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Re: Extending my question. Was: The relational model and relational algebra - why did SQL become the industry standard?

From: Bob Badour <bbadour_at_golden.net>
Date: Sun, 9 Mar 2003 19:29:59 -0500
Message-ID: <ruRaa.432$t_2.63367261@mantis.golden.net>


"Jan Hidders" <jan.hidders_at_REMOVE.THIS.ua.ac.be> wrote in message news:3e6bd183.0_at_news.ruca.ua.ac.be...
> Bob Badour wrote:
> >"Jan Hidders" <jan.hidders_at_REMOVE.THIS.ua.ac.be> wrote in message
> >news:3e620dec.0_at_news.ruca.ua.ac.be...
> >>
> >> Lauri Pietarinen wrote:
> >> >
> >> >>>So actually your views are quite close to Date's, it seems! What
you
> >> >>>are describing sounds like Tutorial-D, to me. Does "SQL with set
> >> >>>semantics" mean what I think it means?
> >> >>
> >> >>Yes, it does.
> >> >>
> >> >Maybe I am missing something here, but what is it _exactly_ you are
> >> >criticizing Date of? Is it the issue of equating SQL with bag
algebra?
> >>
> >> Yes, that too, but mainly that he overestimates the complexity that is
> >> added to the optimizer when bags are exposed to the user.
> >
> >The optimizer may not be any more complex, but it is nowhere near as
> >effective either.

>
> No. It can be just as efective.

If that is the case, why haven't they?

> >> >>>The query
> >> >>>
> >> >>>SELECT CITY
> >> >>> FROM P
> >> >>>
> >> >>>answers the question
> >> >>>"what cities do parts come from?"
> >> >>
> >> >>Yes, it does, but under SQL semantics it also contains the
information
> >> >>how many parts come from each city.
> >> >>
> >> >In a sence, yes, but then you have to have a program to tidy up the
> >> >result or count the duplicates. Why would you want to do that in a
> >> >program?
> >>
> >> If that is what the user asked then that is what he or she will get. If
> >> they wanted something else, they would have asked a different query.
> >
> >Are you not now requiring that all users have expert level knowledge of
the
> >dbms internals?

>
> No.

What level of knowledge would a user need to predict which of the 12 formulations of the query in Date's example will give the user the correct answer out of the 9 different answers received?

> >When I look at your statement above, I think: "Well that totally
> >invalidates the argument that duplicate removal costs too much in
> >performance."

>
> Why do you think that?

I answered that in the part you snipped. The user will just have to formulate and execute multiple queries until the dbms delivers the answer the user needs.

> >> >Well, it was not used because users were advised against using it:
> >> >"Don't use 'DISTINCT', it will result in a sort!". So it was kind of
a
> >> >vicious circle.
> >>
> >> That's nonsense. DISTINCT is usually not used because you don't need
to,
> >> because for example the SELECT clause contains a candidate key. It is
> >> very rare in my experience that users deliberately accept duplicates.
> >
> >Jan, with all due respect, I cannot count how many times I have heard
> >alleged database experts tell users to "Never use DISTINCT." If the
result
> >is already distinct, the keyword should have no cost.

>

> Yes. *should* is the right word. Deriving that "at compile time" is not a
> trival problem.

It is trivial in a system based on sets and requiring logical identity.

> >> What makes you think that while-loops raise the level of abstraction?
> >
> >What makes you think they don't?

>
> They are both imperative programming constructs.

What does that have to do with the level of abstraction?

> >> >Let's say for sake of argument that P has 1 row, and SP has 1000000
rows.
> >>
> >> That assumption makes the example extremely unrealistic because if that
> >> were the case it is very unlikely that the user would accept that many
> >> duplicates in the answer.
> >
> >Why should they accept any duplicates?
>
> I didn't say they should.

You implied that users would more likely accept duplicates with lower cardinality. Received on Sun Mar 09 2003 - 18:29:59 CST

Original text of this message

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