Question: What
does the 10g qb_name hint do? Can you describe when to
use the qb_name hint?
Answer: The cost-based optimizer
tries to eliminate multiple select statements in a complex
query by merging in-line views and subqueries into a single
SQL statement. In Oracle parlance, each in-line view
and subquery is a separate and distinct "query block".
Solely for debugging purposes, Oracle has created the
query block name (qb_name) hint to allow you to clearly see
the individual query blocks in a complex SQL query that has
subqueries (or in-line views).
For example, Oracle un-nests some correlated subqueries
and replaces it with a single anti-join operation. Ti
see this, you can add a qb_name hint and then reference this
nased query block in a no_unnest hint:
select /*+
no_unnest(@nested_query) */
*
from
t1
where
not exists
(
select
/*+ qb_name(nested_query) */
null
from
t1 t1_inner
where
t1_inner.id = t1.id
);
Oracle experts use the qb_name hint to plant "markers".
These markers have two purposes:
1 - The markers make reading complex
execution plans (subqueries) easier to read.
2 - The qb_name hint allows hints to
be applied to subqueries.
|
|
|
|
|
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 - 2016
All rights reserved by
Burleson
Oracle ®
is the registered trademark of Oracle Corporation.
|
|
