模拟验证一致性非锁定读

基本信息：

MariaDB [(none)]> select version();

+---------------------+

| version() |

+---------------------+

| 10.0.20-MariaDB-log |

+---------------------+

1 row in set (0.00 sec)

MariaDB [(none)]> show variables like 'binlog_format';

+---------------+-------+

| Variable_name | Value |

+---------------+-------+

| binlog_format | ROW |

+---------------+-------+

1 row in set (0.01 sec)

场景一：在RC级别下验证非一致性锁定读

MariaDB [(none)]> show variables like 'TX_ISOLATION';

+---------------+----------------+

| Variable_name | Value |

+---------------+----------------+

| tx_isolation | READ-COMMITTED |

+---------------+----------------+

1 row in set (0.00 sec)

数据准备：

use lots

create table test (id int,name varchar(20))

engine=innodb DEFAULT CHARSET=utf8;

insert into test values (1,'zeng1'),(2,'zeng2'),(3,'zeng3');

会话一：用begin开启一个事务，查询id=3的记录.

MariaDB [(none)]> use lots;

Database changed

MariaDB [lots]> begin;

Query OK, 0 rows affected (0.00 sec)

MariaDB [lots]> select * from test where id=3;

+------+-------+

| id | name |

+------+-------+

| 3 | zeng3 |

+------+-------+

1 row in set (0.00 sec)

会话二：用begin开启一个事务，并将id=3的记录更改为id=4.

MariaDB [(none)]> use lots;

Database changed

MariaDB [lots]> begin;

Query OK, 0 rows affected (0.00 sec)

MariaDB [lots]> update test set id=4 where id=3;

Query OK, 1 row affected (0.00 sec)

Rows matched: 1 Changed: 1 Warnings: 0

会话一：上面会话二对id=3的记录执行了update操作，还没有commit，有锁定id=3的记录，此时会话一再执行查询id=3的记录，是从最新的快照中查询，返回如下。

MariaDB [lots]> select * from test where id=3;

+------+-------+

| id | name |

+------+-------+

| 3 | zeng3 |

+------+-------+

1 row in set (0.00 sec)

会话二：执行commit

MariaDB [lots]> commit;

Query OK, 0 rows affected (0.00 sec)

会话一：由于会话二已提交,再查询id=3的记录，是从最新快照中查询，所以返回为空。

MariaDB [lots]> select * from test where id=3;

Empty set (0.00 sec)

场景二：在RR级别下验证非一致性锁定读

MariaDB [lots]> set global tx_isolation='Repeatable-Read';

Query OK, 0 rows affected (0.00 sec)

MariaDB [(none)]> show variables like 'TX_ISOLATION';

+---------------+-----------------+

| Variable_name | Value |

+---------------+-----------------+

| tx_isolation | REPEATABLE-READ |

+---------------+-----------------+

1 row in set (0.00 sec)

会话一：用begin开启一个事务，查询id=3的记录.

MariaDB [(none)]> use lots;

Database changed

MariaDB [lots]> begin;

Query OK, 0 rows affected (0.00 sec)

MariaDB [lots]> select * from test where id=3;

+------+-------+

| id | name |

+------+-------+

| 3 | zeng3 |

+------+-------+

1 row in set (0.00 sec)

会话二：用begin开启一个事务，并将id=3的记录更改为id=4.

MariaDB [(none)]> use lots;

Database changed

MariaDB [lots]> begin;

Query OK, 0 rows affected (0.00 sec)

MariaDB [lots]> update test set id=4 where id=3;

Query OK, 1 row affected (0.00 sec)

Rows matched: 1 Changed: 1 Warnings: 0

会话一：上面会话二对id=3的记录执行了update操作，还没有commit，有锁定id=3的记录，此时会话一再执行查询id=3的记录，是读取事务开始时快照，返回如下。

MariaDB [lots]> select * from test where id=3;

+------+-------+

| id | name |

+------+-------+

| 3 | zeng3 |

+------+-------+

1 row in set (0.00 sec)

会话二：执行commit

MariaDB [lots]> commit;

Query OK, 0 rows affected (0.00 sec)

会话一：由于会话二已提交,再查询id=3的记录，总是读取事务开始时快照，所以返回结果与上面相同。

MariaDB [lots]> select * from test where id=3;

+------+-------+

| id | name |

+------+-------+

| 3 | zeng3 |

+------+-------+

1 row in set (0.00 sec)

总结：非一致性读锁定，在RR和RC下是不同的，RC读取最新快照数据，RR总是读取事务开始时的快照数据。