技术分享 | MySQL 的嵌套事务、自治事务与链式事务

左手的ㄟ右手 2023-07-03 08:21 12阅读 0赞

这篇文章有感于最近支持某客户从 Oracle 迁移到 MySQL 过程中的启示。接下来我们**详细说明 MySQL 中的事务种类。**

**分类**

1. 普通事务

以 begin / start transaction 开始，commit / rollback 结束的事务。或者是带有保存点 savepoint 的事务。

2. 链式事务

一个事务在提交的时候自动将上下文传给下一个事务，也就是说一个事务的提交和下一个事务的开始是原子性的，下一个事务可以看到上一个事务的处理结果。MySQL 的链式事务靠参数 completion\_type 控制，并且回滚和提交的语句后面加上 work 关键词。

3. 嵌套事务

有多个 begin / commit / rollback 这样的事务块的事务，并且有父子关系。子事务的提交完成后不会真的提交，而是等到父事务提交才真正的提交。

4. 自治事务

内部事务的提交不随外部事务的影响，一般用作记录内部事务的异常情况。MySQL 不支持自治事务，但是某些场景可以用 MySQL 的插件式引擎来变相实现。

接下来，我们每种事务用详细例子来说明。

**实例**

**1. 普通事务**

下表 c1，开始一个事务块，有两个保存点 s1 & s2。我们回滚了 s2 之后的所有操作，并且提交了 s2 之前的所有操作，此时 s1 & s2 已经失效。那记录数刚好两条。

{"db":"ytt"},"port":"3320"}-mysql>truncate c1;
    Query OK, 0 rows affected (0.00 sec)
    {"db":"ytt"},"port":"3320"}-mysql>
    {"db":"ytt"},"port":"3320"}-mysql>
    {"db":"ytt"},"port":"3320"}-mysql>use ytt
    Database changed
    {"db":"ytt"},"port":"3320"}-mysql>begin;
    Query OK, 0 rows affected (0.00 sec)
    {"db":"ytt"},"port":"3320"}-mysql>insert into c1 values (1,20,now());
    Query OK, 1 row affected (0.00 sec)
    {"db":"ytt"},"port":"3320"}-mysql>savepoint s1;
    Query OK, 0 rows affected (0.00 sec)
    {"db":"ytt"},"port":"3320"}-mysql>insert into c1 values (2,30,now());
    Query OK, 1 row affected (0.00 sec)
    {"db":"ytt"},"port":"3320"}-mysql>savepoint s2;
    Query OK, 0 rows affected (0.00 sec)
    {"db":"ytt"},"port":"3320"}-mysql>insert into c1 values (3,40,now());
    Query OK, 1 row affected (0.00 sec)
    {"db":"ytt"},"port":"3320"}-mysql>rollback to savepoint s2;
    Query OK, 0 rows affected (0.00 sec)
    {"db":"ytt"},"port":"3320"}-mysql>commit;
    Query OK, 0 rows affected (0.00 sec)
    {"db":"ytt"},"port":"3320"}-mysql>select * from c1;
    +----+------+---------------------+
    | id | c1   | c2                  |
    +----+------+---------------------+
    |  1 |   20 | 2019-12-02 10:07:02 |
    |  2 |   30 | 2019-12-02 10:07:12 |
    +----+------+---------------------+
    2 rows in set (0.00 sec)
    {"db":"ytt"},"port":"3320"}-mysql>

**2. 链式事务**

设置 completion\_type=1，也就是开启了链式事务特征。下面例子，commit work 后的语句是一个隐式事务语句。也就是说语句 rollback 语句执行后，默认的话，sql 2 肯定已经提交了。但是由于继承了上下文，也就是语句 sql 2变为 `begin;SQL2;`　那此时 sql 2 和 rollback 语句其实是一个事务块儿了。最终结果就是只有两条记录。

