Re: In an RDBMS, what does "Data" mean?
Date: Tue, 8 Jun 2004 08:08:07 -0400
Message-ID: <brmdned_eIZNNljdRVn-vg_at_comcast.com>
"Paul" <paul_at_test.com> wrote in message news:BEhxc.12368$NK4.1714909_at_stones.force9.net...
> For a fixed snapshot of a database I guess they don't add anything
> extra, since the tuples satisfy the constraints already. But if you
> think about a database evolving over time, they do add information. For
> example suppose you had a constraint "Age < 60" on some relation/column.
> Then you could ask the question: "Can I add a person aged 65 to my
> database?" Now in current DBMSs I think you'd do that by trying it and
> seeing if you get an error. (or maybe by querying the system tables).
Yes.
In particular, the optimizer can use information made available by the
constraints in order to generate
additional correct strategies, ones that could not be guaranteed to be
correct in the absence of such information.
So a constraint that is known to be valid can be used to advantage, even in the context of a snapshot. Received on Tue Jun 08 2004 - 14:08:07 CEST