对于DML操作来说，索引对于数据库是一个性能负担.假如索引没有被有效的使用,那么其存在性就值得从新考虑.
1. 从Oracle9i开始,Oracle答应你监视索引的使用:SQL> connect scott/tiger@conner
Connected to Oracle9i Enterprise Edition Release 9.2.0.4.0
Connected as scott
SQL> select index_name from user_indexes;
INDEX_NAME
------------------------------
PK_DEPT
PK_EMP
开始监视pk_dept索引:
SQL> alter index pk_dept monitoring usage;
Index altered
在此过程中,假如查询使用索引,将会记录下来:
SQL> select * from dept where deptno=10;
DEPTNO DNAME LOC
------ -------------- -------------
10 ACCOUNTING NEW YORK
停止监视:
SQL> alter index pk_dept nomonitoring usage;
Index altered
查询索引使用情况,YES表示在监视过程中索引被使用到:
SQL> select * from v$object_usage;
INDEX_NAME TABLE_NAME MONITORING USED START_MONITORING END_MONITORING
----------------- ------------------ ---------- ---- ------------------- -------------------
PK_DEPT DEPT NO YES 10/28/2004 10:55:19 10/28/2004 10:55:47
SQL>
2.Oracle9i的Bug在9205之前，假如你不慎监控了SYS.I_OBJAUTH1索引，并且不幸在重起数据库之前没有停止它，那么你的数据库将会无法启动，并且
不会给出任何错误信息。以下这条简单的语句可以轻易再现这个问题:'ALTER INDEX SYS.I_OBJAUTH1 MONITORING USAGE' 假如你有了足够好的备份(严重警告，请不要拿你的生产数据库进行测试)，你可以尝试一下:
[oracle@jumper oradata]$ sqlplus "/ as sysdba"
SQL*Plus: Release 9.2.0.4.0 - ProdUCtion on Sat Dec 4 10:09:30 2004Copyright (c) 1982, 2002, Oracle Corporation. All rights reserved.
Connected to:
Oracle9i Enterprise Edition Release 9.2.0.4.0 - Production
With the Partitioning option
JServer Release 9.2.0.4.0 - ProductionSQL> alter index SYS.I_OBJAUTH1 monitoring usage ;Index altered.SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup
ORACLE instance started.Total System Global Area 80811208 bytes
Fixed Size 451784 bytes
Variable Size 37748736 bytes
Database Buffers 41943040 bytes
Redo Buffers 667648 bytes
Database mounted. 此时，数据库挂起，而且不会有任何提示，在alert<sid>.log文件中，你可以看到:
[oracle@jumper bdump]$ tail -f alert_conner.log
Completed: ALTER DATABASE MOUNT
Sat Dec 4 10:09:49 2004
ALTER DATABASE OPEN
Sat Dec 4 10:09:49 2004
LGWR: Primary database is in CLUSTER CONSISTENT mode
Thread 1 opened at log sequence 54
Current log# 2 seq# 54 mem# 0: /opt/oracle/oradata/conner/redo02.log
Successful open of redo thread 1.
Sat Dec 4 10:09:49 2004
SMON: enabling cache recovery
Sat Dec 4 10:10:33 2004
Restarting dead background process QMN0
QMN0 started with pid=9
然后数据库将会停在此处。假如不知道此bug存在，你可能会一筹莫展的。现在你能做的就是从备份中恢复，或者升级到9.2.0.5。Oracle已经Release了这个Bug，你可以参考Metalink:Note:2934068.8,Oracle声明在9.2.0.5 (Server Patch Set)和 10g Production Base Release中fixed了这个Bug。
[oracle@jumper oradata]$ rm -rf conner
[oracle@jumper oradata]$ cp -R connerbak/ conner
[oracle@jumper oradata]$ sqlplus '/ as sysdba'
SQL*Plus: Release 9.2.0.4.0 - Production on Sat Dec 4 10:19:07 2004Copyright (c) 1982, 2002, Oracle Corporation. All rights reserved.Connected to an idle instance.SQL> startup
ORACLE instance started.Total System Global Area 80811208 bytes
Fixed Size 451784 bytes
Variable Size 37748736 bytes
Database Buffers 41943040 bytes
Redo Buffers 667648 bytes
Database mounted.
Database opened.
SQL> 3. 在非凡的情况下,你可能需要清除这个v$object_usage视图中的信息.
Oracle的说法是,在下一次收集该对象的索引使用情况时会自动覆盖上一次的信息,不提供清除手段.稍微研究了一下.v$object_usage是基于以下基表建立起来的:
create or replace view v$object_usage
(index_name, table_name, monitoring, used, start_monitoring, end_monitoring)
as
select io.name, t.name,
decode(bitand(i.flags, 65536), 0, 'NO', 'YES'),
decode(bitand(ou.flags, 1), 0, 'NO', 'YES'),
ou.start_monitoring,
ou.end_monitoring
from sys.obj$ io, sys.obj$ t, sys.ind$ i, sys.object_usage ou
where io.owner# = userenv('SCHEMAID')
and i.obj# = ou.obj#
and io.obj# = ou.obj#
and t.obj# = i.bo#
/
注重到v$object_usage要害信息来源于OBJECT_USAGE表.
另外我们可以注重一下,此处v$object_usage的查询基于userenv('SCHEMAID')建立.
所以以不同用户登录,你是无法看到其他用户的索引监视信息的,即使是dba,但是可以从object_usage表中得到.
SQL> select * from v$object_usage;
INDEX_NAME TABLE_NAME MON USE START_MONITORING END_MONITORING
------------------------------ ------------------------------ --- --- ------------------- -------------------
PK_DEPT DEPT NO YES 10/28/2004 10:55:19 10/28/2004 10:55:47
SQL> select * from object_usage;
select * from object_usage
*
ERROR at line 1:
ORA-00942: table or view does not exist
SQL> connect /as sysdba
Connected.
SQL> /
OBJ# FLAGS START_MONITORING END_MONITORING
---------- ---------- ------------------- -------------------
6288 1 10/28/2004 10:55:19 10/28/2004 10:55:47
实际上我们清除了object_usage表的记录,实际上也就清空了v$object_usage的信息. SQL> delete from object_usage;
1 row deleted.
SQL> commit;
Commit complete.
SQL> select * from v$object_usage;
no rows selected
此操作对数据库没有潜在的影响,但是请谨慎使用.作为实验目的提供.
本文作者:
eygle,Oracle技术关注者,来自中国最大的Oracle技术论坛itpub.
www.eygle.com是作者的个人站点.你可通过Guoqiang.Gai@gmail.com来联系作者.欢迎技术探讨交流以及链接交换.原文出处:http://www.eygle.com/internal/How.to.Monitor.Index.and.How.to.Clean.out.v$object_usage.htm
