Hi,
I have some SQL that updates a table and I would like to improve it. As you can see from the code below it updates TableA from a select query based on a couple of other tables. This code works but if TableA had 50,000+ lines in it then it would be a slow and inefficient query. Is there a better way of doing this kind of update in Oracle - or is this my only option? Equally is Oracle intelligent when it see's this and does some 'magic' :-)
Any help greatly appreciated
Mike
This Question has been solved and asker verified All Experts Exchange premium technology solutions are available to subscription members.
Experts Exchange has been collecting answers to technology questions since 1996…3 million and counting! If you have a question, chances are we already have your answer.
If you can't find the exact answer you're looking for, ask our exclusive community of 50,000 experts. You’ll get a personalized answer from a trusted professional.
Thousands of free tech tips, tricks, how-to’s and tutorials are available in our peer reviewed articles section. See for yourself how smart our experts are, no login required.
Save solutions to your questions, answers you’ve discovered through searching plus helpful articles in your personal knowledgebase for easy future access.
Access the answers to your technology questions today.
30-day free trial. Register in 60 seconds.
Members of the expert community talk about why the experience at Experts Exchange is different than what you will find anywhere else.
Try it out and discover for yourself.
30-day free trial. Register in 60 seconds.
Join the community of experts here and help other tech pros by answering question in your area of expertise. You can earn FREE access to all Experts Exchange's premium features and resources.
The correlated subqueries are not always evil.
It depends on the kind of the correlated subquery.
For example
SELECT
a.col1, a.col2, (SELECT MIN(b.col1) FROM TABLE_B b where b.col2 = a.col3)
FROM
TABLE_A a
this is bad, because probably it computes the min for every row in A, one at a time.
But sometimes the optimizer can be smart enough to optimize and change the plan to a reasonable one.
But other correlated subqueries are not that bad.
You should always take a look at the execution plan.
Business Accounts
Answer for Membership
by: dragos_craciunPosted on 2008-06-02 at 07:57:43ID: 21692559
Well ... I see nothing wrong or inefficient in this query.
Do you have performance problems?
Did you do any test with "real life" data to test the speed?
Can you post the execution plan?
Alternatively you can use MERGE, something like:
MERGE INTO TableA x
USING
(
select a.fielda, a.fieldb, b.fieldc, b.fieldd, a.season, a.sty_num, a.sty_qual
from pro.sty_defs a, pro.loc_defs b
where a.con_num = b.loc_num
) n
ON
(x.sty_qual = n.sty_qual and x.sty_num = n.sty_num and x.season = n.season)
when matched then update set
x.desta = n.fielda, x.destb = n.fieldb, .......
you should collect statistics and test which is faster.
Also, please not that merge will update only the records in TableA for which there is any corresponding data in pro.sty_defs and pro.loc_defs, while the update writtenn as you wrote it will update all the records, setting the "orphaned" ones to NULL.