死锁

  如果你有两个会话,每个会话都持有另一个会话想要的资源,此时就会出现死锁(deadlock)。例如,



如果我的数据库中有两个A和B,每个表都只有一行,就可以很容易地展示什么是死锁。



我要做的知识打开两个回话(例如,两个SQL*PLUS会话).在会话A中更更新A表,在会话B中更新B。



现在,如果我想在会话B中更新A,就会堵塞。会话A已经锁定了这一行,这不是死锁:只是堵塞而已。



如果我再回到会话A,试图更新表B,这就会导致一个死锁。要在这个会话中选择一个作为牺牲品,



让它的语句回滚。





SQL> select * from v$mystat where rownum<2;



       SID STATISTIC#	   VALUE

---------- ---------- ----------

	35	    0	       0



SQL> create table t3(id int);



Table created.



SQL> insert into t3 values(1);



1 row created.



SQL> insert into t3 values(2);



1 row created.



SQL> commit;



Commit complete.





SQL> update t3 set id=10 where id=1;



1 row updated.



SQL>  update t3 set id=30 where id=2;

 update t3 set id=30 where id=2

        *

ERROR at line 1:

ORA-00060: deadlock detected while waiting for resource







###############################################################

SQL> select * from v$mystat where rownum<2;



       SID STATISTIC#	   VALUE

---------- ---------- ----------

	36	    0	       0





SQL> update t3 set id=20 where id=2;



1 row updated.



SQL> update t3 set id=40 where id=1;



hang-------





查看trace 文件:



----- Information for the OTHER waiting sessions -----

Session 36:

  sid: 36 ser: 145 audsid: 1531179 user: 91/TEST flags: 0x45

  pid: 28 O/S info: user: oracle, term: UNKNOWN, ospid: 29718

    image: oracle@june (TNS V1-V3)

  client details:

    O/S info: user: oracle, term: pts/0, ospid: 29714

    machine: june program: sqlplus@june (TNS V1-V3)

    application name: SQL*Plus, hash value=3669949024

  current SQL:

  update t3 set id=10 where id=1

 

----- End of information for the OTHER waiting sessions -----

 

Information for THIS session:

 

----- Current SQL Statement for this session (sql_id=27jv5jpf0wh5z) -----

 update t3 set id=20 where id=2