Re: By The Dawn's Normal Light

From: Marshall Spight <mspight_at_dnai.com>
Date: Mon, 25 Oct 2004 01:26:30 GMT
Message-ID: <9DYed.523564$8_6.258574_at_attbi_s04>


"Paul" <paul_at_test.com> wrote in message news:417bce83$0$80657$ed2619ec_at_ptn-nntp-reader03.plus.net...
> Marshall Spight wrote:
> >>A value is atomic (with respect to a system) iff that value has no
> >>internal structure from the point of view of that system.
> >
> > All you've really done is push the definition off to the definition
> > of "internal structure." What is that?
>
> I don't see the problem really, don't we all understand and agree what
> internal structure means?

Unfortunately, I would say we don't.

> I think the key point of the definition is
> that atomicity is always with respect to a system rather than an
> absolute concept.

Yeah, that's fair. My terminology would be to say "with respect to a set of operations" but I think we mean the same thing.

> > Does a date have internal structure?
>
> To the relational engine, no. It has no understanding of a date value
> other than its type and how the DBMS represents it internally. It
> doesn't know about month, days, or years. It doesn't know that they come
> in any order. It doesn't even know whether two distinct representations
> are actually the same date.

I think that last one might get you in to trouble, but I can work with the rest.

> To the type engine or whatever it's called (I think I've just invented
> this terminology), ...

The canonical term is "type system."

> ... the date does have internal structure. It can extract
> things like the day, month or year.
> >
> > OTOH its internal representation may simply be a long. But
> > if we can't decide whether it has internal structure without
> > reference to its implementation, then "atomic" is not a
> > logical concept, but an implementation concept, and hence
> > has no part to play in conversations about what happens
> > at the logical level.
>
> Well I guess when we say values are atomic we mean "atomic w.r.t. the
> relational engine". For a standard RDBMS this is almost a tautology: all
> values are by definition atomic. But the problem arises when we think
> about extending the standard relational model with things like nested
> relations that are accessible to the relational engine. Then our values
> cease to become atomic at the logical level. (in the sense mentioned at
> the beginning of this paragraph).

Fair enough.

> > We've had these debates over and over, and no concensus
> > has emerged. I really don't think there's anything useful
> > to be learned from thinking about the phrase "atomic values."
>
> I hate to be so contrary, but I think there is use in the concept at the
> logical level: specifically when considering extending the standard
> relational model to include "nested relations" (in the sense where they
> aren't hidden inside the type engine).
>
> I agree though, that if you're just thinking about the standard
> relational model, then the concept is pretty irrelevant at the logical
> level.

Agreed.

Marshall Received on Mon Oct 25 2004 - 03:26:30 CEST

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