Re: MV Keys

In article <NTXNf.62440$PL5.60906@xxxxxxxxxxxxxxxxxxxxxxxxxx>,
brian@xxxxxxxxxxxxxxxxxxx says...
"Jon Heggland" <heggland@xxxxxxxxxxx> wrote in message
What universe? A concrete example: I can use a varchar as a string
(where, for the sake of the argument, I postulate that the individual
characters "have no meaning"), or as an array of characters (where they
do). The DBMS doesn't know (or care) what meaning I apply to either
varchar---so what is the point of the distinction? Just to say that one
design (the one using varchar as an array) is probably bad, and the
other is not?

I take issue with stuffing nonscalar values into attributes. The universe
of discourse is (at least for relational databases) the set of all possible
values for all relevant domains and a set of rules that describe how those
values can be combined.

I don't really understand. What kind of rules?

It is clearly redundant to have the same value in a
scalar domain and a domain of lists, because undoubtedly, there's an
operator available that can extract that value from the list so that it can
be discussed.

Why is this redundant? It is not very clear to me. Can you show an
example of such redundancy? Is it redundant to have both Strings and
chars?

Is this only an issue with (variable-length) collection types? What
about a phone number with an area code part? Or a location with latitude
and longitude?

I think that redundancy in the universe of discourse is worse
than redundancy in the database, because it undermines the logical
foundation of the database.

How so?

How can you know if you're talking about the
same thing if it values in the universe of discourse don't have identity
with respect to that universe?

Why wouldn't they have?

A database is a logical thing, so it is not relevant what the DBMS knows or
cares.

Then how can it have any practical significance? Can you give any
examples of anomalies or redundancy caused by "stuffing nonscalar values
into attributes"?

My point is that you can't say that a type (e.g. varchar) is scalar or
not a priori; you have to say "the way varchar is used by this operation
in this particular database means it's not a scalar here". Hence,
scalar-ness is a property of some use of some variable of a type, not of
the type as such. I think we actually agree; you do say "scalar /with
respect to the universe of discourse/" (my emphasis).


Whether something is scalar is a logical concept, not a physical one. Also,
it's not the operation in a particular database, but rather the definition
of a domain in the logical universe that determines whether or not something
is scalar.

I think we do agree. Something is scalar with respect to the universe, or
less precisely, with respect to the context in which it is used.

I don't think your "universe" concept is very precise, but ymmv.

Or the other way around. Are substrings of strings components or
transformations? What about subranges, or individual values, of arrays
or lists? What is the difference?

If a string is atomic within the universe, then a substring must be a
transformation. If it's not, then both the string and the substring are
compound entities subject to the rules of combination that are defined in
the universe. I'm using the term entity deliberately here, because such
instances have identity with respect to the database. They're propositions,
not values.

So basically, it is whatever we define it to be? Anyway, whenever the
term "entity" enters a discussion, I leave it to its misery. :)
--
Jon
.

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