Re: satisfies algorithm
Date: Mon, 28 Jul 2008 16:58:50 GMT
Message-ID: <eVmjk.338$rb5.128_at_trnddc04>
"David Portas" <REMOVE_BEFORE_REPLYING_dportas_at_acm.org> wrote in message
news:vJ6dnRSku8yechfVnZ2dneKdnZydnZ2d_at_giganews.com...
> "Brian Selzer" <brian_at_selzer-software.com> wrote in message
> news:92kik.14667$cW3.1512_at_nlpi064.nbdc.sbc.com...
> >
> > 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.
>
True in theory, but very misleading to any newbies who may read it.
A declared primary key with no index to back it up is generally a
performance nightmare.
Enforcing uniqueness of keys without an index requires a full table scan.
For this reason most DBMS builders provide an automatic index when a primary
key is declared.
> --
> David Portas
>
>
Received on Mon Jul 28 2008 - 18:58:50 CEST