Re: Objects and Relations

From: Walt <wamitty_at_verizon.net>
Date: Tue, 20 Feb 2007 15:03:53 GMT
Message-ID: <t5ECh.9879$SR.2795@trndny06>

"David BL" <davidbl_at_iinet.net.au> wrote in message news:1171981093.989824.178580_at_t69g2000cwt.googlegroups.com...
> On Feb 20, 11:36 am, "JOG" <j..._at_cs.nott.ac.uk> wrote:
> > On Feb 20, 1:22 am, "David BL" <davi..._at_iinet.net.au> wrote:
> >
> > > On Feb 19, 10:42 pm, "JOG" <j..._at_cs.nott.ac.uk> wrote:
> > > [snip for length]
> > > > It seems a lot of the communication
> > > > problems here are because of you using different terminology to that
> > > > which is accepted in the field.
> >
> > > I don't know what terminology is used in the field because I'm a
> > > systems programmer. However I would say that thinking entity means a
> > > type of thing is quite strange.
> >
> > I rarely hear the term entity set. I hear people talk of entity types,
> > but more often than not this seems to be shortened to just
> > 'entity' (as opposed to an instance). But who cares. At least I see
> > where the miscommunication stems from.
> >
> >
> >
> >
> >
> >
> >
> > > > Either way, it is better to recognize
> > > > this gap and bridge it, agreed?
> >
> > > Agreed.
> >
> > > > > It was in reference to
> > > > > your book example. For the purposes of stating facts about books,
an
> > > > > entity would be a particular book, not some type "book".
> > > > > Classification is not necessary. It is irrelevant to the
discussion.
> >
> > > > > > > Why are you doing that?
> >
> > > > > > I am trying to help you to a better definition, from which the
limits
> > > > > > of thinking in terms of 'entities' becomes clearer.
> >
> > > > > I can't understand your point. From my perspective you appear to
make
> > > > > the simple mistake of thinking that because it's difficult to
classify
> > > > > things, things don't exist!
> >
> > > > Well I can only quote you "Are you making a real attempt to
understand
> > > > me?". I mean you keep quoting "entities are illusionary" , even
though
> > > > we have clarified pages ago the intention of this statement. I think
> > > > it adequately infers the mistake people often made in assuming that
> > > > entities are anything but arbitrary concoctions.
> > > > But furthermore I have already offered that your confusion with the
> > > > statement is just semantics, and you have ignored this. To clarify
one
> > > > more time, consider an analogy.
> >
> > > > I am saying: "Unicorns don't exist."
> > > > You are saying "Unicorns do exist, look, I just imagined one"
> > > > To which I reply "ok unicorns do exist, but they are something just
> > > > made up in your mind".
> >
> > > I not sure what's point you're making. If you're going to create a
> > > fiction then you're going to store fiction (in the DB) no matter how
> > > you think about.
> >
> > Well the idea is to highlight that your complaint of "entities being
> > real as opposed to illusionary" is all just semantics, and it just
> > depends what "real" means. You keep bringing this up, so it was worth
> > addressing. Hopefully that can be hit on the head now.
> >
> > I'd say a more important point is that because they are all just made
> > up, we can't ever measure where an entity starts and finishes, where
> > it ovelaps with other entities, and so on. There is no correct way of
> > determing what it is composed of, or what it is part of (In a world of
> > entities do written_papers have an authors property, or the other way
> > around?).So why /ever/ try and wrap that fiction up as an entity type
> > then, with remits, boundaries, and qualified attributes then? Just
> > state propositions (always). I see no added value in ever turning any
> > information into an object.

> 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. 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. 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?

> 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

>
>
>

In casual conversation, ER modelers will tend to use the simple word "entity" in place of either "entity set" or "entity instance", leaving the listener to disambiguate by means of the context in whuch the word appears.

By analogy, in casual conversation, object oriented programmers will use the simple word "object" in place of either "object class" or "object instance", similarly leaving disambiguation up to the listener.

The problem comes when the listener is not familiar with the underlying mode of thinking. In that case, the listener will sometimes disambiguate incorrectly. That is why introductory tutorials on object oriented programming tend to spell out "object class" or "object instance", at least until the reader can be presumed to have gotten accustomed to the mode of thinking. I'm sure you will have noticed this, if you've gone back and read some introductory material after gaining proficiency.

Similarly, introductory material on ER modeling should spell out when we are talking about the "set of all vehicles" and when we are talking about "a particular vehicle". Some such material does this.

Discourse in this newgroup tends to be a little more formal than casual conversation, but far less formal than introductory tutorials.

Hope this helps. Received on Tue Feb 20 2007 - 09:03:53 CST