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Re: Teach SELECT DISTINCT first!

From: Mikito Harakiri <mikharakiri_at_iahu.com>
Date: Tue, 27 Apr 2004 17:31:53 -0700
Message-ID: <b6Djc.50$BD6.662@news.oracle.com>

"Paul" <paul_at_test.com> wrote in message
news:_mCjc.36950$Y%6.5035445_at_wards.force9.net...
> Mikito Harakiri wrote:
> >> OK here's a pathological example: Define a function f on the
> >> integers by: f(i) = i (if Fermat's Last Theorem is true
> >> = 0 (if Fermat's Last Theorem is false
> >
> > This is not closed-form expression. Normally you supply a formula
> > with arithmetic operations and elementary functions.
>
> I'm sure you could expand it out in terms of arithmetic operations and
> elementary functions. It might be very long though...

No, you would need quantifiers.

> > CAS have their own problems even within practical realm of
> > closed-form expressions, there is no need to go as far as FLT. The
> > idea is however, that in vast majority of the cases CAS does an
> > excellent work.
>
> OK so say for example I do "SELECT i from R WHERE i*i < 0"
> where i is an integer column, then the DBMS engine should know that the
> square of an integer is never less than zero?

Is it really different from today's RDBMS primitive predicate deduction capabilities when optimizer derives

x = 5

from

x = y and y = 5 ?

> Maybe the DBMS could store statistics about what actual ranges are used
> for a function in practice and physically store them which would help in
> 99% of cases?

Well, likewise, finite precision arithmentics works in 99% of the cases, but as you have seen below I'm not really excited with that kind of approach.

> Yes, I guess the problem is it's very complicated to write just a plain
> DBMS, let alone one with fancy optimizations, unlimited precision and
> computer algebra systems etc. built in.

But, you know what? Writing clean RDBMS engine is not even on vendor's priority list. They are extremely busy chasing X, G and other fads. Received on Tue Apr 27 2004 - 19:31:53 CDT

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