Oracle可延迟约束Deferable的使用

1 deferrable介绍
1.1 deferrable的两个选项区别
deferrable表示该约束是可延迟验证的. 它有两个选项:
Initially immediate(默认): 立即验证, 执行完一个sql后就进行验证；
Initially deferred: 延迟验证, 当事务提交时或调用set constraint[s] immediate语句时才验证.
区别是: 事务提交时验证不通过, 则立即回滚事务; set constraint[s] immediate时只验证, 不回滚事务.
1.2 not deferrable与deferrable区别
区别就在于: “立即验证的可延迟约束” 是可以根据需要设置成 “延迟验证的可延迟约束”的, 而“不可延迟验证”是不能改变的.
2 deferrable实例
2.1 建表
create table test1(a number(1) constraint check_a check(a
> 0) deferrable
                   initially immediate,
                   b number(1) constraint check_b check(b
> 0) deferrable
                   initially deferred);
2.2 正常插入，没问题
SQL> insert into test1 values(1, 1);
1 row inserted
2.3 检验立即验证：数据不能插入
SQL> insert into test1 values(-1, 1);
insert into test1 values(-1, 1)
ORA-02290: 违反检查约束条件 (MYHR.CHECK_A)
2.4 检验延迟验证：可以执行
SQL> insert into test1 values(1, -1);
1 row inserted
SQL> select * from test1;
 A  B
-- --
 1  1
 1 -1
2.5 提交延迟验证(commit)：验证失败，自动回滚
SQL> commit;
 commit
 ORA-02091: 事务处理已回退
ORA-02290: 违反检查约束条件 (MYHR.CHECK_B)
2.6 提交延迟验证(set constraint immediate)：验证失败，不回滚
SQL> insert into test1 values(1, -1);
1 row inserted
SQL> set constraint check_b immediate;
set constraint check_b immediate
ORA-02290: 违反检查约束条件 (MYHR.CHECK_B)
或者将所有的约束做修改: alter session set constraints = immediate;
2.7 将延迟验证设置为立即验证：则在插入时出错
SQL> set constraint check_b immediate;
Constraints set
SQL> insert into test1 values(1,-1);
insert into test1 values(1,-1)
ORA-02290: 违反检查约束条件 (MYHR.CHECK_B)
3 deferrable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 SNAME                        TNO
----------- -------------------- -----------
          1 stu_1                          1
          2 stu_2                          1
 
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 SNAME                        TNO
----------- -------------------- -----------
          1 stu_1                          1
          2 stu_2                          1
 
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 SNAME                        TNO
----------- -------------------- -----------
          1 stu_1                         11
          2 stu_2                         11
 
SQL> select a.constraint_name, a.deferrable, a.deferred from user_constraints a where a.constraint_name like '%TNO%';
CONSTRAINT_NAME                DEFERRABLE     DEFERRED
------------------------------ -------------- ---------
FK_S_TNO                       DEFERRABLE     IMMEDIATE
PK_S_TNO                       NOT DEFERRABLE IMMEDIATE
PK_T_TNO                       NOT DEFERRABLE IMMEDIATE