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ROLLBACK相关的统计信息

王朝other·作者佚名  2008-05-31
窄屏简体版  字體: |||超大  

与ROLLBACK相关的统计信息有如下几个,我们关注什么情况下这些统计信息会增加。

SQL> select s.name,m.value from v$mystat m,v$statname s where s.statistic#=m.statistic# and name like '%rollback%';

NAME VALUE

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

user rollbacks 0

transaction tables consistent read rollbacks 0

rollbacks only - consistent read gets 0

cleanouts and rollbacks - consistent read gets 0

rollback changes - undo records applied 0

transaction rollbacks 0

初始情况下:

SQL> select s.name,m.value from v$mystat m,v$statname s where s.statistic#=m.statistic# and name like '%rollback%';

NAME VALUE

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

user rollbacks 0

transaction tables consistent read rollbacks 0

rollbacks only - consistent read gets 0

cleanouts and rollbacks - consistent read gets 0

rollback changes - undo records applied 0

transaction rollbacks 0

6 rows selected.

未作任何改动,执行rollback

SQL> rollback;

Rollback complete.

SQL> select s.name,m.value from v$mystat m,v$statname s where s.statistic#=m.statistic# and name like '%rollback%';

NAME VALUE

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

user rollbacks 1

transaction tables consistent read rollbacks 0

rollbacks only - consistent read gets 0

cleanouts and rollbacks - consistent read gets 0

rollback changes - undo records applied 0

transaction rollbacks 0

6 rows selected.

统计信息user rollbacks 增加,即使没有做任何改动(没有什么可以rollback的),user rollbacks也会增加。

SQL> connect /

SQL> select s.name,m.value from v$mystat m,v$statname s where s.statistic#=m.statistic# and upper(name) like '%ROLLBACK%';

NAME VALUE

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

user rollbacks 0

transaction tables consistent read rollbacks 0

rollbacks only - consistent read gets 0

cleanouts and rollbacks - consistent read gets 0

rollback changes - undo records applied 0

transaction rollbacks 0

6 rows selected.

插入一行纪录,然后回滚

SQL> insert into t values(1);

1 row created.

SQL> rollback;

Rollback complete.

SQL> select s.name,m.value from v$mystat m,v$statname s where s.statistic#=m.statistic# and upper(name) like '%ROLLBACK%';

NAME VALUE

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

user rollbacks 1

transaction tables consistent read rollbacks 0

rollbacks only - consistent read gets 0

cleanouts and rollbacks - consistent read gets 0

rollback changes - undo records applied 2

transaction rollbacks 1

6 rows selected.

除了user rollbacks增加外,当有活动的事务回滚时候,因为有实际工作可做,transaction rollbacks增加,且因为回滚记录的应用,rollback changes - undo records applied增加。

SQL> insert into t values(1);

1 row created.

SQL> insert into t values(1);

1 row created.

SQL> commit;

commit

*

ERROR at line 1:

ORA-02091: transaction rolled back

ORA-00001: unique constraint (Oracle.SYS_C0020256) violated

SQL> select s.name,m.value from v$mystat m,v$statname s where s.statistic#=m.statistic# and upper(name) like '%ROLLBACK%';

NAME VALUE

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

user rollbacks 1

transaction tables consistent read rollbacks 0

rollbacks only - consistent read gets 0

cleanouts and rollbacks - consistent read gets 0

rollback changes - undo records applied 8

transaction rollbacks 2

6 rows selected.

如上,因为发生unique constraint violated错误,Statement自动回滚(到默认的savepoint),有回滚工作可做,则transaction rollbacks 和rollback changes - undo records applied都增加。

SQL> select * from t;

no rows selected

SQL> rollback;

Rollback complete.

SQL> select s.name,m.value from v$mystat m,v$statname s where s.statistic#=m.statistic# and upper(name) like '%ROLLBACK%';

NAME VALUE

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

user rollbacks 2

transaction tables consistent read rollbacks 0

rollbacks only - consistent read gets 0

cleanouts and rollbacks - consistent read gets 0

rollback changes - undo records applied 8

transaction rollbacks 2

6 rows selected.

因为Statement已经回滚,没有实际的回滚工作量,因此只有user rollbacks增加。

SQL> connect / 如下 测试因为Write consistent导致的mini-rollback

SQL> select s.name,m.value from v$mystat m,v$statname s where s.statistic#=m.sta

NAME VALUE

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

user rollbacks 0

transaction tables consistent read rollbacks 0

rollbacks only - consistent read gets 0

cleanouts and rollbacks - consistent read gets 0

rollback changes - undo records applied 0

transaction rollbacks 0

6 rows selected.

SQL> update t set x=x*10 where x in (1,5,10);

2 rows updated. 之前该session被另外的一个session在更新x=5处被阻塞;

SQL> select s.name,m.value from v$mystat m,v$statname s where s.statistic#=m.statistic# and upper(name) like '%ROLLBACK%';

NAME VALUE

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

user rollbacks 0

transaction tables consistent read rollbacks 0

rollbacks only - consistent read gets 0

cleanouts and rollbacks - consistent read gets 1

rollback changes - undo records applied 3

transaction rollbacks 0

6 rows selected.

如上的mini-rollback 会导致cleanouts and rollbacks - consistent read gets 增加且带往返滚(rollback changes - undo records applied)。

结论是:

1) 'user rollbacks' 当用户执行rollback的时候每次递增,而不管是否有实际的回滚工作量要做。

2) 'transaction rollbacks'在如下3种情况下会递增

当用户执行 'rollback' 回滚了之前的DML操作,因此有实际的回滚工作要做。

进程中断 terminated or killed, PMON 代替执行回滚。

DML 发生错误,如. unique constraint violation,执行Statement级别的语句回滚

3) 'user rollbacks' 和'transaction rollbacks' 不会因为"write consistency" mini-rollback 的原因增加。

4)mini-rollback会导致cleanouts and rollbacks - consistent read gets增加。当发生mini-rollback的时候,update/delete语句会释放掉所有已更新的block的锁(cleanout),然后再以select for update(v$lock.mode=2) 先“悲观”锁住要更新的纪录,在执行update/delete.(v$lock.mode=3)

 
 
 
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