约束的DEFERRABLE, NOT DEFERRABLE, INITIALLY IMMEDIATE 和 INITIALLY DEFERRED

[ CONSTRAINT constraint_name ]
{ NOT NULL |
  NULL |
  CHECK ( expression ) [ NO INHERIT ] |
  DEFAULT default_expr |
  UNIQUE index_parameters |
  PRIMARY KEY index_parameters |
  REFERENCES reftable [ ( refcolumn ) ] [ MATCH FULL | MATCH PARTIAL | MATCH SIMPLE ]
    [ ON DELETE action ] [ ON UPDATE action ] }
[ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ]

DEFERRABLE(可以推迟的约束检查)：可以推迟到事务结尾。目前仅UNIQUE, PRIMARY KEY, EXCLUDE和FOREIGN KEY才支持DEFERRABLE。NOT NULL和CHECK约束不支持。

NOT DEFERRABLE(不可推迟的约束检查)：在每一个命令后会立即验证是否满足约束条件。缺省是NOT DEFERRABLE。

如果约束是INITIALLY IMMEDIATE，那么每条语句之后就检查它。这个是缺省。

如果约束是INITIALLY DEFERRED，那么直到事务完成才检查。

检查的时间可以用以下命令修改：

SET CONSTRAINTS [ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ]

例子：

• 默认是NOT DEFERRABLE INITIALLY IMMEDIATE

create table t1 (id number, name char(10));

alter table t1 modify id number primary key;

SQL> insert into t1 values(1,'Tough1');
已创建 1 行。

SQL> insert into t1 values(1,'Tough1');
insert into t1 values(1,'Tough1')
*
第 1 行出现错误:
ORA-00001: 违反唯一约束条件 (SCOTT.SYS_C005428)

• DEFERRABLE或INITIALLY DEFERRED的情况：

create table t2 (id number, name char(10));

alter table t2 modify id number primary key INITIALLY DEFERRED;

或

alter table t2 modify id number primary key DEFERRABLE INITIALLY DEFERRED;

SQL> insert into t2 values(1,'Tough1');

已创建 1 行。

SQL> insert into t2 values(1,'Tough1');

已创建 1 行。

SQL> commit;
commit
*
第 1 行出现错误:
ORA-02091: 事务处理已回退
ORA-00001: 违反唯一约束条件 (SCOTT.SYS_C005435)

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