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

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.

>> >>>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.

>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?

>> >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.

>> 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.

>> >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.

• Jan Hidders