Oracle外键不加索引会引起死锁问题
转载链接：http://www.jb51.net/article/50161.htm
--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 constraint pk_pktable primary key (OBJECT_ID);
--创建子表的外键
alter table FK_t add constraint 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 where object_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 where object_id=200;
delete from pk_t where object_id=200;
--回到session1:死锁马上发生
delete from pk_t where object_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 where object_id=100;
delete from pk_t where object_id=100;
--session2
delete from fk_t where object_id=200;
delete from pk_t where object_id=200;
--回到session1不会发生死锁
delete from pk_t where object_id=100;
关于外键不加索引引起死锁的详细分析参考http://blog.csdn.net/robinson1988/article/details/38421991
经过验证，如果外键不加索引的确会出现死锁。因此，外键务必添加索引。
