Re: object algebra
Date: Wed, 10 Mar 2004 04:23:43 GMT
Message-ID: <iLw3c.224401$uV3.930481_at_attbi_s51>
"Neo" <neo55592_at_hotmail.com> wrote in message news:4b45d3ad.0403090914.ee60a90_at_posting.google.com...
> > > We agree that there is always a schema and ultimately the system, not
> > > necessarily the user, needs to know it at run-time.
> >
> > The schema is what the data *means.*
>
> First your definition does not match others. Per dictionary, a schema
> is an underlying organization pattern or scheme.
Yes. It is this underlying organizational pattern or schema that determines what the data means.
> Looking thru several
> db books, I could not find a standardized definition (like that for
> relation) of what a schema is.
That's a shame, don't you think? It seems we ought to at least be able to have a common vocabulary, even if we are disagreeing. It's impossible to have a meaningful conversation in the absence of common definitions.
Come to think of it, this paragraph is self-referential. A definition is the semantics for a word; a schema is the semantics for a database.
> Date's book don't mention it in the
> index. Another book, defines several types of schemas, but never
> schema itself. A third book, distinguishes database schema (aka
> meta-data) as the description of the database as opposed to the data
> itself.
That's the best one so far.
> They say schema is specified during design phase and is not
> expected to change frequently. IMO, the last definition seems the most
> appropriate with respect to RDM and contrary to yours given above.
(That's less a definition and more of a partial functional description.) Nothing contradictory that I can see.
> > The user has to know the schema, too. If the user doesn't know
> > what the data means, the user can't use the system.
>
> Using the last definition above, the level to which the user "has to
> know the schema" is dependent on what he is trying to do, the db's
> design, the code which interfaces user to db, etc. For somethings,
> user may not need to know anything about a db's schema. At the other
> extreme, user may need to know nearly every detail of a db's schema.
These two statements:
- > In TDM/XDb1, there is no meta-data about the data in the db.
- > Data added to the db doesn't have to conform to any design-time
> "schema" but the added data itself defines the current "schema".
contradict each other. I believe the second one, but I don't believe the first one.
Marshall Received on Wed Mar 10 2004 - 05:23:43 CET