Re: Surrogate Keys: an Implementation Issue
From: paul c <toledobythesea_at_oohay.ac>
Date: Sun, 30 Jul 2006 18:17:54 GMT
Message-ID: <mJ6zg.277632$Mn5.224019_at_pd7tw3no>
>> Brian Selzer wrote:
>> For some people, unfortunately, that is a matter of belief. In the
>> context of RT, I'd say it is mere willfullness. To see why, try to avoid
>> the words 'before' and 'after', using say, x and y instead to stand for
>> two relations (which might be the same). To ask which one (or two) does
>> the db state at this moment in time is the same as asking does it state x
>> or does it state y. But to ask which one(s) was stated yesterday requires
>> that we deliberately add time information. This is a choice that allows
>> us to ask the second question. The point is that we may not be interested
>> in the second question and choose not to state information about time
>> which makes the use of a time concept arbitrary.
Date: Sun, 30 Jul 2006 18:17:54 GMT
Message-ID: <mJ6zg.277632$Mn5.224019_at_pd7tw3no>
Brian Selzer wrote:
> "paul c" <toledobythesea_at_oohay.ac> wrote in message > news:qt4zg.281144$IK3.267521_at_pd7tw1no...
>> Brian Selzer wrote:
>>> "paul c" <toledobythesea_at_oohay.ac> wrote in message >>> news:zTOyg.269253$IK3.233927_at_pd7tw1no... >>>> Brian Selzer wrote: >>>>> "paul c" <toledobythesea_at_oohay.ac> wrote in message >>>>> news:g9Nyg.263082$iF6.250988_at_pd7tw2no... >>>>>> Brian Selzer wrote: >>>>>>> What's the point of a database if it doesn't reflect some aspect of >>>>>>> reality. ... >>>>>> To talk precisely about whatever we want to talk about. Nothing more. >>>>>> Doesn't need to be real. >>>>>> >>>>> Agreed. But even a conceived universe subsumes certain absolutes, such >>>>> as time. >>>>> >>>> In that case, the statements in the database should talk about time, >>>> ie., aspects of time. These are different from statements about the >>>> time it takes the database to say something. >>>> >>> There can be no discussion without time. Relational assignment cannot >>> exist without the concepts of before and after. ...
>> For some people, unfortunately, that is a matter of belief. In the
>> context of RT, I'd say it is mere willfullness. To see why, try to avoid
>> the words 'before' and 'after', using say, x and y instead to stand for
>> two relations (which might be the same). To ask which one (or two) does
>> the db state at this moment in time is the same as asking does it state x
>> or does it state y. But to ask which one(s) was stated yesterday requires
>> that we deliberately add time information. This is a choice that allows
>> us to ask the second question. The point is that we may not be interested
>> in the second question and choose not to state information about time
>> which makes the use of a time concept arbitrary.
>> > > But the assignment operation by its nature determines the order of the > states x and y: one must always precede the other because the operation > itself produces that other. > ...
I think you are pre-supposing a particular imperative implementation.
>> Maybe the confusion arises from Codd mentioning 'time-varying relations'.
>> In the interest of making his main points in an efficient way, I suspect
>> he didn't want to dot every last 'i'. As soon as you talk about
>> relational assignment you are talking about variable replacement, aka
>> pointer replacement. Logical replacement doesn't require any notion of
>> time. Nor does algebraic difference.
>> > > Could you please elaborate on that? Replacement, as I understand it, > subsumes that something exists and will be superceded by something else. > Algebraic difference isn't even related to assignment. Difference is an > operator, assignment is an operation. I don't understand why you included > it. Perhaps you're conflating the concepts of operator and operation. >
p Received on Sun Jul 30 2006 - 20:17:54 CEST