Re: globals besides relvars?
Date: Mon, 28 Jul 2003 15:07:24 -0400
"Marshall Spight" <mspight_at_dnai.com> wrote in message
> "Paul Vernon" <paul.vernon_at_ukk.ibmm.comm> wrote in message
> > "Marshall Spight" <mspight_at_dnai.com> wrote in message
> > news:eX0Va.143552$GL4.37155_at_rwcrnsc53...
> > > So what if you have some function that requires some storage?
> > No such thing. Functions take values, do some computation on those
values, and return
> > some other values. They don't store values (at least not ones that
change), that is
> > what assignment is for.
> This is true for functions in the strict mathematical sense, but in
> most programming languages, functions have access to global
> data. In many programming languages, functions have access to
> function-local state as well.
I can only think of one function that ordinarily requires access to some kind of global data, and one could just as easily replace this function with a value in a relvar.
> > > For example, imagine a function that returns the current timezone.
> > > It needs to store this somewhere.
> > current timezone is not a function. It is just a value of some relvar.
> "A function" is a perfectly good name for "a value of some relvar."
> (Well, a part of a relvar, anyway.) It more typically refers to some
> code, but the above still works.
> > Same goes for all of the SQL 'special registers'. By which I mean, in a
> > relational dbms, there would not be any special registers, or functions
> > values. Such things would violate the information principle
> > IMO A good catalog model would include relvars to holds things like
> > and current USER.
> Okay, but why do these things have to be relvars? I mean, the current
> is not a relation, it's a value from the timezone domain.
Nobody said it has to be a relation. It is a value in a relation. All values in relations are values from one domain or another.
> Current user is a
> string or a User value or something. It's not a relation. So why must it
> into a relation?
So that we can directly apply logic for its management.
> I'm already aware that Codd so asserted, but I don't
> see any good theoretical or even a good design reason for it.
You don't see any good theoretical use for requiring a representation whereby one can use logic directly?
> Worse, if I make a relvar Timezone, I can issue nonsensical updates
> to it. What does it mean to insert into Timezone?
You presuppose a nonsensical relvar. Why should subsequent nonsense surprise or alarm anyone? It's just a straw man.
> Okay, so maybe
> you say the fact that I have a no-columns key for the table makes
> that insert fail. Or there's some kind of constraint that prevents
> a delete from succeeding. But it would be better if the structure
> made it the case that these nonsensical updates weren't even
Don't use a TimeZone relvar--use a sensible relvar instead.
> That's what you get if you make Timezone a global
> *non-relation* variable.
A global non-relation TimeZone variable is just as nonsensical as the relvar you propose above. If one allows the global distribution of a database (and I see no reason to prohibit distribution), the database does not have single, distinct time zone. Received on Mon Jul 28 2003 - 21:07:24 CEST