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;