3deferrable用途

3.1物化视图

物化视图(快照)，这是它的主要用途。这些视图会使用延迟约束来进行视图刷新。在刷新物化视图的过程中，可能会破坏完整性，而且将不能逐句检验约束。但到执行COMMIT时，数据完整性就没问题了，而且能满足约束。没有延迟约束，物化视图的约束可能会使刷新过程不能成功进行。

3.2级联更新

当预测是否需要更新父/子关系中的主键时，它有助于级联更新。看一下实际的例子：

SQL> create table t(tno number(10) constraint pk_t_tno primary key, tname varchar2(20));

SQL> create table s(sno number(10) constraint pk_s_tno primary key, sname varchar2(20), tno number(10));

SQL> alter table s add constraint fk_s_tno foreign key (tno) references t(tno);

SQL> insert into t values(1,'yuechaotian');

SQL> insert into t values(2,'tianyuechao');

SQL> commit;

SQL> insert into s values(1,'stu_1', 1);

SQL> insert into s values(2,'stu_2', 1);

SQL> commit;

SQL> update t set tno=22 where tno=2;

SQL> update t set tno=11 where tno=1;

update t set tno=11 where tno=1

ORA-02292:违反完整约束条件(MYHR.FK_S_TNO) -已找到子记录

SQL> select * from t;

TNO TNAME

----------- --------------------

1 yuechaotian

22 tianyuechao

SQL> select *from s;

SNO SNAMETNO

----------- -------------------- -----------

1 stu_11

2 stu_21

SQL> rollback;

SQL> alter table s drop constraint fk_s_tno;

SQL> alter table s add constraint fk_s_tno foreign key (tno) references t(tno) deferrable initially immediate;

SQL> select * from t;

TNO TNAME

----------- --------------------

1 yuechaotian

2 tianyuechao

SQL> select * from s;

SNO SNAMETNO

----------- -------------------- -----------

1 stu_11

2 stu_21

SQL> set constraint fk_s_tno deferred;

Constraints set

SQL> update t set tno=22 where tno=2;

SQL> update t set tno=11 where tno=1;

SQL> update s set tno = 11 where tno = 1;

SQL> commit;

SQL> select * from t;

TNO TNAME

----------- --------------------

11 yuechaotian

22 tianyuechao

SQL> select * from s;

SNO SNAMETNO

----------- -------------------- -----------

1 stu_111

2 stu_211

SQL> select a.constraint_name, a.deferrable, a.deferred from user_constraints a where a.constraint_name like '%TNO%';

CONSTRAINT_NAMEDEFERRABLEDEFERRED

------------------------------ -------------- ---------

FK_S_TNODEFERRABLEIMMEDIATE

PK_S_TNONOT DEFERRABLE IMMEDIATE

PK_T_TNONOT DEFERRABLE IMMEDIATE