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Re: Constraints and Functional Dependencies

From: Marshall <marshall.spight_at_gmail.com>
Date: 24 Feb 2007 21:41:18 -0800
Message-ID: <1172382078.860707.11350@q2g2000cwa.googlegroups.com>


On Feb 24, 1:29 pm, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:

>

> UPDATE, INSERT and DELETE are all simply short-hands for popular
> assignment statements with hooks for statement-specific triggered
> procedures. UPDATE is logically equivalent to DELETE/INSERT (generally
> without triggering delete or insert triggered procedures but a different
> triggered procedure instead.)

I've always been uncomfortable with the use of the word "short hand" in this context. Admittedly I can't point to a formal definition of the
term, but nonetheless it seems to me to connote that the mutative operators will have not only the same semantics as the associated assignment would, but actually expand into exactly that, and thus have the same operational characteristics. Which I think would suck. So I think it's important to be clear that we mean that the two have the same semantics, but not necessarily the same operational characteristics. Which I think is what *you* have in mind when you say that; I think "shorthand" is your shorthand for "same semantics but not necessarily anything else." (Correct me if wrong.) But I'm not sure everyone will come across with the same idea.

> One wonders what a general assignment statement should
> trigger--especially assignments with logically equivalent results.

Yeah, that's a nasty question. Essentially you have to re-verify every constraint from scratch. And transition constraints are out the window, if I understand correctly, because there is no unique way to assign correspondence between elements of the old and new value.

(Not that I am particularly attached to transition constraints; notice my FOL constraint system doesn't support them.)

Certainly assignment is the fundamental imperative operation; it can do anything. However, the same is true for the pair (insert, delete.) Admittedly that pair is not minimal, but nonetheless I am leaning in the direction of favoring support for insert and delete, and not for unrestrained assignment. Even if you have assignment, you still want insert and delete because they have cleaner, more transparent operational characteristics. (That is, it is easy to reason about their performance.) If we accept that we want insert and delete, then assignment becomes redundant. I do not expect that everyone will agree with my reasoning.

Marshall Received on Sat Feb 24 2007 - 23:41:18 CST

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