PostgreSQL事務特性之嵌套事務

嵌套事務的實現是基於SAVEPOINT、ROLLBACK TO SAVEPOINT和RELEASE SAVEPOINT的，也就是設置一個保存點，能夠回滾到保存點和釋放保存點。

測試表的初始狀態以下：

postgres=# \d test Table "public.test" Column | Type | Modifiers --------+---------+----------- id | integer | name | text | postgres=# select * from test ; id | name ----+------ (0 rows)

 

開始測試

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)

 

如今定義了兩個SAVEPOINT，而且插入了3條數據，如今測試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)

 

可見回滾到前面定義的保存點成功了。

若是回滾到前面的保存點，後面的更改就丟失了，包括保存點，好比回滾到insert_a,那麼在insert_a以後的數據就沒有了，insert_b這個保存點也不存在了。

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

 

測試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

 

保存點被釋放後就不能再回滾到該保存點了。