骤4- SESSION A 在SESSION B执行commit的动作，则SESSION B的删除操作可以执行通过，但注意class表的数据两个SESSION中查看到的是不一样的
- 步骤5- 此时SESSION B执行commit,否则后面session A 更新数据时也会阻塞。此时如果SESSION A不执行commit，查看class表的结果也是不一样的，如步骤中的情况
- 步骤6- SESSION A 开启事务，更新users 表中c_id字段存在于class表中的记录，结果为3条记录更新成功，并将c_note内容更新为 t2，另外2条记录虽然本此时查看class表中存在对应的c_id，但是不会更新，此时提交事务，然后再次查看class的内容，结果和SESSION B 查看的结果一致了（幻读）
- 步骤7- 在从库查看users、class表中的内容，数据与主库一致
步 骤 | SESSION A | SESSION B |
|---|---|---|
1 | mysql>show variables like '%iso%'; +-----------------------+-----------------+ | Variable_name | Value | +-----------------------+-----------------+ | transaction_isolation | REPEATABLE-READ | | tx_isolation | REPEATABLE-READ | +-----------------------+-----------------+ 2 rows in set (0.01 sec) mysql>show variables like '%binlog_format%'; +---------------+-----------+ | Variable_name | Value | +---------------+-----------+ | binlog_format | STATEMENT | +---------------+-----------+ 1 row in set (0.01 sec) | mysql>show variables like '%iso%'; +-----------------------+-----------------+ | Variable_name | Value | +-----------------------+-----------------+ | transaction_isolation | REPEATABLE-READ | | tx_isolation | REPEATABLE-READ | +-----------------------+-----------------+ 2 rows in set (0.01 sec) mysql>show variables like '%binlog_format%'; +---------------+-----------+ | Variable_name | Value | +---------------+-----------+ | binlog_format | STATEMENT | +---------------+-----------+ 1 row in set (0.01 sec) |
2 | root@testdb:3306 12:37:04>set autocommit=0; Query OK, 0 rows affected (0.00 sec) root@testdb:3306 12:37:17>update users set c_note='t1' where c_id in (select c_id from class); Query OK, 5 rows affected, 1 warning (0.00 sec) Rows matched: 5 Changed: 5 Warnings: 1 | |
3 | root@testdb:3306 12:28:25>set autocommit=0; Query OK, 0 rows affected (0.00 sec) root@testdb:3306 12:38:06>delete from class where c_id=2; Query OK, 1 row affected (4.74 sec) | |
4 | root@testdb:3306 12:38:09>commit; Query OK, 0 rows affected (0.00 sec) root@testdb:3306 12:38:13>select * from users; +----+-----------+------+--------+ | id | user_name | c_id | c_note | +----+-----------+------+--------+ | 1 | 刘备 | 2 | t1 | | 2 | 曹操 | 1 | t1 | | 3 | 孙 权 | 3 | t1 | | 4 | 关羽 | 2 | t1 | | 5 | 司马懿 | 1 | t1 | +----+-----------+------+--------+ 5 rows in set (0.00 sec) root@testdb:3306 12:39:07>select * from class; +------+--------+--------+ | c_id | c_name | c_note | +------+--------+--------+ | 1 | 魏 | NULL | | 2 | 蜀 | NULL | | 3 | 吴 | NULL | | 4 | 晋 | | +------+--------+--------+ 4 rows in set (0.00 sec) | |
5 | root@testdb:3306 12:38:13>commit; Query OK, 0 rows affected (0.00 sec) root@testdb:3306 12:39:56>select * from class ; +------+--------+--------+ | c_id | c_name | c_note | +------+--------+--------+ | 1 | 魏 | NULL | | 3 | 吴 | NULL | | 4 | 晋 | | +------+--------+--------+ 3 rows in set (0.00 sec) | |
6 | root@testdb:3306 12:52:23>update users set c_note='t2' where c_id in (select c_id from class); Query OK, 3 rows affected, 1 warning (0.00 sec) Rows matched: 3 Changed: 3 Warnings: 1 root@testdb:3306 12:52:45>select * from class; +------+--------+--------+ | c_id | c_name | c_note | +------+--------+--------+ | 1 | 魏 | NULL | | 2 | 蜀 | NULL | | 3 | 吴 | NULL | | 4 | 晋 | | +------+--------+--------+ 4 rows in set (0.00 sec) root@testdb:3306 12:52:49>select * from users; +----+-----------+------+--------+ | id | user_name | c_id | c_note | +----+-----------+------+--------+ | 1 | 刘备 | 2 | t1 | | 2 | 曹操 | 1 | t2 | | 3 | 孙 权 | 3 | t2 | | 4 | 关羽 | 2 | t1 | | 5 | 司马懿 | 1 | t2 | +----+-----------+------+--------+ 5 rows in