PostgreSQL事务特性之嵌套事务

嵌套事务的实现是基于SAVEPOINT、ROLLBACK TO SAVEPOINT和RELEASE SAVEPOINT的，也就是设置一个保存点，可以回滚到保存点和释放保存点。

测试表的初始状态如下：

postgres=# \d test
     Table "public.test"
 Column |  Type   | Modifiers
--------+---------+-----------
 id     | integer |
 name   | text    | 

postgres=# select * from test ;
 id | name
----+------
(0 rows)

开始测试

postgres=# begin ;
BEGIN
postgres=# insert into test values (1, 'a');
INSERT 0 1
postgres=# savepoint insert_a;
SAVEPOINT
postgres=# select * from test ;
 id | name
----+------
  1 | a
(1 row)

postgres=# insert into test values (2, 'b');
INSERT 0 1
postgres=# savepoint insert_b;
SAVEPOINT
postgres=# select * from test ;
 id | name
----+------
  1 | a
  2 | b
(2 rows)

postgres=# insert into test values (3, 'c');
INSERT 0 1
postgres=# select * from test ;
 id | name
----+------
  1 | a
  2 | b
  3 | c
(3 rows)

现在定义了两个SAVEPOINT，并且插入了3条数据，现在测试ROLLBACK TO SAVEPOINT

postgres=# rollback to insert_b;
ROLLBACK
postgres=# select * from test ;
 id | name
----+------
  1 | a
  2 | b
(2 rows)

postgres=# rollback to insert_a;
ROLLBACK
postgres=# select * from test ;
 id | name
----+------
  1 | a
(1 row)

可见回滚到前面定义的保存点成功了。

如果回滚到前面的保存点，后面的更改就丢失了，包括保存点，比如回滚到insert_a,那么在insert_a之后的数据就没有了，insert_b这个保存点也不存在了。

postgres=# rollback to insert_a;
ROLLBACK
postgres=# select * from test ;
 id | name
----+------
  1 | a
(1 row)

postgres=# rollback to insert_b;
ERROR:  no such savepoint

测试RELEASE SAVEPOINT

postgres=# select * from test ;
 id | name
----+------
(0 rows)

postgres=# begin ;
BEGIN
postgres=# insert into test values (1, 'a');
INSERT 0 1
postgres=# savepoint insert_a;
SAVEPOINT
postgres=# select * from test ;
 id | name
----+------
  1 | a
(1 row)

postgres=# insert into test values (2, 'b');
INSERT 0 1
postgres=# savepoint insert_b;
SAVEPOINT
postgres=# select * from test ;
 id | name
----+------
  1 | a
  2 | b
(2 rows)

postgres=# release insert_a;
RELEASE
postgres=# select * from test ;
 id | name
----+------
  1 | a
  2 | b
(2 rows)

postgres=# rollback to insert_a;
ERROR:  no such savepoint

保存点被释放后就不能再回滚到该保存点了。