Re: Relation Schemata vs. Relation Variables
Date: Wed, 06 Sep 2006 23:25:35 GMT
Message-ID: <PNILg.8455$yO7.4096_at_newssvr14.news.prodigy.com>
"JOG" <jog_at_cs.nott.ac.uk> wrote in message
news:1157554419.796440.287750_at_e3g2000cwe.googlegroups.com...
> Bob Badour wrote:
>> JOG wrote:
>> > Brian Selzer wrote:
>> >
>> >>"Jan Hidders" <hidders_at_gmail.com> wrote in message
>> >>news:1157532864.768886.10750_at_d34g2000cwd.googlegroups.com...
>> >>
>> >>>Brian Selzer wrote:
>> >>>
>> >>>>"Jan Hidders" <hidders_at_gmail.com> wrote in message
>> >>>>news:1157457516.222077.154380_at_b28g2000cwb.googlegroups.com...
>> >>>>
>> >>>>>Sets of facts can and do change, and transitional constraints
>> >>>>>restrict
>> >>>>>wich transitions from one set of fact to another are allowed. I
>> >>>>>don't
>> >>>>>see a fundamental problem here. Note btw. that they are a strict
>> >>>>>subclass of the restrictions that might be expressed by some kind of
>> >>>>>temporal logic.
>> >>>>
>> >>>>I don't understand what you mean. Are you saying that transition
>> >>>>constraints can be expressed as state constraints?
>> >>>
>> >>>A transitional constraint is a binary predicate over states. One
>> >>>argument is the old state and the other the new state. Or, put in
>> >>>another way, a transition constraint constrains the transitions. This,
>> >>>I would say, is pretty much the definition of the term.
>> >>>
>> >>>Or did I misunderstand your question and are you asking about temporal
>> >>>logics?
>> >>>
>> >>
>> >>No. I just wanted to be sure that we're on the same page.
>> >>
>> >>The point that I was making in the original post is that because keys
>> >>can
>> >>change, there isn't enough information given only the old state and the
>> >>new
>> >>state to pair up the values in the old state with those in the new
>> >>state for
>> >>comparison.
>> >
>> > You cannot pair up values David. You can only compare the sets as a
>> > whole.
>>
>> I think you misspoke. I draw your attention yet again to Date's
>> _Principle of Incoherence_: "It is very difficult to respond coherently
>> to that which is incoherent."
> > guilty as charged. Very difficult indeed. I'll rephrase: > > There is no transition between individual tuples in different relation > values Brian, and it is illogical to try and compare them as if there > were. There is only a transition from one set of tuples to another, as > a whole. (This is because, as a variable, a relvar posesses an identity > outside of its current value). >
That's interesting. Today I dug out my old copy of Date's "An Introduction to Database Systems, Seventh Edition," and on page 256-257 he describes transition constraints exactly as I understand them, and although he doesn't address the issue of "key updates," he writes about joining S and S' and picking out the tuples that violate a particular constraint. In addition, his description hasn't changed since 2000, because he describes them in exactly the same way in "Database in Depth" on page 130. So, if my understanding of transition constraints is faulty, then so is Date's.
I don't think that pairing up tuples for comparison is illogical at all. What is illogical is expecting R join R' to give consistent results without object identifiers or tuple identifiers. What is illogical is discarding information supplied by the user in order to fit what are obviously more expressive modification operations into the mold of relational assignment. What is illogical is assuming that keys won't change.
I really don't understand what you mean about a variable having identity.
>>
>> One can pair up values any number of ways: least, greatest, lesser,
>> greater, least greater, greatest lesser etc.
>>
>> Cartesian product and restrict have the effect that one can pair up
>> tuples within relations ie. elements within sets.
> Received on Thu Sep 07 2006 - 01:25:35 CEST