對于DML操作來說，索引對于數(shù)據(jù)庫是一個性能負擔.假如索引沒有被有效的使用,那么其存在性就值得從新考慮.
1. 從Oracle9i開始,Oracle答應(yīng)你監(jiān)視索引的使用:
SQL> connect scott/tiger@connerConnected to Oracle9i Enterprise Edition Release 9.2.0.4.0 Connected as scottSQL> select index_name from user_indexes;INDEX_NAME------------------------------PK_DEPTPK_EMP開始監(jiān)視pk_dept索引:SQL> alter index pk_dept monitoring usage;Index altered在此過程中,假如查詢使用索引,將會記錄下來:SQL> select * from dept where deptno=10;DEPTNO DNAME LOC------ -------------- ------------- 10 ACCOUNTING NEW YORK停止監(jiān)視:SQL> alter index pk_dept nomonitoring usage;Index altered查詢索引使用情況,YES表示在監(jiān)視過程中索引被使用到:SQL> select * from v$object_usage;INDEX_NAME TABLE_NAME MONITORING USED START_MONITORING END_MONITORING----------------- ------------------ ---------- ---- ------------------- -------------------PK_DEPT DEPT NO YES 10/28/2004 10:55:19 10/28/2004 10:55:47SQL>
2.Oracle9i的Bug
在9205之前，假如你不慎監(jiān)控了SYS.I_OBJAUTH1索引，并且不幸在重起數(shù)據(jù)庫之前沒有停止它，那么你的數(shù)據(jù)庫將會無法啟動，并且不會給出任何錯誤信息。
以下這條簡單的語句可以輕易再現(xiàn)這個問題:
'ALTER INDEX SYS.I_OBJAUTH1 MONITORING USAGE'
假如你有了足夠好的備份(嚴重警告，請不要拿你的生產(chǎn)數(shù)據(jù)庫進行測試)，你可以嘗試一下:
[oracle@jumper oradata]$ 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 - ProductionWith the Partitioning optionJServer 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> startupORACLE instance started.
Total System Global Area 80811208 bytesFixed Size 451784 bytesVariable Size 37748736 bytesDatabase Buffers 41943040 bytesRedo Buffers 667648 bytesDatabase mounted.
此時，數(shù)據(jù)庫掛起，而且不會有任何提示，在alert<sid>.log文件中，你可以看到:
[oracle@jumper bdump]$ tail -f alert_conner.log Completed: ALTER DATABASE MOUNTSat Dec 4 10:09:49 2004ALTER DATABASE OPENSat Dec 4 10:09:49 2004LGWR: Primary database is in CLUSTER CONSISTENT modeThread 1 opened at log sequence 54 Current log# 2 seq# 54 mem# 0: /opt/oracle/oradata/conner/redo02.logSuccessful open of redo thread 1.Sat Dec 4 10:09:49 2004SMON: enabling cache recoverySat Dec 4 10:10:33 2004Restarting dead background process QMN0QMN0 started with pid=9
然后數(shù)據(jù)庫將會停在此處。
假如不知道此bug存在，你可能會一籌莫展的。
現(xiàn)在你能做的就是從備份中恢復(fù)，或者升級到9.2.0.5。
Oracle已經(jīng)Release了這個Bug，你可以參考Metalink:Note:2934068.8,Oracle聲明在9.2.0.5 (Server Patch Set)和 10g Production Base Release中fixed了這個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 2004
Copyright (c) 1982, 2002, Oracle Corporation. All rights reserved.
Connected to an idle instance.
SQL> startupORACLE instance started.
Total System Global Area 80811208 bytesFixed Size 451784 bytesVariable Size 37748736 bytesDatabase Buffers 41943040 bytesRedo Buffers 667648 bytesDatabase mounted.Database opened.SQL>
3. 在非凡的情況下,你可能需要清除這個v$object_usage視圖中的信息.
Oracle的說法是,在下一次收集該對象的索引使用情況時會自動覆蓋上一次的信息,不提供清除手段.
稍微研究了一下.
v$object_usage是基于以下基表建立起來的:
create or replace view v$object_usage(index_name, table_name, monitoring, used, start_monitoring, end_monitoring)asselect io.name, t.name, decode(bitand(i.flags, 65536), 0, 'NO', 'YES'), decode(bitand(ou.flags, 1), 0, 'NO', 'YES'), ou.start_monitoring, ou.end_monitoringfrom sys.obj$ io, sys.obj$ t, sys.ind$ i, sys.object_usage ouwhere io.owner# = userenv('SCHEMAID') and i.obj# = ou.obj# and io.obj# = ou.obj# and t.obj# = i.bo#/
注重到v$object_usage要害信息來源于OBJECT_USAGE表.另外我們可以注重一下,此處v$object_usage的查詢基于userenv('SCHEMAID')建立.所以以不同用戶登錄,你是無法看到其他用戶的索引監(jiān)視信息的,即使是dba,但是可以從object_usage表中得到.
SQL> select * from v$object_usage;INDEX_NAME TABLE_NAME MON USE START_MONITORING END_MONITORING------------------------------ ------------------------------ --- --- ------------------- -------------------PK_DEPT DEPT NO YES 10/28/2004 10:55:19 10/28/2004 10:55:47SQL> select * from object_usage;select * from object_usage *ERROR at line 1:ORA-00942: table or view does not existSQL> connect /as sysdbaConnected.SQL> / OBJ# FLAGS START_MONITORING END_MONITORING---------- ---------- ------------------- ------------------- 6288 1 10/28/2004 10:55:19 10/28/2004 10:55:47
實際上我們清除了object_usage表的記錄,實際上也就清空了v$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
此操作對數(shù)據(jù)庫沒有潛在的影響,但是請謹慎使用.作為實驗?zāi)康奶峁?
