Re: Objects and Relations

On Feb 20, 2:18 pm, "David BL" <davi..._at_iinet.net.au> wrote:
> [snip]
> I still think we have a quite different understanding of what (in
> practical terms) we mean by "entity". I think you're still
> associating it with ERMs (even if you distinguish instance from type)
> while I'm not. Let me explain what I mean by that...
>
> You question whether written papers have an authors property or vice
> versa. For me that question only arises at the point where you try
> to create an ERM.

Well yes I do see that. But I /don't/ think those issues start at the ERM, so I'm more than happy to address things outside the E/RM, at a 'real world' level (apologetic quotes for obvious reasons).

> Since we agree it is generally better to go
> directly to relational schema design it seems a moot point. We
> understand that the many to many relationship between authors and
> written papers is nicely represented in its own relation.
>
> What I'm saying is that it doesn't indicate any problem with our
> understanding of the entities as we perceive and understand them in
> real life.

Ok I've got to stop you there for a moment, because I need to get over one hurdle first - I am still not clear at all what the definition of your 'entity' concept is? And at the low level we're addressing this, it is vital I understand what you define the term to mean otherwise we just end up talking past each other again.

Currently I realise it is interchangeable with 'thing', and I also know that you _don't_ define it is just a collection of attributes and values, but above that I only have the statement that "if you point at a set of atoms and call it Fred, then that would be an entity" from a post way back in the thread.

Obviously I think there are serious problems with the 'pointing at atoms' description, but instead of jumping on it (I realise this is usenet and it was probably a quick post) I'd appreciate if if you could expand your defintion.

> This distinction comes down to the difference between
> model and what is modelled. The question of where to put an
> attribute is yet another one of the adhoc "features" of the ER
> modelling process.
>
> If one is going to represent the fact that a particular author has
> written a particular paper, surely one is comfortable with being able
> to identify the human and the paper in real life. Otherwise why
> bother storing the fact in the first place?

I'm not ignoring your points here, I just need that definition first.

>
> Obviously the actual human and written paper are quite different from
> the models of those things associated with an ERM. As I see it, the
> various and substantial limitations of ER models should not be taken
> as evidence that the actual entities for which we want to store
> knowledge about are "arbitrary concoctions". I have a feeling you
> are throwing out the baby with the bath water!
>
> Nevertheless entities like humans are too complex for us to ever
> expect to reasonably characterise with simple mathematical models.
> In that sense I completely agree with you that the right way to store
> information about them is through relations, and expect many tuples in
> many relations to be used to provide detailed information about a
> single person.
>
>
>
> > (or to bring it back to the OP, why would a struct ever be preferable
> > to a relational encoding?)
>
> > > > Does that make more sense as to the semantic difference that you are
> > > > (perhaps) obsessing over, but now seems to preventing further
> > > > conversation?
>
> > > I would suggest we avoid generalised statements and use examples to
> > > clarify what is meant. I agree that definitions of terms is a likely
> > > cause of difference of opinion.
>
> > I imagine it probably is.
>
> > ('Course, don't forget I didn't think your intiial conjecture was
> > testable, or that its comparison made sense.)
>
> > > > > I ask again, what's you point about the difficulties of
> > > > > classification?
>
> > > > What difficulties?
>
> > > You pointed out (correctly) that it is difficult to have a type called
> > > "book" and to know what attributes it should have.
>
> > > I regard that as a classification problem. It doesn't imply that a
> > > particular entity is illusionary or subjective.
>
> > > > > > If it helps given the E/R-style 'entity' terminology you are holding
> > > > > > onto, you might consider that I view /everything/ as an "associative
> > > > > > entity". But of course I would not call it that.
>
> > > > > > > It is well known that classification of entities is
> > > > > > > adhoc. Fortunately In DB systems we tend to state facts about
> > > > > > > particular things far more often than sets of things.
>
> > > > > > > If I were to place an actual book in front of you, you could think of
> > > > > > > hundreds of objective propositions about it. Actually the number of
> > > > > > > possible propositions you could state about the book would seem almost
> > > > > > > unlimited.
>
> > > > > > > If you were given a different book, again there would be countless
> > > > > > > propositions you could state about it. Now the book may have some
> > > > > > > fundamental differences. Therefore attributes relevant to the first
> > > > > > > book may not make sense for the second book and vice versa. This
> > > > > > > makes classification of books difficult. However we both agree that
> > > > > > > the RM copes well with that because it can represent knowledge about a
> > > > > > > single book across lots of different relations. RM has no need to
> > > > > > > develop a class hierarchy in the manner of OO (or indeed E/R
> > > > > > > diagrams).
> > > > > > It is good we are agreed of the benefit there, and an important point
> > > > > > not to forget in all of this.
>
> > > > > > > What is more fundamental - facts about a particular entity, or the
> > > > > > > entity itself? Surely the facts are secondary - at least for physical
> > > > > > > entities.
>
> > > > > > Well of course I don't accept there is anything but facts and values,
> > > > > > so your question is nonsensical to me.
>
> > > > > I guess your statement that entities are illusionary is nonsensical to
> > > > > yourself as well then.
>
> > > > > > Remember that there are practical evidence of this standpoint having
> > > > > > merit. For instance Symbolic AI died in the 1970's - a very real,
> > > > > > practical example of how tyring to manipulate these elusive 'entities'
> > > > > > results in failure. Situated and Nouvelle AI was born from this and
> > > > > > I'd encourage you to check this area out - "Elephants don't play
> > > > > > chess" by Brooks, is a good starting point.
>
> > > > No comment on this? I've offered to points indicating to how thinking
> > > > in terms of entities can be unproductive - the lack of success of E/R
> > > > modelling in replacing RM (which was its original goal), and the
> > > > collapse of the entity-based manipulation of Classical AI in the 70's.
> > > > There is insight in both of these.
>
> > > IMO the insight in the first is the difficulty of an objective
> > > classification of things.
>
> > > The second I cannot properly comment on - I'm out of my depth. My
> > > impression is that AI in general hasn't lived anywhere near to the
> > > original promises and there are more explanations than you can poke a
> > > stick at. One explanation I favor is that it is difficult to close
> > > the association between reasoning and meta-reasoning, leading to the
> > > so-called ghost in the machine. I find it telling that it's not
> > > generally possible to apply one's own high level understanding of
> > > "truth" to the very sentences that are used to reason about those
> > > truths. As a simple example, 3+4 and 7 are equal and yet not equal.
>
> > If you get chance, that Brooks "Elephants don't play chess" paper is
> > extremely good. "The Owl and the Electric Encyclopedia" (1991 Journal
> > of AI) is also recommended.
>
> I intend to have a look when I get the time
Received on Wed Feb 21 2007 - 22:09:19 CET