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 


 

 

 


 

 

 
 

  Oracle descending index tips

Oracle Database Tips by Donald BurlesonMarch 25, 2015

Question:  If Oracle can read indexes in both directions, what is the purpose of the Oracle descending index?  How to I implement an Oracle descending index?  Are there special execution plans which use descending indexes?

Answer:  The Oracle database supports descending indexes and they have special min/max execution plans associated with their usage.  With a descending index you will see the execution plan reference the index:

INDEX RANGE SCAN DESCENDING

It is interesting to note that when you create an index with the "desc" option, it creates a index as a composite function based index.  The choice of ascending or descending index when defining an index would depend on the kind of SQL queries that you want to answer using that index.

For example, a query that asks for the key values in descending order would benefit from a descending index. Any query with an ORDER BY DESC would use a descending index to remove the need for a back-end sort operation.

Note that in some releases the descending index is buggy and causes sub-optimal execution plans.  See MOSC bug 11072246 (Oracle 11gr2) and MOSC Note titled ""non optimal index is used when index column is in desc order".

Also, remember that an ascending index can be read in descending order, so there are VERY few benefits to creating a index with the DESC clause.  In most cases a "vanilla" ascending index will be able to handle requests for data in descending order, using the index and sorting the result set, much faster and more efficiently than the descending index.

   
Oracle Training from Don Burleson 

The best on site "Oracle training classes" are just a phone call away! You can get personalized Oracle training by Donald Burleson, right at your shop!

Oracle training
 
 


 

 

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

 

 

��  
 
 
Oracle Training at Sea
 
 
 
 
oracle dba poster
 

 
Follow us on Twitter 
 
Oracle performance tuning software 
 
Oracle Linux poster