Oracle9i的索引监视及注意事项

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对于DML操作来说，索引对于数据库是一个性能负担.如果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允许你监视索引的使用:

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,如果查询使用索引,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，如果你不慎监控了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'

如果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 2004

Copyright (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 - Production

SQL> 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。如果不知道此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。

[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 2004

Copyright (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. 在特殊的情况下,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#

/

注意到v$object_usage关键信息来源于OBJECT_USAGE表.另外我们可以注意一下,此处v$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.