事务保存点

SAVEPOINT 语句可以在事务过程中标记一个"保存点"，事务可以选择回滚到这个点。保存点是可选的，一个事务过程中也可以有多个保存点。

示例：将一个事务回滚到一个保存点

下面示例展示了一个事务中包含多个 DML 语句和多个保存点，然后回滚到其中一个保存点，只丢弃了保存点后面的那部份修改。

查看表当前记录

obclient> select * from t_insert;
+----+------+-------+---------------------+
| id | name | value | gmt_create          |
+----+------+-------+---------------------+
|  1 | CN   | 10001 | 2020-04-03 16:05:45 |
|  2 | US   | 10002 | 2020-04-03 16:05:54 |
|  3 | UK   | 10003 | 2020-04-03 16:05:54 |
+----+------+-------+---------------------+
3 rows in set (0.00 sec)

开启一个事务，设置多个保存点信息。

obclient> set session autocommit=off;
Query OK, 0 rows affected (0.00 sec)

obclient> begin;
Query OK, 0 rows affected (0.00 sec)

obclient> insert into t_insert(id, name) values(6,'FR');
Query OK, 1 row affected (0.00 sec)

obclient> savepoint fr;
Query OK, 0 rows affected (0.00 sec)

obclient> insert into t_insert(id, name) values(7,'RU');
Query OK, 1 row affected (0.00 sec)

obclient> savepoint ru;
Query OK, 0 rows affected (0.00 sec)

obclient> insert into t_insert(id, name) values(8,'CA');
Query OK, 1 row affected (0.00 sec)

obclient> savepoint ca;
Query OK, 0 rows affected (0.00 sec)

当前会话能看到事务未提交的所有修改。

obclient> select * from t_insert;
+----+------+-------+---------------------+
| id | name | value | gmt_create          |
+----+------+-------+---------------------+
|  1 | CN   | 10001 | 2020-04-03 16:05:45 |
|  2 | US   | 10002 | 2020-04-03 16:05:54 |
|  3 | UK   | 10003 | 2020-04-03 16:05:54 |
|  6 | FR   |  NULL | 2020-04-03 16:26:22 |
|  7 | RU   |  NULL | 2020-04-03 16:26:32 |
|  8 | CA   |  NULL | 2020-04-03 16:26:42 |
+----+------+-------+---------------------+
6 rows in set (0.00 sec)

回滚事务到其中一个保存点。

obclient> rollback to savepoint ru;
Query OK, 0 rows affected (0.00 sec)

obclient> select * from t_insert;
+----+------+-------+---------------------+
| id | name | value | gmt_create          |
+----+------+-------+---------------------+
|  1 | CN   | 10001 | 2020-04-03 16:05:45 |
|  2 | US   | 10002 | 2020-04-03 16:05:54 |
|  3 | UK   | 10003 | 2020-04-03 16:05:54 |
|  6 | FR   |  NULL | 2020-04-03 16:26:22 |
|  7 | RU   |  NULL | 2020-04-03 16:26:32 |
+----+------+-------+---------------------+
5 rows in set (0.01 sec)

提交事务，确认表最新修改包含保存点之前的修改。

obclient> commit;
Query OK, 0 rows affected (0.00 sec)
obclient> select * from t_insert;
+----+------+-------+---------------------+
| id | name | value | gmt_create          |
+----+------+-------+---------------------+
|  1 | CN   | 10001 | 2020-04-03 16:05:45 |
|  2 | US   | 10002 | 2020-04-03 16:05:54 |
|  3 | UK   | 10003 | 2020-04-03 16:05:54 |
|  6 | FR   |  NULL | 2020-04-03 16:26:22 |
|  7 | RU   |  NULL | 2020-04-03 16:26:32 |
+----+------+-------+---------------------+
5 rows in set (0.00 sec)

obclient>

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事务保存点

示例：将一个事务回滚到一个保存点

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