--創(chuàng)建一個(gè)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(chuàng)建一個(gè)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(chuàng)建父表的主鍵

alter table PK_t add constraintpk_pktable primary key (OBJECT_ID);

--創(chuàng)建子表的外鍵

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

--session1:執(zhí)行一個(gè)刪除操作，這時(shí)候在子表和父表上都加了一個(gè)Row-S(SX)鎖

delete from fk_t whereobject_id=100;

delete from pk_t where object_id=100;

--session2:執(zhí)行另一個(gè)刪除操作，發(fā)現(xiàn)這時(shí)候第二個(gè)刪除語(yǔ)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(fā)生

delete from pk_t whereobject_id=100;

session2中報(bào)錯(cuò)：

SQL> delete from pk_table where object_id=200;
delete from pk_table where object_id=200
*
第 1 行出現(xiàn)錯(cuò)誤:

ORA-00060: 等待資源時(shí)檢測(cè)到死鎖

當(dāng)對(duì)子表的外鍵列添加索引后，死鎖被消除，因?yàn)檫@時(shí)刪除父表記錄不需要對(duì)子表加表級(jí)鎖。

--為外鍵建立索引

create index ind_pk_object_id on fk_t(object_id) nologging;

--重復(fù)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(huì)發(fā)生死鎖

delete from pk_t whereobject_id=100;

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