Re: Ping: dawn, some mvl questions
Date: Thu, 25 May 2006 04:34:17 GMT
Message-ID: <dtadg.12650$A26.300258_at_ursa-nb00s0.nbnet.nb.ca>
Keith H Duggar wrote:
>>And isn't this a physical or implementation issue as >>opposed to a logical issue?
>
> mAsterdam wrote:
>
>>(The second "this" referred to "/using/ or >>/manipulation/ of a list representation".)
>
> Ah, oops, I see my words were ambiguous (as natural
> language so often can be). With "this" I was actually
> referring to
>
> mAsterdam wrote:
>
>>The drawback would be that the preservation of the >>listness is not garantueed. Whithout additional >>constraints there is no stopping anyone from inserting >>nodes which make the described graph more complex.
>
> In other words, isn't preservation of "listness" a physical
> or implementation issue? When I wrote using and manipulation
> (a vague sentence I should have left out I guess) I had in
> mind the /physical/ manipulation of the data. Not an
> abstract set of operations or algebra defined for them.
Constraints are logical. For instance, if one must have a continuous sequence of integers to maintain "listness", one can declare those constraints more-or-less as "every integer's successor appears in the relation else it is the largest in the relation, and every integer's predecessor appears in the relation else it is the smallest in the relation".
One could further constrain the smallest to be one or zero or some other integer as one sees fit.
Thus a list is a short-hand for a particular kind of relation with a specific set of constraints.
However, if that is a list, what is an array? It seems to me that a list is something else.
Because the RM is predicate calculus, one uses well-formed formulae (wff's) or their set algebra equivalents to declare constraints to the dbms that it can then enforce regardless of how one physically structures the data. The logical data model deals well with the concern for correctness while the physical encodings deal with the separate and important concern for performance. The dbms automates and manages the translation between the two. Received on Thu May 25 2006 - 06:34:17 CEST