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
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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;