set (0.01 sec) root@testdb:3306 12:53:03>commit; Query OK, 0 rows affected (0.00 sec) root@testdb:3306 12:53:06>select * from users; +----+-----------+------+--------+ | id | user_name | c_id | c_note | +----+-----------+------+--------+ | 1 | 刘备 | 2 | t1 | | 2 | 曹操 | 1 | t2 | | 3 | 孙 权 | 3 | t2 | | 4 | 关羽 | 2 | t1 | | 5 | 司马懿 | 1 | t2 | +----+-----------+------+--------+ 5 rows in set (0.00 sec) root@testdb:3306 12:53:11>select * from class; +------+--------+--------+ | c_id | c_name | c_note | +------+--------+--------+ | 1 | 魏 | NULL | | 3 | 吴 | NULL | | 4 | 晋 | | +------+--------+--------+ 3 rows in set (0.00 sec) | |
7 | 查看从库数据 root@testdb:3307 12:44:22>select * from class; +------+--------+--------+ | c_id | c_name | c_note | +------+--------+--------+ | 1 | 魏 | NULL | | 3 | 吴 | NULL | | 4 | 晋 | | +------+--------+--------+ 3 rows in set (0.01 sec) root@testdb:3307 12:57:07>select * from users; +----+-----------+------+--------+ | id | user_name | c_id | c_note | +----+-----------+------+--------+ | 1 | 刘备 | 2 | t1 | | 2 | 曹操 | 1 | t2 | | 3 | 孙 权 | 3 | t2 | | 4 | 关羽 | 2 | t1 | | 5 | 司马懿 | 1 | t2 | +----+-----------+------+--------+ 5 rows in set (0.00 sec) | |
也就是此时主从结果也是一致的，原因在于，binlog里存储的语句顺序如下：
binlog里的顺序 | 语句内容 |
|---|---|
1 | update users set c_note='t1' where c_id in (select c_id from class); |
2 | delete from class where c_id=2; |
3 | update users set c_note='t2' where c_id in (select c_id from class); |
与主库执行的顺序是一致的，因此，主从的结果是一致的。
3、 RC隔离级别
3.1 ROW格式
为了和之前的步骤一致，先初始化数据
root@testdb:3306 12:14:27>truncate table users;
Query OK, 0 rows affected (0.08 sec)
root@testdb:3306 12:14:29>truncate table class;
Query OK, 0 rows affected (0.04 sec)
root@testdb:3306 12:14:50>insert into users values(1,'刘备',2,null),(2,'曹操',1,null),(3,'孙 权',3,null),(4,'关羽',2,null),(5,'司马懿',1,null);
Query OK, 5 rows affected (0.00 sec)
Records: 5 Duplicates: 0 Warnings: 0
root@testdb:3306 12:15:10>insert into class values(1,'魏',null),(2,'蜀',null),(3,'吴',null),(4,'晋','');
Query OK, 4 rows affected (0.00 sec)
Records: 4 Duplicates: 0 Warnings: 0再将binlog日志格式改为STATAMENT格式（全局及会话级都改一下，或者修改全局变量后重新登录也行，当然 只改会话级别的也可以测试），然后 再次进行测试。
步骤说明如下：
- 步骤1 - 分别查看两个会话中的事务隔离级别及binlog格式（隔离级别均为RC，binlog为ROW格式）
- 步骤2 - SESSION A 开启事务，更新users 表中c_id字段存在于class表中的记录，结果为5条记录均更新，并将c_note内容更新为 t1
- 步骤3- SESSION B 开启事务，准备删除class表中 c_id等于2的记录，此时不会像RR事务隔离级别那样处于阻塞状态，而是可以直接执行通过
- 步骤4- 此时SESSION A查看class数据还是删除前的，因为session B 暂未提交
- 步骤5- SESSION B 提交事务，
- 步骤6- 更新users 表中c_id字段存在于class表中的记录，结果为3条记录更新成功，并将c_note内容更新为 t2
- 步骤7- 在从库查看users、class表中的内容，数据与主库一致
步 骤 | SESSION A | SESSION B |
|---|---|---|
1 | root@testdb:3306 01:25:24>show variables like '%iso%'; +-----------------------+----------------+ | Variable_name | Value | +-----------------------+----------------+ | transaction_isolation | READ-COMMITTED | | tx_isolation | READ-COMMITTED | +-----------------------+----------------+ 2 rows in set (0.01 sec) root@testdb:3306 01:25:36>show variables like '%binlog_format%'; +---------------+-------+ | Variable_name | Value | +---------------+-------+ | binlog_format | ROW | +---------------+-------+ 1 row in set (0.01 sec) | root@testdb:3306 01:24:57>show variables like '%iso%'; +-----------------------+----------------+ | Variable_name | Value | +-----------------------+----------------+ | transaction_isolation | READ-COMMITTED | | tx_isolation | READ-COMMITTED | +-----------------------+----------------+ 2 rows in set (0.01 sec) root@testdb:3306 01:25:39>show variables like '%binlog_format%'; +---------------+-------+ | Variable_name | Value | +---------------+-------+ | binlog_format | ROW | +---------------+-------+ 1 row in set (0.00 sec) |
