Re: object algebra

> > 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. Looking thru several db books, I could not find a stardardized definition (like that for relation) of what a schema is. 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. 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.

> 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.

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". Received on Tue Mar 09 2004 - 18:14:13 CET