Oracle外键不加索引引起死锁示例

–创建一个表，此表作为子表

create table fk_t as select *from user_objects;

delete from fk_t where object_id is null;

commit;

–创建一个表，此表作为父表

create table pk_t as select *from user_objects;

delete from pk_t where object_id is null;

commit;

–创建父表的主键

alter table PK_t add constraintpk_pktable primary key (OBJECT_ID);

–创建子表的外键

alter table FK_t addconstraint fk_fktable foreign key (OBJECT_ID) references pk_t (OBJECT_ID);

–session1:执行一个删除操作，这时候在子表和父表上都加了一个Row-S(SX)锁

delete from fk_t whereobject_id=100;

delete from pk_t where object_id=100;

–session2:执行另一个删除操作，发现这时候第二个删除语句等待

delete from fk_t whereobject_id=200;

delete from pk_t whereobject_id=200;

–回到session1:死锁马上发生

delete from pk_t whereobject_id=100;

session2中报错：

SQL> delete from pk_table where object_id=200;
delete from pk_table where object_id=200
*
第 1 行出现错误:

ORA-00060: 等待资源时检测到死锁

当对子表的外键列添加索引后，死锁被消除，因为这时删除父表记录不需要对子表加表级锁。

–为外键建立索引

create index ind_pk_object_id on fk_t(object_id) nologging;

–重复上面的操作session1

delete from fk_t whereobject_id=100;

delete from pk_t whereobject_id=100;

–session2

delete from fk_t whereobject_id=200;

delete from pk_t whereobject_id=200;

–回到session1不会发生死锁

delete from pk_t whereobject_id=100;