ORACLE中,约束分deferred 跟 immediate 2种:
deferred:如果 Oracle 在事务提交(commit)时才对约束执行检查,则称此约束是延迟的(deferred)。如果数据违反了延迟约束,提交操作将导致事务被回滚(undo)。
immediate:如果约束是即时的(immediate)(非延迟的),则此约束将在语句执行结束后进行检查。如果数据违反了延迟约束,语句将被立即回滚。
解释一下,即:将约束检验延迟到了执行COMMIT的时候。以下是具体的解释和举例。 定义为可延迟(deferrable)的约束可以指定为:
1. initially immediate(初始化立即执行)或
2. initially deferred(初始化延迟执行)。
解释二者的区别。初始化立即执行/延迟执行规定了在默认情况下应该如何执行约束:
初始化立即执行--在每条语句执行结束时检验约束
初始化延迟执行--一直等到事务完成后(或者调用set constraint immediate语句时)才检验约束
一般情况下,我们用的约束初始都是immediate型的(默认),而且不好转为deferred型。但是如果初始是deferrable(需要手动指定),那deferred跟immediate 2种状态可以随意转换。
此外,约束有以下4种状态:
ENABLE(启用)确保所有输入的数据都遵从约束(constraint)
DISABLE(禁用)总是允许输入数据,无论数据是否遵从约束
VALIDATE(验证)确保已存在的数据遵从约束
NOVALIDATE(无验证)允许已存在的数据不遵从约束
ENABLE VALIDATE 与 ENABLE 相同。Oracle 将检查约束,并保证所有数据均遵从约束。
ENABLE NOVALIDATE 表示所有新插入或被修改的数据都必须遵从约束,但允许已存在的数据不遵从约束。
DISABLE NOVALIDATE 与 DISABLE 相同。Oracle 不会检查约束.
DISABLE VALIDATE 将禁用约束,移除约束使用的索引,并禁止修改约束键的数据。
约束不论哪种类型,要能够生效,必须状态是enable才行。
--deferred 跟 immediate的对比试验-----------------------------
SQL> drop table aa purge;
Table dropped.
SQL> create table aa ( id number,name varchar2(20),constraint pk primary key(id));
Table created.
SQL> col constraint_name for a11
SQL> select CONSTRAINT_NAME ,CONSTRAINT_TYPE,TABLE_NAME ,STATUS,DEFERRABLE,DEFERRED,validated from u
ser_constraints where table_name='AA';
CONSTRAINT_ C TABLE STATUS DEFERRABLE DEFERRED VALIDATED
----------- - ----- -------- -------------- --------- -------------
PK P AA ENABLED NOT DEFERRABLE IMMEDIATE VALIDATED
--可以看到,默认的是NOT DEFERRABLE,下面我们将它转为immediate试试
SQL> set constraint pk immediate;
set constraint pk immediate
*
ERROR at line 1:
ORA-02447: cannot defer a constraint that is not deferrable
--增加一个uk,指定deferrable initially immidiate|deferred
SQL> alter table aa add constraint uk unique (name) deferrable initially immediate;
Table altered.
UK U AA ENABLED DEFERRABLE IMMEDIATE VALIDATED
SQL> select * from aa;
no rows selected
SQL> insert into aa values(1,'SDF');
1 row created.
SQL> insert into aa values(2,'SDF');
insert into aa values(2,'SDF')
ORA-00001: unique constraint (LYN.UK) violated
SQL> set constraints uk deferred;
Constraint set.
SQL> commit;
commit
ORA-02091: transaction rolled back
此外,可以在SESSION里设定所有约束是立即检查(immediate)还是延迟检查(deferred),当然,这只影响初始时指定可延迟的(deferrable)约束。
SQL> alter session set constraint=immediate;
Session altered.
SQL> alter session set constraints=deferred;
--4种状态对比-----------------
SQL> create table aa(id number primary key);
--enable validate(默认)
SQL> select CONSTRAINT_NAME ,CONSTRAINT_TYPE,TABLE_NAME ,STATUS,DEFERRABLE,DEFERRED,validated from
user_constraints where table_name='AA';
SYS_C002829 P AA ENABLED NOT DEFERRABLE IMMEDIATE VALIDATED
SQL> insert into aa select 2010 from dual;
insert into aa select 2010 from dual
ORA-00001: unique constraint (LYN.SYS_C002829) violated
--disable novalidate
SQL> alter table aa modify constraints SYS_C002829 disable;
SYS_C002829 P AA DISABLED NOT DEFERRABLE IMMEDIATE NOT VALIDATED
ID
----------
2010
Commit complete.
--disable validate
SQL> alter table aa modify constraints SYS_C002829 disable validate;
alter table aa modify constraints SYS_C002829 disable validate
ORA-02437: cannot validate (LYN.SYS_C002829) - primary key violated
SQL> delete aa where rownum<3;
2 rows deleted.
SQL> select rownum,id from aa;
ROWNUM ID
---------- ----------
1 2010
SQL> update aa set id=2012;
update aa set id=2012
ORA-25128: No insert/update/delete on table with constraint (LYN.SYS_C002829) disabled and validated
SQL> insert into aa select 2012 from dual;
insert into aa select 2012 from dual
SQL> delete from aa;
delete from aa
SQL> truncate table aa;
truncate table aa
--enable novalidate