Oracle FAQ | Your Portal to the Oracle Knowledge Grid |
Home -> Community -> Usenet -> comp.databases.theory -> Re: Extending my question. Was: The relational model and relational algebra - why did SQL become the industry standard?
"Bob Badour" <bbadour_at_golden.net> wrote in message
news:mnF6a.332$QR3.51347581_at_mantis.golden.net...
> "Paul Vernon" <paul.vernon_at_ukk.ibmm.comm> wrote in message
> news:b3dr5r$sss$1_at_sp15at20.hursley.ibm.com...
> > "Bob Badour" <bbadour_at_golden.net> wrote in message
> > news:zst6a.297$9O1.43758648_at_mantis.golden.net...
> > > > Let me put it this way. If a type has 2 possiable representations,
> then
> > > > A) the number of values represented by PosRep1 must equal the
> number
> > > > of values represented by PosRep2
> > > > B) each value represented by PosRep1 must 'map' to exactly one
> value
> > > > represented bt PosRep2, and versa.
> > >
> > > While desirable whenever possible and while certainly possible in an
> ideal
> > > machine, I do not require this of physical implementations.
> >
> > Yes and that is my worry. I'm not comfortable with such a fudge. If the
> > logical model cannot be implemented correctly then is there not something
> > wrong with the model?
>
That does seem to be the received wisdom. I don't take people's word for it however.
The received wisdom seems to suggest that it is OK to assume an infinite and continuous universe. Rubbish! Any logical model that needs to assume that is never going to be a true model of the world.
> Fourth and finally, I must ask: What other logical data
> model do you propose that addresses the specific issue of cartesian and
> polar coordinates and possible rounding in the internal physical
> representation?
Well, lets have a go shall we.
In the case of polar and catesian points, something like the following type hierarchy suggests itself to me
POINT_Real / \ / \ Point_Int32_Polar Point_Int32_Cart \ / \ /
Here POINT is an abstract type with both poss reps both using the abstact type
REAL
for the components of the pos reps.
Point_Int32_Polar is the set of POINTS representable using signed 32 bit integers in the r & thera polar poss rep components. This type has only one poss rep.
Point_Int32_Cart is the set of POINTS representable using signed 32 bit integers in the r & thera cartiesian poss rep components. This type has only one poss rep.
Then Point_Int32_Polar_isect_Int32_Cart is the intersection of Point_Int32_Polar and Point_Int32_Cart. This would include all *exactly equal* points in the two representations.
e.g. (a:0, b:0) = (r:0, theta:0), (a:1,b:0) = (r:1, theta:0) etc etc. This type has two poss reps.
Regards
Paul Vernon
Business Intelligence, IBM Global Services
Received on Tue Feb 25 2003 - 07:11:50 CST