死锁

<pre name="code" class="sql">  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。

创建2张测试表:
SQL> select * from t1;

	ID NAME
---------- ----------
	 1 a
	 2 b

SQL> select * from t2;

	ID NAME
---------- ----------
	 1 a
	 2 b



第一种情况 :

两个会话持有同一对象的不同记录

---SESSION 2187
SQL> select * from v$mystat where rownum<2;

       SID STATISTIC#	   VALUE
---------- ---------- ----------
      2187	    0	       0

SQL> select * from t1;

	ID NAME
---------- ----------
	 1 a
	 2 b

SQL> update t1 set id=3 where id=1 and 1=1;

1 row updated.


--SESSION 98
SQL> show user
USER is "DWF"
SQL> select * from v$mystat where rownum<2;

       SID STATISTIC#	   VALUE
---------- ---------- ----------
	98	    0	       0

SQL> update t1 set id=4 where id=2 and 2=2;

1 row updated.



SESSION 2187 执行:

SQL> update t1 set id=4 where id=2 and 3=3;

hang住


SESSION 98 

执行SQL>  update t1 set id=3 where id=1 and 4=4;


此时SESSION 2187报死锁错误
SQL> update t1 set id=4 where id=2 and 3=3;
update t1 set id=4 where id=2 and 3=3
       *
ERROR at line 1:
ORA-00060: deadlock detected while waiting for resource



查看trace文件

Deadlock graph:
                       ---------Blocker(s)--------  ---------Waiter(s)---------
Resource Name          process session holds waits  process session holds waits
TX-000a0008-000033cd        23    2187     X             25      98           X
TX-00090011-0000155c        25      98     X             23    2187           X

session 2187: DID 0001-0017-000003E4    session 98: DID 0001-0019-000000F1
session 98: DID 0001-0019-000000F1      session 2187: DID 0001-0017-000003E4

Rows waited on:
  Session 2187: obj - rowid = 00019B31 - AAAZsxAAxAAA5jWAAB
  (dictionary objn - 105265, file - 49, block - 235734, slot - 1)
  Session 98: obj - rowid = 00019B31 - AAAZsxAAxAAA5jWAAA
  (dictionary objn - 105265, file - 49, block - 235734, slot - 0)

----- Information for the OTHER waiting sessions -----
Session 98:
  sid: 98 ser: 347 audsid: 103737 user: 91/DWF
    flags: (0x45) USR/- flags_idl: (0x1) BSY/-/-/-/-/-
    flags2: (0x40009) -/-/INC
  pid: 25 O/S info: user: oracle, term: UNKNOWN, ospid: 11545
    image: oracle@dwh1 (TNS V1-V3)
  client details:
    O/S info: user: oracle, term: pts/4, ospid: 11544
    machine: dwh1 program: sqlplus@dwh1 (TNS V1-V3)
    application name: SQL*Plus, hash value=3669949024
  current SQL:
  update t1 set id=3 where id=1 and 4=4

[oracle@dwh1 trace]$ grep update /oracle/app/diag/rdbms/dwh1/dwh1/trace/dwh1_ora_11486.trc | grep t1
  update t1 set id=3 where id=1 and 4=4
update t1 set id=4 where id=2 and 3=3
        ObjectName:  Name=update t1 set id=4 where id=2 and 3=3 
  ObjectName:  Name=update t1 set id=4 where id=2 and 3=3 
update t1 set id=4 where id=2 and 3=3
sql=update t1 set id=4 where id=2 and 3=3



trc文件里只有引起死锁的会话和产生死锁的会话


第二种情况:

两个会话持有不同对象的记录

--SESSION 2187

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

       SID STATISTIC#	   VALUE
---------- ---------- ----------
      2187	    0	       0

SQL> update t2 set id=3 where id=1 and 1=1;

1 row updated.

--SESSION 98
SQL> select * from v$mystat where rownum<2;

       SID STATISTIC#	   VALUE
---------- ---------- ----------
	98	    0	       0

SQL> update t1 set id=3 where id=1 and 2=2;

1 row updated.


SESSION 2187执行:
SQL>  update t1 set id=3 where id=1 and 3=3;

此时HANG

SESSION 98 执行
SQL> update t2 set id=3 where id=1  and 4=4;

此时SESION 2187
SQL>  update t1 set id=3 where id=1 and 3=3;


 update t1 set id=3 where id=1 and 3=3
        *
ERROR at line 1:
ORA-00060: deadlock detected while waiting for resource


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