为什么MySQL默认事务隔离级别是RR
2020/04/09 06:05:29 来源:Linux社区 作者:Linux

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,'',null),(2,'',null),(3,'',null),(4,'','');
Query OK, 4 rows affected (0.00 sec)
Records: 4  Duplicates: 0  Warnings: 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:

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,'',null),(2,'',null),(3,'',null),(4,'','');
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 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,'',null),(2,'',null),(3,'',null),(4,'','');
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 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。


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