Re: A real world example

"Keith H Duggar" <duggar_at_alum.mit.edu> wrote in message news:1155685951.008054.41480_at_m73g2000cwd.googlegroups.com...
> JOG wrote:
> [snip excellent post]
>
> It's interesting. I came to a similar conclusion back when
> Brian responded to my question. Ie that it's either a poor
> choice of "universe" or an attempt to redefine the RM notion
> of "identity". I wrote this long reply then decided it was
> too metaphysical to belong in cdt so I didn't post. But what
> the hell, it's written might as well post it. Feel free to
> ignore what lies below.
>
> ------
>
> The bulk of your post was a philosophical and metaphysical
> discussion. While this helped me understand (I think) your
> points it does not constitute a "real-world example". Can
> you please provide a real-world (though perhaps cleaned and
> simplified) example? No rush but when you have time I think
> it will help clarify some of your concerns.
>
> Brian Selzer wrote:
>> Keith H Duggar wrote:
>> > > My point is that here is a real-world example where the
>> > > universes of two databases overlap, but the set of
>> > > attributes used to identify something common to the two
>> > > universes is different for different databases.
>> >
>> > So isn't the actual problem that the databases employ
>> > different models of the same data? In other words, it
>> > seems in that example (and in "the" problem), that the
>> > problem is not different /values/ but different
>> > /models/?
>>
>> No. It's not the data that's the same: it's that the
>> things that the data is about are the same.
>
> The terms "universe", "overlap", "something common", and
> "things" are all very unclear to me. By "universe" do you
> mean a "relation universe" as in the set of all relations
> matching the given headers? By thing do you mean a tuple? Or
> do you mean some metaphysical concept? etc. Perhaps using
> clearer or more formal language would be helpful.
>
>> A database contains information about things in the
>> universe. In a relational database, this information is
>> organized into relations, but that doesn't change the fact
>> that each proposition describes things in the universe.
>
> Again what universe? If this is a philosophical discussion
> of how the relational model fairs when modeling a particular
> notion of the physical or metaphysical /Universe/ (capital U
> there) then I think you should first expound more accurately
> your philosophical position.
>
> For example:
>
>> The problem is that a thing can have multiple sets of
>> identifying properties, that some of these identifying
>> properties can have different values in different
>> situations, and that when propositions about a thing refer
>> to it by using identifying properties that can be
>> different in different situations, there's not enough
>> information available to know that you're talking about
>> the same thing.
>
> If here by "thing" you are referring to metaphysical things
> then the notions of identity I'm familiar with are first-
> order and second-order variations of Leibniz identity (ie x
> is y if all that is true of x is true of y). In this case it
> makes no sense to claim "a thing can have multiple sets of
> identifying properties" since identity is the value of
> /all/ properties.
>
> If by thing you mean mean tuple then the set theoretic
> definition of identity is again the obvious Leibniz "all
> elements (or attributes) are identical". On the other hand
> and if I understand correctly, the relational model uses a
> weaker notion of identity based on a /primary key/? That is
> identity is determined by a subset of attributes holding as
> a /candidate/ key and designated /primary/. So in this case,
> by definition there is one and only one "set of identifying
> properties", the /primary key/.
>
> Is this correct? If so then I admit I'm confused as to what
> precisely the primary key identifies. I sorta understand
> the "guaranteed access rule" from an "access" perspective
> but not an "identity" perspective. It seems either a little
> physical to me or if a primary key "identifies" a tuple
> then I'm having trouble escaping the usual Leibniz notion.
>
> Finally, much of your previous discussion has focused on
> identity over time: both diachronic and more recently
> synchronic. You seem to take a perdurance perspective. That
> is "things" have an identity over all time that /persists/
> over time. Hence your language such as "how can we tell if
> they /are the same thing/". However, logically this view
> requires that time is essentially yet another property. This
> fits well in the RM if you store time as an attribute. Ie
> something like a log file that has already been suggested to
> you before. However, by either Leibniz or key identity the
> tuples cannot be "the same thing" and hence the "thing" that
> you are "identifying" must become a set of tuples not a
> single tuple. Example in plain language I Keith am the set
> off all my states past present and future.
>
> If you do not maintain time as an attribute then since a
> relation is set, only a single /present/ time slice of the
> entities entirety is ever present. Hence the RM simply does
> maintain the complete perdurance concept of identity you
> hold unless time is an attribute.
>
> So in RM it seems either you need to store time as an
> attribute or ditch the concept that "things" have identity
> beyond their key. Thus the tuple is the "thing". You could
> see these tuples as "temporal parts" of a "thing" that the
> database never actually holds in it's entirety. This is the
> endurance concept of diachronic identity. Though this is
> the "usual" way people view time as a "special" dimension
> over which "things" can change yet maintain a meta-identity,
> it simply does not seem to fit well in the RM given that the
> "thing" cannot be an element of a relational domain lacking
> a time attribute and hence cannot be discussed.
>
> -- Keith -- Fraud 6
>

I guess it's time to wax philosophical. (I'm going to try to use the correct terms, but I'm not a philosopher, so please, be gentle.) The universe of discourse is the set of all things that are interesting. (By "things," I mean also people and places.) "Interesting" limits the universe to only those things that can be relevant. Interesting things can be discussed, relevant things are being discussed. Things have properties that define and describe them. Properties can be interesting; they can also be relevant. There are two categories of properties: individual and universal. An individual property defines the essence of something, either by itself or when combined with other individual properties. Individual properties remain constant throughout something's lifetime because their values depend only on the existence of the thing that they define. A universal property, on the other hand, is descriptive in nature and its value depends on a state of the universe (a situation). The value of a universal property of something can be different in different situations. The distinction between individual and universal properties implies that the universe can change. Things persist, and they have both endurant and purdurent properties. Individual properties are endurant; universal properties are purdurent. Events change the universe. They can alter the appearance of things, or they can affect the existence of things. They can create new things; they can transform things; they can destroy things. Events shape the universe, unraveling it into a succession of developing situations. Events imply order (Something must exist before it can be destroyed.), but not necessarily duration. Events can occur simultaneously, each affecting a different set of things, or they can occur in a definite sequence, each affecting the same set of things. Each event marks the temporal boundary between one situation and its successor. The effect of an event is embodied in three sets: the set of things destroyed, the set of things changed, and the set of things created. For things created, the event is an individual property that provides a link to the circumstances that brought them into being. For things destroyed, the event relegates them to history. For things altered, universal properties will have new values, while the old values become history. Each thing that exists has a universal property that identifies the event that caused it to appear as it does. Each thing that is history has no universal properties: it's individual properties include all of the properties of the thing for which it became history, their values originating from the situation whose end was bounded by the event that created it. Thus, an interval exists within the individual properties of a history that is bounded by the event that caused the originating thing to have the values that became history and by the event that created the history.

What is known about the universe is markedly different from what the universe is. What is known about the universe is part of the universe. The universe is the set of all interesting things that can be discussed; a database records the discussion. Events precede knowledge: a situation must arise before it can be discussed. A database consists of a schema, a predicate, and a succession of database instances separated by a series of informative statements. Each database instance contains knowledge about things in the universe that was relevant to the discussion when that instance came into being. This knowledge is a set of propositions about things in the universe. A relational database instance is organized into a set of relation values containing sets of tuples that correspond to and represent the set of propositions. A database instance contains only true propositions (at least they're assumed to be true) about relevant things. A relational database instance is one where history is not relevant; a temporal database instance is one where history is relevant. Received on Wed Aug 16 2006 - 10:31:10 CEST