Oracle FAQ Your Portal to the Oracle Knowledge Grid
HOME | ASK QUESTION | ADD INFO | SEARCH | E-MAIL US
 

Home -> Community -> Usenet -> comp.databases.theory -> Re: Stored fields ordered left to right

Re: Stored fields ordered left to right

From: Bob Badour <bbadour_at_golden.net>
Date: Mon, 29 Dec 2003 00:11:11 -0500
Message-ID: <Ws6dnWXlao4FKnKiRVn-tA@golden.net>


"Marshall Spight" <mspight_at_dnai.com> wrote in message news:dKIHb.681736$Tr4.1704642_at_attbi_s03...
> "Bob Badour" <bbadour_at_golden.net> wrote in message
news:v6mdnQYXkoaxl3OiRVn-uw_at_golden.net...
> > "Marshall Spight" <mspight_at_dnai.com> wrote in message
> > news:VBhHb.671183$Tr4.1678668_at_attbi_s03...
> > >
> > > The question is, how does one distinguish the attributes in the
relation?
> > > There are two choices: numerically/positionally, or by name. That is,
> > > one either has a mapping from 1, 2, ... n to attribute, or one has
> > > a mapping from name1, name2, ... namen to attribute. To me, it's
> > > not all that great a difference. It's just a question of what the
> > > application is.
> >
> > Physical dependence vs. physical independence is not that big a
> > difference?!?
>
> I was speaking of logically distinguishing attributes. I don't
> see how the physical level is even relevant here.

You don't see how logically distinguishing attributes by physical position violates physical independence and confuses logical and physical issues?!?

> > > If one is writing a page of equations, the convenience of using
> > > positional identification is high.
> >
> > You are confusing an external physical representation with a logical
> > representation.
>
> Interesting distinction, but not one that I can follow without further
> information. Do you have a reference for further reading?

Um, everything that has ever been written on logical data models and the relational model in particular. What exactly do you not understand? Do you understand external vs. internal? Do you understand physical vs. logical? Actually, you don't have to answer the last question because it is clear you do not.

> > > It doesn't affect the semantics of relations or relational operators;
> > > it just affects how attributes are identified.
> >
> > Huh? Of course it affects the semantics if positional ordering has
meaning!
>
> Mumble. Operations like union, intersection, difference, are identical
> either way. Join needs some work, but it's not what I'd call a huge issue.

No, they are not identical. Consider the following:

R1 = { { A=1, B=2 } }
and
R2 = { { B=2, A=1 } }

What is R3 = R1 union R2?
What is R4 = R1 intersect R2?
What is R5 = R1 minus R2?

If position matters, the answers are:

R3 = { { A=1, B=2 }, { A=2, B=1 } }
R4 = { }
R5 = { { A=1, B=2 } }

If position does not matter, the answers are:

R3 = { { A=1, B=2 } }
R4 = { { A=1, B=2 } }
R5 = { }
Received on Sun Dec 28 2003 - 23:11:11 CST

Original text of this message

HOME | ASK QUESTION | ADD INFO | SEARCH | E-MAIL US