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Re: RM and abstract syntax trees

From: Marshall <marshall.spight_at_gmail.com>
Date: Thu, 01 Nov 2007 04:59:53 -0000
Message-ID: <1193893193.228589.125410@z24g2000prh.googlegroups.com>


On Oct 31, 8:31 pm, Jonathan Leffler <jleff..._at_earthlink.net> wrote:
> Marshall wrote:
> > On Oct 31, 7:50 pm, David BL <davi..._at_iinet.net.au> wrote:
> >> Isn't it helpful to see the analogy with a pointer dereference?
>
> >> I'll leave it up to you as to whether you dislike the analogy between
> >> node identifiers and pointer values, and the idea that a join can be
> >> compared to a pointer dereference. Perhaps you are right and the
> >> analogy creates confusion.
>
> > I think if we are clear about it being an analogy we are on solid
> > ground. But as soon as we start thinking pointers and references
> > are the *same* thing we are in trouble, because now we can't
> > see the differences anymore.
>
> > I think Date actually nails this issue. He says (roughly) that
> > pointers add complexity but don't add any expressive power.
>
> Isn't the other 'point' that 'pointers point somewhere' but values
> stored in a relation don't - that relational database bases work on
> associative addressing. In particular, even in a foreign key, the value
> doesn't point to the referenced primary key; it merely contains the same
> value as some entry in the referenced table. It may also contain the
> same value as a large number of other places in the database.

Yes, well put.

I mentioned previously that pointers are only meaningful or functional within the context of a specific, typically non-portable address space. Without the address space, pointers might as well be random numbers. So if you are going to pack up some data and move it from, say, one machine to another, you have to go through all sorts of contortions to preserve some semblance of pointer semantics. And as anyone who's dug through the code for Java serialization, this is quite ugly and error prone. Whereas if you want to copy some relations from one place to another no transformations of any kind are necessary. Which rather highlights how the relational form is a logical form, and not a physical form.

Marshall Received on Wed Oct 31 2007 - 23:59:53 CDT

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