Re: v$object_usage: anyone have bad experience with it?

The index monitoring feature simply flags an index as used when the optimizer generates an execution plan that includes that index.

This means that any use of the index that is not instigated by the optimization process is simply 'forgotten'.

It also means that when the optimizer produces a 'bad plan', you may have a misleading report identifying a 'used, therefore useful' index.

For a good strategy on index monitoring, you might like to read the following article, which has some useful comments on the
issue:

http://www.dizwell.com/html/useful_indexes.html

Regards

Jonathan Lewis

http://www.jlcomp.demon.co.uk/faq/ind_faq.html The Co-operative Oracle Users' FAQ

http://www.jlcomp.demon.co.uk/seminar.html Public Appearances - schedule updated Jan 21st 2005

• Original Message ----- From: "zhu chao" <zhuchao_at_gmail.com> To: "ORACLE-L" <oracle-l_at_freelists.org> Sent: Saturday, February 05, 2005 9:52 AM Subject: v$object_usage: anyone have bad experience with it?

Hi, all,
  We have a database which maxed its CPU capacity. and I did some SQL tuning /index creation. I happened to think some index should be useless and I enabled index monitoring for some index and they do showed used='NO' in v$object_usage.
  Later I enabled index monitoring in all the indexes for this user, and I see:
SQL> select used,count(*) from v$object_usage group by used; USE COUNT(*)
--- ----------

NO 160
YES 108
  and these indexes used 36G space:
1 select uniqueness, sum(bytes) from user_segments a, user_indexes b where segment_name in
2 (select index_name from v$object_usage where used='NO') 3 and a.segment_name=b.index_name
4* group by uniqueness
SQL> /
UNIQUENES SUM(BYTES)
--------- ----------------

NONUNIQUE 36,488,478,720
UNIQUE 272,760,832
    I plan to drop those indexes. But I am not sure whether there is bugs/issues with the v$object_usage that it does not report some used index, or under some circumstance, even SQL don't use the index , we have to keep these indexes.

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http://www.freelists.org/webpage/oracle-l Received on Sat Feb 05 2005 - 11:27:34 CST