Re: Temporal database - no end date

From: V.J. Kumar <vjkmail_at_gmail.com>
Date: Fri, 19 Jan 2007 22:27:42 +0100 (CET)
Message-ID: <Xns98BDA7BB2C069vdghher@194.177.96.26>

"DBMS_Plumber" <paul_geoffrey_brown_at_yahoo.com> wrote in news:1169239340.938970.215230_at_s34g2000cwa.googlegroups.com:

>
> V.J. Kumar wrote:

>> So,  how do you model your continuous time if all you have is at best
>> a subset of rational numbers (IEEE 754) ?

>
> Le sigh.
>
> I am making a very, very narrow claim. I would appreciate it if you
> read what I wrote, and addressed your questions to it.
>
> My technical observation is that using chronons to discretize time
> implies that mathematical operations over temporal quantities lose
> information.

Could you show with an example how the loss occurs ? You may be right, but let's see.

> My snarky remark was to the effect that many people on
> this list are perfectly happy to invoke the 'but the physical
> computer' dodge when it suits them, while objecting to it's use at
> other times.
>

>> >Models of time that divide the continium into
>> > discrete units, and then force all intervals, aggregations and the
>> > results of any operation into that model, just don't work.
>>
>> Really ?  How come that people do it all the time by using digital
>> computers that do not have real numbers ?  All the computers have are
>> a subset of integers and a subset of rationals ?

>
> They either don't, or they do it badly. How do you propose to
> compute
> the variance of a temporal random variable using integers?

You've answered that yourself below: by using home-made rational numbers or the prepackaged floating point numbers.

>This is no

> academic question, or corner case from obscure scientific
> applications. It's a basic question that comes up regularly in
> industrial and OR applications all the time.
>
> I'm perfectly happy to make the necessary engineering compromises
> and
> use doubles precision types in my algorithms, mindful of the joys that
> come with 'thinking about precision'. Alternatively I could-- and have
> before -- opt to use one of the quite sophisticated open source math
> packages that support arbitrarily large integers, and whatever
> rational numbers can be expressed using ( INTEGER / INTEGER ) ( a
> perfectly reasonably domain which can be implemented in most of the
> modern SQL DBMS engines).
>
> NONE of which is relevant to the question of whether my temporal
> domain is better off modelled as a sequence of discrete units, or
> points on a continium.

Here you go again. You do not have "points on a continium" in your computer, you must use integers or rational numbers, both of which are really the same in the sense of being able to approximate the continuum.

But you maybe right about chronons as inadequate approximations although their inadequatness has got nada to do with Zeno, the continuum and other cool sounding nonsense.

>
>
Received on Fri Jan 19 2007 - 15:27:42 CST