Re: A real world example

"Bob Badour" <bbadour_at_pei.sympatico.ca> wrote in message news:zTLEg.49101$pu3.577586_at_ursa-nb00s0.nbnet.nb.ca...
> JOG wrote:
>
>> Brian Selzer wrote:
>>
>>>[snippage]
>>>I disagree. The definition of a key is critical. Because its scope is a
>>>single database instance, it cannot permanently identify things in a
>>>universe that can change.
>
> Please note that the self-aggrandizing ignorant, Selzer, argues for
> stability and then denies the possible benefits of familiarity. He doesn't
> address irreducibility or simplicity at all.
>
>
>> Ok, I understand your point there - you want a form of key that is
>> consistent across relation values. I agree that is useful, and
>> terminology is sketchy. However given that the definition of a database
>> relation is still argued (date/pascal for example), I think we have to
>> concede there is also ambiguity at the very edges of the term key. As
>> ever as long as we are talking about the same thing we'll be ok.
>>
>>
>>>[snippage]
>>>
>>>>It is important to see that "Me now" is a completely different entity
>>>>to "Me over my whole lifetime". The temporal issue is irrelevant, all
>>>>that matters is to recognise they are just different entites. I know
>>>>this is initially seems an obtuse philosophical point, but it has
>>>>_real_ consequences for how to model those entities.
>>>
>>>I see the difference, but I can't see how you can shrug off the temporal
>>>issue.
>>
>> Temporal differences are just a context change.
>> Geographical/Cultural/etc may be another generating exactly the same
>> problems. There is no way I'm shrugging it off, I'm just saying we
>> can't just externalise it - its integral to the universe of discourse
>> we are concerned with.
>
> In fact, the differences serve to define the universe of discourse much in
> the same way immovable objects and irresistible forces define their
> universes.
>
>
>>>They must have a common property because they're related. I would
>>>argue that "Me now" is part of "Me over my whole lifetime."
>>
>> "Me now" and "Me over my lifetime" could have a common property, but
>> they don't _have to_ as they are entities from separate domains. Again
>> it's the universe of discourse under concern that matters. At some
>> point the chances are they will have common properties, but a DNA value
>> might not be recorded for "Me now" and it still be perfectly valid in
>> its domain.
>
> You overestimate the use and importance of DNA. That will change in coming
> decades.
>
>
>>>>Something must remain constant to compare entities - something must
>>>>identify them. If nothing remains constant the things being compared,
>>>>by liebniz equality, are different things full stop. This is what
>>>>mathematical logic is grounded in, we can't just avoid it. You seem to
>>>>be saying it is possible that "every attribute of something has
>>>>changed, yet it is still the same thing". Surely that's logical
>>>>nonsense!
>
> "This hammer has been in our family for over 150 years. In that time, we
> replaced the handle five times and the head twice!"
>
>
>>>In the mathematical sense, if *any* attribute is different, then they're
>>>different, but that's beside the point. I'm not saying that it is
>>>possible
>>>that "every attribute of something has changed, yet it is still the same
>>>thing" What I'm saying is that "every universal property of something
>>>can
>>>change, yet it is still the same thing." I know I'm going to get beat up
>>>about this, but as I see it, there is a difference between a property
>>>that
>>>defines something and a property that describes something. In an earlier
>>>post, I used the terms, "individual" and "universal" to describe these
>>>categories of properties. Individual properties define the essence of
>>>something and remain constant throughout its lifetime.
>
> Thus any change in the property separates lifetimes. This then describes
> any property.
>

For a thing that persists, doesn't an endurant part overlap the same spatial location as thing of which it is a part during the entire interval that the thing exists? And doesn't a purdurent temporal part overlap the same spatial location as the thing of which it is a part only during particular intervals or instants during a thing's lifetime?

