Oracle外键不加索引引起死锁
--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;