Re: Reminder, blatant ad

From: Jan Hidders <jan.hidders_at_REMOVETHIS.pandora.be>
Date: Tue, 14 Feb 2006 21:02:26 GMT
Message-ID: <CzrIf.257293$3P4.7890037_at_phobos.telenet-ops.be>

dawn wrote:
> Jan Hidders wrote:
>

>> dawn wrote:
>> 
>>> Jan Hidders wrote:
>>> 
>>>> dawn wrote:
>>>> 
>>>>> I'd like to better understand data independence in terms of 
>>>>> functionality.
>>>>> 
>>>>> [...]
>>>> 
>>>> It means that you can make some changes in the physical model
>>>> w/o having to change the logical model. Note that this is
>>>> different from what you just said because you were talking
>>>> about the logical model.
>>> 
>>> I thought that was what I was saying in the first answer.  How is
>>> this different?
>> 
>> The version of the DBMS tool doesn't change. You should be able to 
>> change the physical model w/o changing the tool.

>
> Surely some software somewhere (the OS, the DBMS?) is getting a new
> version in order to make for this "change"?

No. If I decide to restructure the indexes or cluster differently, I'm still using the same version of the DBMS.

>>> [...]  Why would you think that a change in the physical model of
>>> a non-relational DBMS (e.g. PICK) would require reworking the
>>> logical model?
>> 
>> This is not necessarily true for all such systems but in many of
>> those the logical and physical model are closely tied together and
>> their efficiency in fact often depends on this close relationship.

>
> What is the test for determining whether they are too tightly
> coupled?

You check if many of the changes in the physical model that might be needed in the near future will change the logical model.

> What is it that changes (the "physical model" doesn't resonate with
> me, I want to know what software components would change) that should
> not require a change in the logical model but does in some
> non-relational products?

None of the software components change. Are you serious about not understanding the distinction between the physical model and the logical model? Try looking in Date's introduction for 'internal level' or 'physical level'.

• Jan Hidders

Received on Tue Feb 14 2006 - 22:02:26 CET