>
> Universal properties
>>>depend on the state of the universe during that lifetime. Universal
>>>properties are the only ones that can change.
>>
>> I've wrestled with definitional and descriptional properties too, and
>> I've come to the conclusion (painfully) from reading the philosophy
>> that there isn't any difference. Consider two towers, identical in
>> every way, one in paris and one in london. The only difference to
>> identify them is their geographical position. Then, what we may have
>> thought of as a descriptional property originally. is now necessarily
>> definitional. Its a murky distinction. I'll post a better example at
>> some point, as I think this also contributes to why we shouldn't hide
>> surrogates.
>>
>>
>>>"Are you the same person you were when you were 10?" depends on the
>>>context
>>>of the query, not necessarily the data, but in either context, you must
>>>be
>>>able to correlate the state when you were 10 to the state you are now in
>>>order to do the comparison. In the one context, you would compare the
>>>individual properties; in the other you would compare the universal
>>>properties.
>>
>> No! You have already presumed they are the same person if you do that.
>>
>> All one needs to do is see if the two entities _do correlate or not_
>> (And not assume they do a priori). If they don't, as in the identity is
>> not the same, I conclude they are different people. If they do, I can
>> say they are the same person, but in different states. I apologise for
>> banging on about liebniz all the time, but thats exactly how we
>> distinguish whether something is the same thing or not (conceding that
>> functional dependency means we can rely on the key for identity in RM).
>> I think this is a big point where we are differing.
>
> Given that Selzer makes up his own meanings for words, what makes you
> think you have any insight into what he thinks?
>
>
>>>>>>This is exactly the same as the basic philosophical question of
>>>>>>change.
>>>>>>Are you the same person you were when you were 10? In one context the
>>>>>>answer is absolutely not, so an appropriate identifier is not DNA. In
>>>>>>another yes you are the same person, so DNA is perfect. It depends on
>>>>>>the domain you are considering, and one must pick the correct key
>>>>>>accordingly.
>>>>
>>>>I worry you have glazed over this, but it highlights how the context
>>>>information is more complex than we initially think, and we must
>>>>understand that context to make the right key choices (especially if
>>>>you want to make inter-relation value comparisons).
>>>>
>>>>My DNA don't change over my lifetime, so its a good key for that
>>>>conceptual entity (n.b. the representative label to record the DNA
>>>>might change, but the value is the same.)
>
> Actually, your DNA change constantly. The length of the 10 year old's
> telomeres, for example, are longer. (But size isn't everything--or so I've
> been told.)
>
>
>>>>>>>Here's a simple example of what can happen:
>>>>>>>
>>>>>>>create table P
>>>>>>>(
>>>>>>> x int primary key,
>>>>>>> y int
>>>>>>>)
>>>>>>>create table F
>>>>>>>(
>>>>>>> x int primary key references P(x),
>>>>>>> z int
>>>>>>>)
>>>>>>>insert P (x, y) values (1, 1)
>>>>>>>insert P (x, y) values (2, 3)
>>>>>>>insert P (x, y) values (3, 2)
>>>>>>>insert F (x, z) values (1, 5)
>>>>>>>insert F (x, z) values (2, 4)
>>>>>>>select * from P join F on (p.x = F.x)
>>>>>>>update P
>>>>>>> set x = case x
>>>>>>> when 1 then 2
>>>>>>> when 2 then 1
>>>>>>> end
>>>>>>> where x in (1, 2)
>>>>>>>select * from P join F on (p.x = f.x)
>>>>>>>
>>>>>>>RESULTS:
>>>>>>>
>>>>>>>Before:
>>>>>>>x y x z
>>>>>>>------ -------- ------- --------
>>>>>>>1 1 1 5
>>>>>>>2 3 2 4
>>>>>>>
>>>>>>>After:
>>>>>>>x y x z
>>>>>>>------ ------- -------- --------
>>>>>>>1 3 1 5
>>>>>>>2 1 2 4
>>>>>>>
>>>>>>>
>>>>>>>Should the new values for x in P have cascaded into F?
>
> Of course not. Otherwise, you would have included "ON UPDATE CASCADE"
> somewhere in the foreign key declaration.
>
>
> Assume that
>>>>>>>the
>>>>>>>system only has the before image and the after image (the first two
>>>>>>>columns
>>>>>>>in RESULTS) in order to complete the update. How can such a system
>>>>>>>differentiate between the above update and the following update?
>>>>>>>
>>>>>>>update P
>>>>>>> set y = case y
>>>>>>> when 1 then 3
>>>>>>> when 3 then 1
>>>>>>> end
>>>>>>> where x in (1, 2)
>>>>>>>
>>>>>>>Clearly first update affects a key, and consequently, the changes
>>>>>>>should
>>>>>>>cascade,
>
> Actually, they should not cascade as explained above. However, if they
> were to cascade, how they do so would depend on the implementation of the
> dbms. Hopefully, the dbms implementers would at least document the
> implementation.
>
>
> but with the information available (both updates produce the
>>>>>>>exact
>>>>>>>same before and after images), the system cannot differentiate
>>>>>>>between
>>>>>>>the
>>>>>>>two updates; therefore, it cannot determine whether or not to cascade
>>>>>>>the
>>>>>>>changes. Updates within the Relational Model are are constrained in
>>>>>>>the
>>>>>>>same way as this hypothetical system. All that is available is the
>>>>>>>preceding instance and the succeeding instance, and if the only key
>>>>>>>can
>>>>>>>change, then there is no way to correlate tuples.
>
> I don't recall where either set algebra or predicate calculus cover
> cascading updates for assignment statements. Perhaps Selzer could cite his
> source.
>

In the above example and the next example I'm trying to illustrate the problem with correlating tuples in the preceding and succeeding database instances, because that's what a hypothetical implementation that faithfully implemented the Relational Model would be presented with. Relational assignment is set-based, meaning that the hypothetical implementation would be presented with the current database instance and a proposed relation value, and then must be able to determine whether or not to reject the proposed relation value.

>
>>>>>>I'll consider the example in due course. RL work to do in the
>>>>>>meantime,
>>>>>>sigh. All best, Jim.
>>>>>
>>>>>Another example is trying to write an update trigger in Sql Server to
>>>>>log
>>>>>changes to one table in another. The deleted and inserted pseudotables
>>>>>can
>>>>>be thought of as the preceding and succeeding database instances
>>>>>respectively. Unless you constrain updates to a single row, or reject
>>>>>updates that affect both key and non-key columns, or use a surrogate,
>>>>>you
>>>>>can't tell what changed to a particular row. In fact, it's possible
>>>>>for
>>>>>it
>>>>>to appear that nothing at all has changed, even though new key values
>>>>>have
>>>>>been propogated into referencing tables. (Sql Server does not
>>>>>faithfully
>>>>>implement the relational model. It can tell the difference between the
>>>>>two
>>>>>updates above, even though within a trigger on P, the deleted and
>>>>>inserted
>>>>>pseudotables would appear identical.)
>
> Since when does the RM say anything about the implementation of triggered
> procedures? Perhaps Selzer could cite his reference.
Received on Thu Aug 17 2006 - 01:39:39 CEST