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，可以回滾到保存點和釋放保存點。
測試表的初始狀態如下：
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)
現在定義了兩個SAVEPOINT，並且插入了3條數據，現在測試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。
如果回滾到前面的保存點，后面的更改就丟失了，包括保存點，比如回滾到insert_a,那么在insert_a之后的數據就沒有了，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
保存點被釋放后就不能再回滾到該保存點了。