RE: SELECT Output Default Ordering ?

This is not so much a response to Binley's comment as a general note on the whole thread.

Not using ORDER BY when you need it is a huge risk... It doesn't matter what hint you have, if someone drops the index, you're hosed. That's what the ORDER BY is for. If you use the ORDER BY clause with the right index, then performance will be spectacular. The Oracle query optimizers are smart enough not to sort if the rows are coming from an index guaranteed to provide the specified ordering.

If the index is missing, then of course performance will blow, but at least the application will produce the correct answer. The users screaming about the performance problem will clue you in that you have to fix something. But at least the users won't ever get the wrong answer back from the application.

The huge problem with using the index hint without the ORDER BY clause is that situations can easily occur in which users might go on for *months* getting wrong data out of the database without ever knowing there's a functional bug in their application.

Cary Millsap
Hotsos Enterprises, Ltd.
http://www.hotsos.com

Upcoming events:

- Hotsos Clinic 101 in Dallas, Washington, Denver, Sydney
- Hotsos Symposium 2004, March 7-10 Dallas
- Visit www.hotsos.com for schedule details...

-----Original Message-----
Binley Lim
Sent: Wednesday, June 25, 2003 6:51 PM
To: Multiple recipients of list ORACLE-L

In cases where the SQL is an important (ie frequently used) part of the application, you do get significant gains with the index hint technique.

The ORDER BY is a guarantee - you have to pay your insurance premiums (ie additional sort).

The index hint is a trade-off with the devil - you have to understand what you are doing, or it may take a few years off your life.

• Original Message ----- To: Multiple recipients of list ORACLE-L <[EMAIL PROTECTED]> Sent: Thursday, June 26, 2003 9:27 AM

> Beware, though, that without explicit ORDER BY clauses, you're not
> guaranteed to get the results in the order you expect (I think you
mentioned
> this yourself, for example, when an index is missing). It's not just a
> performance problem. In some applications, you'll get the *wrong answer*
if
> you don't use ORDER BY. Example:
>
> SELECT stuff, score
> FROM tables
> WHERE conditions AND ROWNUM<=10
> ORDER BY score DESC
>
>
> Cary Millsap
> Hotsos Enterprises, Ltd.
> http://www.hotsos.com
>
> Upcoming events:
> - Hotsos Clinic 101 in Dallas, Washington, Denver, Sydney
> - Hotsos Symposium 2004, March 7-10 Dallas
> - Visit www.hotsos.com for schedule details...
>
>
> -----Original Message-----
> DENNIS WILLIAMS
> Sent: Wednesday, June 25, 2003 3:58 PM
> To: Multiple recipients of list ORACLE-L
>
> Tanel
> No ORDER BY. It upsets we DBAs, but some sites purchase Oracle and
don't
> hire a DBA. Especially smaller sites. Naturally Oracle doesn't want to
make
> it sound like they must have a DBA or the customer will probably buy MS
SQL.
> So as a vendor you must have your application run reasonably well with no
> tuning if you want to sell to small sites. Otherwise the customer will
blame
> you for a fussy application. If you add an ORDER BY, sometimes Oracle will
> decide it needs to sort, or at least that is the fear. The method I
describe
> has worked pretty well over many Oracle versions (I think one of the
Oracle
> 7.1 versions ignored hints). And yes, if an index is missing, it doesn't
> blow up, it just gets really sloooooow. But it makes sense to the customer
> that if they dropped an index that things foul up. Then they don't blame
the
> vendor, just recreate the index and remember to not get so creative next
> time.
> I think the lesson here is that as a DBA you need to support certain
> applications and understand WHY the vendor did certain things a particular
> way. Right now my learning project is E.piphany so if anyone on the list
> works with that, please drop me a private note.
>
> Dennis Williams
> DBA, 80%OCP, 100% DBA
> Lifetouch, Inc.
> [EMAIL PROTECTED]
>
>
> -----Original Message-----
> Sent: Wednesday, June 25, 2003 1:05 PM
> To: Multiple recipients of list ORACLE-L
>
>
> > Vivek
> > If you want the data returned in an order, you can create an index
with
> > the order you want, and in your query provide a hint for Oracle to use
> that
> > index. If your query is such that Oracle actually uses that index, the
> data
> > will be returned in that order. I work with a large application that
> > entirely depends on this principle. Crude but nevertheless effective.
>
> Ouch!!!
>
> Or did you mean that you still use ORDER BY, but index scan helps to avoid
> sorting?
>
> What if this index becomes invalid for some reason, your hint won't be
used,
> and if you don't have order by, your application will fail big time!
>
> Tanel.
>
>
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-- 
Author: Binley Lim
  INET: [EMAIL PROTECTED]

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