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From: David BL <davidbl@iinet.net.au>
Newsgroups: comp.databases.theory
Subject: Re: More on identifiers
Date: Sat, 6 Jun 2009 03:24:17 -0700 (PDT)
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On Jun 6, 12:19 pm, "Joe Thurbon" <use...@thurbon.com> wrote:
> On Fri, 05 Jun 2009 14:52:27 +1000, David BL <davi...@iinet.net.au> wrote:
>
> [...]
>
>
>
> > Due to the projection, all the abstract identifiers have disappeared
> > from every relation.  In a way, it's like seeing a database within a
> > database!  The value of the "inner" database records all the facts in
> > the /context/ of just one of the items, and therefore has no need for
> > abstract identifiers to glue things together.
>
> I may have misunderstood, but haven't you just moved the 'problem'?

I don't understand what you mean.

> That
> is, each abstract identifier that you want to get rid of ends up with 'its
> own database'.

Yes, the intention is to eliminate the abstract identifiers from the
logical model, by instead using a database /value/ as a descriptor for
each item.  This value is only a function of the recorded visible
properties of the item.

> Two database values that came from entities that differed
> only by their abstract identifier will not be distinguishable.

That is a good thing!  It means that two items that are
indistinguishable in terms of the /recorded/ visible properties have
the same descriptor value.

> So to
> distinguish between them, the abstract identifiers and up being arbitary
> names of databases.

The idea is not to distinguish them.  That's an advantage of
completely eliminating the abstract identifiers from the logical model
by using DVAs.  If there are duplicates then the "outer" database can
record the number of duplicates.  Of course more generally the
descriptors could take part in all sorts of relations in the "outer"
database.

A binding between a name and a value can be regarded as a variable
(something that "holds" or "encodes" a value).  The elimination of
abstract identifiers can be regarded as the elimination of variables
from the logical model.


> By the way, I really liked your motivating example and analysis.

Thanks