{"db":"ytt"},"port":"3320"}-mysql>truncate table c1;
    Query OK, 0 rows affected (0.01 sec)
    {"db":"ytt"},"port":"3320"}-mysql>set completion_type=1;
    Query OK, 0 rows affected (0.00 sec)
    {"db":"ytt"},"port":"3320"}-mysql>insert into c1 values (4,50,now());
    Query OK, 1 row affected (0.00 sec)
    {"db":"ytt"},"port":"3320"}-mysql>insert into c1 values (5,60,now());
    Query OK, 1 row affected (0.00 sec)
    -- sql 1
    {"db":"ytt"},"port":"3320"}-mysql>commit work;
    Query OK, 0 rows affected (0.00 sec)
    -- sql 2
    {"db":"ytt"},"port":"3320"}-mysql>insert into c1 values (6,70,now());
    Query OK, 1 row affected (0.00 sec)
    -- sql 3
    {"db":"ytt"},"port":"3320"}-mysql>rollback;
    Query OK, 0 rows affected (0.01 sec)
    {"db":"ytt"},"port":"3320"}-mysql>select * from c1;
    +----+------+---------------------+
    | id | c1   | c2                  |
    +----+------+---------------------+
    |  4 |   50 | 2019-12-02 10:14:16 |
    |  5 |   60 | 2019-12-02 10:14:31 |
    +----+------+---------------------+
    2 rows in set (0.00 sec)

**3. 嵌套事务**

其实严格意义上来说，MySQL 是不支持嵌套事务的。MySQL 的每个事务块的开始默认的会提交掉之前的事务。比如下面的例子，第二个 begin 语句默认会变为 `commit;begin;` 那之后的 rollback 其实只回滚了一条记录。最终记录数为 ID=7 这条。

{"db":"ytt"},"port":"3320"}-mysql>truncate table c1;
    Query OK, 0 rows affected (0.01 sec)
    {"db":"ytt"},"port":"3320"}-mysql>begin;
    Query OK, 0 rows affected (0.00 sec)
    {"db":"ytt"},"port":"3320"}-mysql>insert into c1 values (7,80,now());
    Query OK, 1 row affected (0.00 sec)
    {"db":"ytt"},"port":"3320"}-mysql>begin;
    Query OK, 0 rows affected (0.01 sec)
    {"db":"ytt"},"port":"3320"}-mysql>insert into c1 values (8,90,now());
    Query OK, 1 row affected (0.00 sec)
    {"db":"ytt"},"port":"3320"}-mysql>rollback;
    Query OK, 0 rows affected (0.01 sec)
    {"db":"ytt"},"port":"3320"}-mysql>select * from c1;
    +----+------+---------------------+
    | id | c1   | c2                  |
    +----+------+---------------------+
    |  7 |   80 | 2019-12-02 10:24:44 |
    +----+------+---------------------+
    1 row in set (0.00 sec)

**4. 自治事务**

其实 MySQL 本来不支持自治事务，但是基于 MySQL 先天的可插拔架构来说，也可以变相的实现自治事务。比如可以把记录日志的表变为非事务引擎表，比如 MyISAM。

{"db":"(none)"},"port":"3326"}-mysql>use ytt
    Database changed
    {"db":"ytt"},"port":"3326"}-mysql>create table log(err_msg varchar(200))engine myisam;
    Query OK, 0 rows affected (0.01 sec)
    {"db":"ytt"},"port":"3326"}-mysql>begin;
    Query OK, 0 rows affected (0.00 sec)
    {"db":"ytt"},"port":"3326"}-mysql>insert into t1 values (100);
    Query OK, 1 row affected (0.01 sec)
    {"db":"ytt"},"port":"3326"}-mysql>insert into log values ('这个记录不应该插入进来');
    Query OK, 1 row affected (0.00 sec)
    {"db":"ytt"},"port":"3326"}-mysql>select * from t1;
    +------+
    | id   |
    +------+
    |  100 |
    +------+
    1 row in set (0.00 sec)
    {"db":"ytt"},"port":"3326"}-mysql>rollback;
    Query OK, 0 rows affected, 1 warning (0.00 sec)
    {"db":"ytt"},"port":"3326"}-mysql>select * from log;
    +-----------------------------------+
    | err_msg                           |
    +-----------------------------------+
    | 这个记录不应该插入进来            |
    +-----------------------------------+
    1 row in set (0.00 sec)

**总结**

本篇内容主要把 MySQL 的事务类别简单介绍了下，针对了日常使用的几种场景做了简单的 SQL 演示，希望对大家有帮助。

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