Re: Re[2]: to_number question

  • From: "Jonathan Lewis" <jonathan@xxxxxxxxxxxxxxxxxx>
  • To: <oracle-l@xxxxxxxxxxxxx>
  • Date: Sat, 17 Jul 2004 13:09:45 +0100

Think about this one:

create table t1 (
    v1 varchar2(1),
    v2 varchar2(1)
);

insert into t1 values (1,'a');
insert into t1 values ('a',1);
insert into t1 values(2,2);

commit;

select * 
from t1 
where
        to_number(v1) = 2
and    to_number(v2) = 2
;

If you handled the first two rows differently
from each other (which, in effect, is what relational 
theory says you should do - since column ordering
is not supposed to be significant), you can get the
right result set.  But there is no current mechanism in
Oracle that could find it.

From the relational point of view, the answer to this
'problem' is easy.  You've made a mistake in your
analysis and got the column domain wrong - you
need a domain which includes numbers and characters
and includes a "to_number" function that has meaning 
when applied to a character-like object in that domain.


Regards

Jonathan Lewis

http://www.jlcomp.demon.co.uk/faq/ind_faq.html
The Co-operative Oracle Users' FAQ

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----- Original Message ----- -----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx
[mailto:oracle-l-bounce@xxxxxxxxxxxxx]On Behalf Of Jonathan Gennick
Sent: Friday, July 16, 2004 14:32
To: Stephen.Lee@xxxxxxxx
Cc: oracle-l@xxxxxxxxxxxxx
Subject: Re[2]: to_number question


Well put Stephen. I agree with your post.

SLDC> The comments were an expression of disbelief (not lack
SLDC> of understanding) that the specifications concerning
SLDC> subqueries would be so loose and open ended as to
SLDC> allow this level of unpredictability.

Yes. And there's a lot more to this issue than just the
specific Oracle behavior we've been talking about. I have
many questions, most of which I'll just have to research
off-list. For example:

* What would relational theory have to say about this
subquery behavior? What would Codd say? What would Date say?
Do any of their writings bear directly on subquery
execution?

* What about the SQL language designers? What did they have
in mind when they spec'd out subqueries? What's their
conceptual vision for how subqueries execute? Did they write
anything into the standard in detail that bears on the
behavior we've been discussing?

* It would seem that the mental model of FROM-clause
subquery results becoming the source of data for the main
query is flawed. What then, is the correct mental model to
use when thinking about and writing such subqueries? (this
last is a *very* important question)

* Clearly we have a case where an optimization changes the
behavior of a statement. Without optimization the statement
succeeds. With optimization the statement fails. Even though
I understand what is happening, this troubles me. It raises
an interesting question: should an optimization be allowed
to change the results? Why/why not?

* Related to the above, it would now seem that in order to
write a statement containing a subquery, that you must be
aware of, and think about, how that statement's execution is
optimized. Yet databases are supposed to free us from
worrying about optimization and physical access and the
like.

* What about other database vendors? Is this an Oracle-only
issue? How does this scenario play out under MySQL? under
DB2? under SQL Server? Is there consensus among vendors? Or
do those who design optimizers take different, fundamental
approaches to the issue we've been discussing? (I'd really
like to talk to the developers behind the various database
optimizers about this issue)

* Oh, and this *is* important, I haven't had time yet to put
together a test case that reproduces Stephen's error. It's
on my to-do list though. My one attempt so far failed to
reproduce the error. If I can't put together a test case,
then I have nothing to research.

Now that I see what Oracle is doing in this one case, I want
to go deeper, find out about the intent (of theorists,
language designers, optimizer writers), come up with a
mental model that encompasses the behavior we've seen, and
so forth. There's a lot of drilling down to be done here,
and probably a good article to be written afterward.

Best regards,

Jonathan Gennick --- Brighten the corner where you are


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