Tuesday, February 19, 2008

Oracle Index Monitoring Benefits


There are many new features available in Oracle 9i and 10g database releases. Out of the these many new features, a few standout from the crowd and 'Index Monitoring' is one of them. It is introduced in the 9i release and its one of my favorites in Oracle. Whereas the benefits are enormous, the risks are minimal to zero.

All database indexes need to be maintained in Oracle whether they are being used by a SQL query execution plan. The primary purpose of creating indexes is to reduce the cost of an sql operation thereby increasing performance and response times in the database. Sometimes database developers create indexes that will be never be used by Oracle.

Example 1 :- Creating index on a column that will be used only in the select clause and never used in the where clause of an sql statement (non-driving columns).

Example 2 :- Another common example is when the cost based optimizer decides that a table access or an alternate index/composite index is a better access path than using the index in question .

One of the most easy to implement Oracle 9i features (and 10g) has been 'Index Monitoring'. The feature identifies all unused indexes in the database and updates a data dictionary view with this information. A DBA in coordination with the development team can review these results and proceed to drop these indexes.

Benefits of dropping unused indexes
1. Faster DML operations
Database table insert/update/delete statements can be executed quickly without the need for Oracle to insert/update/delete the corresponding indexes in the background.
2. Reduced Database size
Indexes occupy the next highest storage after tables in most transactional databases. If large indexes are found unused, they can help reduce the overall database size.
3. Less RedoLog consumption
All indexes update/insert/delete get logged (if they are created with the default LOGGING option - which is the norm rather than an exception ) in the redologs. If unwanted/redundant indexes are dropped, less redologs are switched per day thereby increasing overall database performance by a good multiple.
4. Reduced archive space
More redologs caused by unused indexes mean more archivelog files.
5. Faster database backups
Dropping unused indexes will decrease the database size. Lesser sized databases take less time for their backup.
6. Reduced recovery time
Less archive logs translates to lesser archive logs to apply in database recovery operations.
Implementation Procedure
1. Identify all hot custom indexes which are of greater than a threshold size (e.g 100MB)
2. For each of the above indexes, the following command need to be executed
alter index monitoring usage;

3. Let Oracle monitor the indexes usage for 24 hrs. The data dictionary view v$object_usage gets updated if the index has been used atleast once in the last 24 hrs. (Here 24 hrs is just an example. Some sites may need 48 hrs of monitoring or even upto 7 days of monitoring. )

4. Turn off index monitoring using executing the following command
alter index nomonitoring usage;

5. Get all unused indexes reported in the view v$object_usage
Are there any dependencies ? Is downtime needed for Index Monitoring ?
Index Monitoring has no dependencies with any database operations. It can be dynamically implemented with a very little overhead. There is also no downtime required for the implementation.

1. Implementing Index Monitoring in development and test databases may not yield good results or may not be used as a basis for production implementation. Whereas several indexes may be shown as unused in development databases, the same indexes may be used more frequently in production because of the higher transaction rate.

2. Only custom indexes need to be considered for index monitoring. Its not advisable to monitor indexes that come in a pre-seeded database similar to the Oracle 11i E-Business Suite database.

3. The actual Index Monitoring is entirely done by Oracle without a DBA intervention. In ideal cases, Oracle may not even report any unused/redundant index. But this is very rare and happens only in perfect database design cases. So the index monitoring exercise may end up without any unused/redundant indexes in perfect design databases.

4. Sometimes an index is used exclusively for a weekend job or a monthend job to improve its performance. Exercise sufficient caution before dropping unused indexes reported in v$object_usage. Confirmation has to be taken from the index owner that there is no special purpose for the concerned index.
Database upgrades and migrations to new releases have to be performed with a pro-active frame of mind instead of a reactive frame of mind. I have seen many Oracle upgrades and migrations performed just to meet Oracle support deadlines or to meet a prerequisite for other software components. New features like Index Monitoring come with the least risks and lot of benefits. If you are starting a 'reorg' exercise in your database, Index Monitoring can be a good starting point.


Anonymous said...

This article on Index monitoring is very informative.

Anonymous said...

This was a very good article, thank you for linking me to this in my OTN discussion of the same topic.

The one thing I would be very careful about is waiting only 24 hours. I'd check with your user community first before dropping indexes in just 24 hours. We have a lot of monthly and even yearly jobs here that rely on some of these indexes. I'd also create a spot to store the DDL of these dropped indexes so they can be re-created easily if performance issues do crop up.

I am actually turning on this monitoring and letting it run for a year before doing anything. I am collecting monthly stats from the v$object_usage table and will do a thorough analysis at year end to compile a list of index candidates to be dropped.

-- G. Chaika
Cornell University

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