Re: satisfies algorithm

From: David Portas <>
Date: Sat, 26 Jul 2008 19:40:45 +0100
Message-ID: <>

"Brian Selzer" <> wrote in message news:PoFik.34002$
> "David Portas" <> wrote in message
>> "Brian Selzer" <> wrote in message
>> news:92kik.14667$
>>> Yes. Normalize. A schema that is in BCNF does not have any nontrivial
>>> functional dependencies where the determinant is not also a key. Where
>>> there is a key, there should also be a unique index of some sort, making
>>> it impossible for there to be two tuples with the same determinant.
>> Unique indexes have nothing to do with keys. A key is a logical construct
>> whereas an index is merely one possible physical structure used by some
>> DBMSs. A key does not require an index.
> Keys have the uniqueness property. Don't you agree that the uniqueness
> property should be enforced by whatever implementation is chosen?

Yes of course.

> I think you would be hard pressed with today's technology to find a more
> efficient implementation method to enforce the uniqueness property than
> maintaining an index--especially when a relation has more than one key.

This is a different thing from saying there "should" be a unique index for a key. Some DBMSs don't even have the concept of indexes (Netezza comes to mind). Whether a database has indexes or not has nothing to do with whether keys are enforced.

David Portas
Received on Sat Jul 26 2008 - 13:40:45 CDT

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