技术分享 | MySQL 的嵌套事务、自治事务与链式事务

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mb5fe55a71c1d08 2021-01-25 09:27:18 ©著作权

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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)