–創建一個表，此表作為子表

create table fk_t as select *from user_objects;

delete from fk_t where object_id is null;

commit;

–創建一個表，此表作為父表

create table pk_t as select *from user_objects;

delete from pk_t where object_id is null;

commit;

–創建父表的主鍵

alter table PK_t add constraintpk_pktable primary key (OBJECT_ID);

–創建子表的外鍵

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

–session1:執行一個刪除操作，這時候在子表和父表上都加了一個Row-S(SX)鎖

delete from fk_t whereobject_id=100;

delete from pk_t where object_id=100;

–session2:執行另一個刪除操作，發現這時候第二個刪除語句等待

delete from fk_t whereobject_id=200;

delete from pk_t whereobject_id=200;

–回到session1:死鎖馬上發生

delete from pk_t whereobject_id=100;

session2中報錯：

SQL> delete from pk_table where object_id=200;

delete from pk_table where object_id=200

*

第 1 行出現錯誤:

ORA-00060: 等待資源時檢測到死鎖

當對子表的外鍵列添加索引后，死鎖被消除，因為這時刪除父表記錄不需要對子表加表級鎖。

–為外鍵建立索引

create index ind_pk_object_id on fk_t(object_id) nologging;

–重復上面的操作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不會發生死鎖

delete from pk_t whereobject_id=100;

總結

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