2 | root@testdb:3306 01:27:55>set autocommit=0; Query OK, 0 rows affected (0.00 sec) root@testdb:3306 01:28:27>update users set c_note='t1' where c_id in (select c_id from class); Query OK, 5 rows affected (0.00 sec) Rows matched: 5 Changed: 5 Warnings: 0 | |
3 | root@testdb:3306 01:26:07>set autocommit=0; Query OK, 0 rows affected (0.00 sec) root@testdb:3306 01:28:37>delete from class where c_id=2; Query OK, 1 row affected (0.00 sec) | |
4 | root@testdb:3306 01:28:27>select * from class; +------+--------+--------+ | c_id | c_name | c_note | +------+--------+--------+ | 1 | 魏 | NULL | | 2 | 蜀 | NULL | | 3 | 吴 | NULL | | 4 | 晋 | | +------+--------+--------+ 4 rows in set (0.00 sec) | |
5 | root@testdb:3306 01:28:41>commit; Query OK, 0 rows affected (0.00 sec) | |
6 | root@testdb:3306 01:28:59>select * from class; +------+--------+--------+ | c_id | c_name | c_note | +------+--------+--------+ | 1 | 魏 | NULL | | 3 | 吴 | NULL | | 4 | 晋 | | +------+--------+--------+ 3 rows in set (0.01 sec) root@testdb:3306 01:29:13>update users set c_note='t2' where c_id in (select c_id from class); Query OK, 3 rows affected (0.00 sec) Rows matched: 3 Changed: 3 Warnings: 0 root@testdb:3306 01:29:26>select * from class; +------+--------+--------+ | c_id | c_name | c_note | +------+--------+--------+ | 1 | 魏 | NULL | | 3 | 吴 | NULL | | 4 | 晋 | | +------+--------+--------+ 3 rows in set (0.00 sec) root@testdb:3306 01:29:31>select * from users; +----+-----------+------+--------+ | id | user_name | c_id | c_note | +----+-----------+------+--------+ | 1 | 刘备 | 2 | t1 | | 2 | 曹操 | 1 | t2 | | 3 | 孙 权 | 3 | t2 | | 4 | 关羽 | 2 | t1 | | 5 | 司马懿 | 1 | t2 | +----+-----------+------+--------+ 5 rows in set (0.00 sec) root@testdb:3306 01:29:38>commit; | |
7 | 查看从库数据 root@testdb:3307 01:40:32>select * from users; +----+-----------+------+--------+ | id | user_name | c_id | c_note | +----+-----------+------+--------+ | 1 | 刘备 | 2 | t1 | | 2 | 曹操 | 1 | t2 | | 3 | 孙 权 | 3 | t2 | | 4 | 关羽 | 2 | t1 | | 5 | 司马懿 | 1 | t2 | +----+-----------+------+--------+ 5 rows in set (0.00 sec) root@testdb:3307 01:40:35>select * from class; +------+--------+--------+ | c_id | c_name | c_note | +------+--------+--------+ | 1 | 魏 | NULL | | 3 | 吴 | NULL | | 4 | 晋 | | +------+--------+--------+ 3 rows in set (0.00 sec) | |
也就是此时主从结果也是一致的。
3.2 STATEMENT格式
因为当前版本已经不支持RC+STATEMENT组合下数据的操作，否则将报如下错误：
Cannot execute statement: impossible to write to binary log since BINLOG_FORMAT = STATEMENT and at least one table uses a storage engine limited to row-based logging. InnoDB is limited to row-logging when transaction isolation level is READ COMMITTED or READ UNCOMMITTED.因此单纯根据步骤讲解
步骤 | SESSION A | SESSION B |
|---|---|---|
1 | mysql>set autocommit=0; mysql>update users set c_note='t1' where c_id in (select c_id from class); | |
2 | mysql>set autocommit=0; mysql>delete from class where c_id=2; mysql>commit; | |
3 | mysql>update users set c_note='t2' where c_id in (select c_id from class); | |
4 | commit； |
因为binlog是按照commit时间的顺序保存，因此上述步骤在binlog里会以如下顺序存储：
binlog里的顺序 | 语句内容 |
|---|---|
1 | delete from class where c_id=2; |
2 | update users set c_note='t1' where c_id in (select c_id from class); |
3 | update users set c_note='t2' where c_id in (select c_id from class); |
从库通过binlog应用后，最终的结果将导致主库的数据不一样（具体案例后续安装低版本后演示）。
因而，此种场景下很容易导致数据不一样。
4、总结
通过上述的实践，可以发现在RR级别下，binlog为任何格式均不会造成主从数据不一致的情况出现，但是当低版本MySQL使用RC+STATEMENT组合时（MySQL5.1.5前只有statement格式）将会导致主从数据不一致。当前这个历史遗漏问题以及解决，大家可以将其设置为RC+ROW组合的方式（例如Oracle等数据库隔离级别就是RC），而不是必须使用RR（会带来更多的锁等待），具体可以视情况选择。