Oracle 约束(constraint)的几个参数的小研究

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. 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
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')
*
ERROR at line 1:
ORA-00001: unique constraint (LYN.UK) violated
SQL> set constraints uk deferred; Constraint set. SQL> select * from aa; no rows selected SQL> insert into aa values(1,'SDF'); 1 row created. SQL> insert into aa values(2,'SDF'); 1 row created. SQL> commit;
commit
*
ERROR at line 1:
ORA-02091: transaction rolled back
ORA-00001: unique constraint (LYN.UK) violated
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;
Session altered. --4种状态对比-----------------
SQL> drop table aa purge; Table dropped. SQL> create table aa(id number primary key); Table created. --enable validate(默认) SQL> select CONSTRAINT_NAME ,CONSTRAINT_TYPE,TABLE_NAME ,STATUS,DEFERRABLE,DEFERRED,validated from
user_constraints where table_name='AA'; CONSTRAINT_ C TABLE STATUS DEFERRABLE DEFERRED VALIDATED
----------- - ----- -------- -------------- --------- -------------
SYS_C002829 P AA ENABLED NOT DEFERRABLE IMMEDIATE VALIDATED SQL> insert into aa select 2010 from dual; 1 row created. SQL> insert into aa select 2010 from dual;
insert into aa select 2010 from dual
*
ERROR at line 1:
ORA-00001: unique constraint (LYN.SYS_C002829) violated --disable novalidate SQL> alter table aa modify constraints SYS_C002829 disable; Table altered. SQL> select CONSTRAINT_NAME ,CONSTRAINT_TYPE,TABLE_NAME ,STATUS,DEFERRABLE,DEFERRED,validated from
user_constraints where table_name='AA'; CONSTRAINT_ C TABLE STATUS DEFERRABLE DEFERRED VALIDATED
----------- - ----- -------- -------------- --------- -------------
SYS_C002829 P AA DISABLED NOT DEFERRABLE IMMEDIATE NOT VALIDATED SQL> select * from aa; ID
----------
2010 SQL> insert into aa select 2010 from dual; 1 row created. SQL> insert into aa select 2010 from dual; 1 row created. SQL> commit; Commit complete. --disable validate SQL> alter table aa modify constraints SYS_C002829 disable validate;
alter table aa modify constraints SYS_C002829 disable validate
*
ERROR at line 1:
ORA-02437: cannot validate (LYN.SYS_C002829) - primary key violated
SQL> select * from aa; ID
----------
2010
2010
2010 SQL> delete aa where rownum<3; 2 rows deleted. SQL> select rownum,id from aa; ROWNUM ID
---------- ----------
1 2010 SQL> alter table aa modify constraints SYS_C002829 disable validate; Table altered. SQL> update aa set id=2012;
update aa set id=2012
*
ERROR at line 1:
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
*
ERROR at line 1:
ORA-25128: No insert/update/delete on table with constraint (LYN.SYS_C002829) disabled and validated
SQL> delete from aa;
delete from aa
*
ERROR at line 1:
ORA-25128: No insert/update/delete on table with constraint (LYN.SYS_C002829) disabled and validated
SQL> truncate table aa;
truncate table aa
*
ERROR at line 1:
ORA-25128: No insert/update/delete on table with constraint (LYN.SYS_C002829) disabled and validated
--enable novalidate
SQL> alter table aa enable novalidate constraint SYS_C002829; Table altered. SQL> insert into aa select 2010 from dual;
insert into aa select 2010 from dual
*
ERROR at line 1:
ORA-00001: unique constraint (LYN.SYS_C002829) violated