oracle 中约束有如下分类：
1、not deferrable(默认)
2、deferrable:
①initially immediate：每条语句执行结束时检查约束
②initially deferred：事务结束时才检查约束（或者设置了set constraint immediate时检查）

not deferrable和deferrable initially immediate 效果一样

实际环境测试结果如下：
操作系统：AIX 5.3
oracle版本：9.2.0.8

SQL> create table a (
2 x int constraint check_x check ( x > 0 )
3 　　deferrable
4 　　initially immediate,
5 y int constraint check_y check ( y > 0 )
6 　　deferrable
7 　　initially deferred
8 );

Table created
SQL> insert into a values (1,1);

1 row inserted

SQL> commit;

Commit complete

SQL> select * from a;

X Y
------------- ---------------
1 1

SQL> insert into a values (-1,1);

insert into a values (-1,1)

ORA-02290: check constraint (SAN.CHECK_X) violated
//立刻检查约束
SQL> select * from a;

X Y
---------- ----------
1 1
//回滚

SQL> insert into a values (1,-1);

1 row inserted

SQL> commit;

commit

ORA-02091: transaction rolled back
ORA-02290: check constraint (SAN.CHECK_Y) violated
//事务结束时检查

SQL> set constraints all deferred;

Constraints set
//手工设置为延迟检查约束
SQL> insert into a values (-1,1);

1 row inserted

SQL> commit;

commit

ORA-02091: transaction rolled back
ORA-02290: check constraint (SAN.CHECK_X) violated

SQL> set constraints all immediate;
//手工设置为立刻检查约束
Constraints set

SQL> insert into a values (1,-1);

insert into a values (1,-1)

ORA-02290: check constraint (SAN.CHECK_Y) violated