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为什么MySQL默认隔离级别是RR

2020-07-02 11:35 · 稿源:数据库干货铺

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>createtableusers(idintprimarykeyauto_increment,user_namevarchar(20),c_idtinyint(4),c_notevarchar(50),keyc_id(c_id))engine=innodb;
QueryOK,0rowsaffected(0.01sec)
mysql>insertintousersvalues(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);
QueryOK,5rowsaffected(0.00sec)
Records:5Duplicates:0Warnings:0mysql>createtableclass(c_idintprimarykey,c_namevarchar(1),c_notevarchar(50))engine=innodb;
QueryOK,0rowsaffected(0.00sec)
mysql>insertintoclassvalues(1,'魏',null),(2,'蜀',null),(3,'吴',null),(4,'晋','');
QueryOK,4rowsaffected(0.00sec)
Records:4Duplicates:0Warnings:0

2、 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:330612:14:27>truncatetableusers;
QueryOK,0rowsaffected(0.08sec)
root@testdb:330612:14:29>truncatetableclass;
QueryOK,0rowsaffected(0.04sec)
root@testdb:330612:14:50>insertintousersvalues(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);
QueryOK,5rowsaffected(0.00sec)
Records:5Duplicates:0Warnings:0root@testdb:330612:15:10>insertintoclassvalues(1,'魏',null),(2,'蜀',null),(3,'吴',null),(4,'晋','');
QueryOK,4rowsaffected(0.00sec)
Records:4Duplicates:0Warnings: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 ASESSION 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);

2delete from class where c_id=2;
3update 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:330612:14:27>truncatetableusers;
QueryOK,0rowsaffected(0.08sec)
root@testdb:330612:14:29>truncatetableclass;
QueryOK,0rowsaffected(0.04sec)
root@testdb:330612:14:50>insertintousersvalues(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);
QueryOK,5rowsaffected(0.00sec)
Records:5Duplicates:0Warnings:0root@testdb:330612:15:10>insertintoclassvalues(1,'魏',null),(2,'蜀',null),(3,'吴',null),(4,'晋','');
QueryOK,4rowsaffected(0.00sec)
Records:4Duplicates:0Warnings: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 ASESSION 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:

Cannotexecutestatement:impossibletowritetobinarylogsinceBINLOG_FORMAT=STATEMENTandatleastonetableusesastorageenginelimitedtorow-basedlogging.InnoDBislimitedtorow-loggingwhentransactionisolationlevelisREADCOMMITTEDorREADUNCOMMITTED.

因此单纯根据步骤讲解

步骤SESSION ASESSION 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;

3mysql>update users set c_note='t2' where c_id in (select c_id from class);
4commit;

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;

2update users set c_note='t1' where c_id in (select c_id from class);
3update 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。

本文转载自微信公众号【数据库干货铺】。

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