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!
|
|
|
|
| |
|
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.
Copyright © 1996 - 2020
All rights reserved by
Burleson
Oracle ®
is the registered trademark of Oracle Corporation.
|
|
