Re: Why all the max length constraints?
Date: 29 May 2006 13:05:27 -0700
Message-ID: <1148933127.734019.296990_at_i40g2000cwc.googlegroups.com>
David Cressey wrote:
> "dawn" <dawnwolthuis_at_gmail.com> wrote in message
> news:1148926992.308719.48250_at_g10g2000cwb.googlegroups.com...
> >
> > David Cressey wrote:
> > > "dawn" <dawnwolthuis_at_gmail.com> wrote in message
> > > news:1148773922.267863.170930_at_j55g2000cwa.googlegroups.com...
> > > > [OK, here is my next "stupid question" as I cut a path in my study of
> > > > the RM. Those teachers who just want to tell this student how
> ignorant
> > > > she is are welcome to sit this out as I really am hoping to
> > > > understand.]
> > > >
> > > > In SQL-DBMS's, like VSAM (and other indexed sequential files before
> > > > them) a lot of attributes are specified with max length constraints.
> > > > While there are some attributes where this constraint is related to a
> > > > conceptual constraint (from the analysis phase), these lengths are
> > > > often introduced for the logical model or implemenation in the DBMS.
> > >
> > > Is VSAM really a DBMS? I looked up VSAM in wikipedia, and the
> definition I
> > > found suggests that VSAM is NOT a DBMS, although both IMS and DB2 are
> > > layered upon it.
> >
> > I agree that VSAM is not a DBMS. I did not intend to imply they would
> > be categorized as one.
>
> OK. I didn't pick up on the comma before the word "like" in your original
> post. I thought you were giving VSAM as an example.
> >
> Now that we've got that nailed down, on to the next issue: The consensus
> of the responders (including me) is that there is nothing inherent in the RM
> that would force declaration of a max length for character strings.
Yes. I'm trusting that, even though I don't yet know the full answer.
> There's nothing inherent in SQL either, afaik.
Good.
> There are plenty of systems
> that have nothing to do with RM or SQL that impose character limits on
> strings.
Yes, that is definitely the case (I gave the example of card decks).
> COBOL is one you should be familiar with.
We were once intimate, yes ;-)
> There are plenty of systems that have nothing to do with MV that permit
> manipulation of variable length strings,
Absolutely. Java for example.
> without an explicint max declared.
> BASIC is one you should be familiar with.
Yes (a variation thereof is the primary update language and 3GL for MV btw)
> With regard to implementation internals, how deep do you want to go?
> With regard to increased marketability of products that work with fixed
> length strings, that's a whole different matter.
Agreed. It is a question about the requirements of the RM as implemented in DBMS's, not about relative merit nor marketability of anything.
> And it's where RM theory,
> and the design choices made by DBMS egineers start to diverge.
I don't think that I have seen anything suggesting that if the max length constraint is an added feature, the RM would be in conflict with such a decision. The RM is typing-agnostic, including length designations, IIRC.
> We could
> have a separate discussion about that.