Re: some ideas about db rheory

From: none <rp_at_raampje.>
Date: 27 Jul 2009 23:46:35 GMT
Message-ID: <4a6e3c5b$0$13418$703f8584_at_news.kpn.nl>


I wrote:

>> [...] if you have no way to track identity across changes
>> in real life, adding it as a modeling feature (either with explicit
>> identities or by distinguishing between updates and deletes+inserts,
>> as Brian proposes) isn't going to help a bit.

Brian replies:

>[...] Either there can be change, which implies that there can be
>things that can change, or there cannot be change, which means that
>there cannot be deletes or inserts, let alone updates.

No, Brian. These deletes, inserts, and updates are about statements of fact about the world, which can change to reflect new or changed observations, even when we haven't identified any objects that these statements are about to the extent you appear to deem necessary. I can observe Mary's goldfish and its medical condition, and truthfully record that in my database, two days in a row, *regardless of* whether I can tell whether we're dealing with the same goldfish in both cases.

We may choose not to care and it won't be a problem.

But for goldfish, at least we know they are identifiable in principle. This is not so easy for other types of objects; say, species of fish, countries of the world, or clouds.

>Once you
>commit to change, a system that has no way to track identity across
>changes is broken.

If you mean: once you commit to being able to identify objects across changes, you're right. But it becomes a rather tautological statement.

>For things that can change to be identical, the
>loci in time or space during which they exist or existed must
>coincide, and all changes in appearance that has been sustained by any
>at any time must have been sustained by all at that same time. There
>can be no discernible difference between them at any time during their
>lifetimes.

I feel you keep confusing things and statements about things. A relational database records statements, usually about things.

-- 
Reinier
Received on Tue Jul 28 2009 - 01:46:35 CEST

Original text of this message