Call now: 252-767-6166  
Oracle Training Oracle Support Development Oracle Apps

 
 Home
 E-mail Us
 Oracle Articles
New Oracle Articles


 Oracle Training
 Oracle Tips

 Oracle Forum
 Class Catalog


 Remote DBA
 Oracle Tuning
 Emergency 911
 RAC Support
 Apps Support
 Analysis
 Design
 Implementation
 Oracle Support


 SQL Tuning
 Security

 Oracle UNIX
 Oracle Linux
 Monitoring
 Remote s
upport
 Remote plans
 Remote
services
 Application Server

 Applications
 Oracle Forms
 Oracle Portal
 App Upgrades
 SQL Server
 Oracle Concepts
 Software Support

 Remote S
upport  
 Development  

 Implementation


 Consulting Staff
 Consulting Prices
 Help Wanted!

 


 Oracle Posters
 Oracle Books

 Oracle Scripts
 Ion
 Excel-DB  

Don Burleson Blog 


 

 

 


 

 

 

 
 

The _use_nosegment_indexes parameter and nosegment keyword

Oracle Tips by Burleson Consulting


Oracle has a special hidden "nosegment" clause to the create index syntax that allows you to create an index definition without actually creating the index.

A virtual index is a "fake" index whose definition exists in the data dictionary, but has no index tress association.  It is used by Oracle developers to test whether a specific index is going to use useful without having to use the disk space associated with the "real" index.  The hidden parameter _use_nosegment_indexes is used by Quest tools and is also use in the Oracle Tuning pack.

Creating a virtual index is easy:

create index
   virtual_idx
on
   emp(emp_id)
nosegment;

If you own the extra cost Oracle Tuning Pack, you will note the "virtual index wizard" area.  According to Oracle, virtual indexes will help you determine how the Oracle cost-based SQL optimizer (CBO) will evaluate and use the potential index.

Because the virtual index is a fake index, Oracle will never be able to use it, but you can use the hidden _use_nosegment_indexes parameter to evaluate execution plans for virtual indexes:

set autotrace on explain;
 
alter session set "_use_nosegment_indexes" = true;

Session altered.

SQL> select ename from emp where ename = 'KING';
 
ENAME
-----
KING

1 rows selected.

OBJECT     OPERATION
---------- ----------------------------------------
                 SELECT STATEMENT()
                  NESTED LOOPS()
EMP                TABLE ACCESS(FULL)
EMP                 TABLE ACCESS(BY INDEX ROWID)
VIRTUAL_INDEX         INDEX(UNIQUE SCAN)

 

I cannot really see a use for this undocumented Oracle features except in cases where you wanted to evaluate how the CBO will change execution plans on very large tables.

 

If you like Oracle tuning, you may enjoy the new book "Oracle Tuning: The Definitive Reference", over 900 pages of BC's favorite tuning tips & scripts. 

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.


 

 

��  
 
 
Oracle Training at Sea
 
 
 
 
oracle dba poster
 

 
Follow us on Twitter 
 
Oracle performance tuning software 
 
Oracle Linux poster
 
 
 

 

Burleson is the American Team

Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

Verify experience! Anyone considering using the services of an Oracle support expert should independently investigate their credentials and experience, and not rely on advertisements and self-proclaimed expertise. All legitimate Oracle experts publish their Oracle qualifications.

Errata?  Oracle technology is changing and we strive to update our BC Oracle support information.  If you find an error or have a suggestion for improving our content, we would appreciate your feedback.  Just  e-mail:  

and include the URL for the page.


                    









Burleson Consulting

The Oracle of Database Support

Oracle Performance Tuning

Remote DBA Services


 

Copyright © 1996 -  2017

All rights reserved by Burleson

Oracle ® is the registered trademark of Oracle Corporation.

Remote Emergency Support provided by Conversational