Re: OO and relation "impedance mismatch"

"Kenneth Downs" <firstinit.lastname_at_lastnameplusfam.net> wrote in message news:fr2vjc.8q4.ln_at_mercury.downsfam.net...

> We may have different definitions of data dictionary. My definition of dd
> is something designed by me and populated by db analysts/designers that is
> used to build databases. Neither SQL Server nor DB/2 uses the term that I
> have ever seen, but I think I've heard people use the term "data
> dictionary" in connection with Oracle to mean the vendor-supplied
> description of an existing database.

AFAIK, your definition of "data dictionary" is the classic definition of a "passive" or "off line" data dictionary.

Oracle uses the term "data dictionary" to refer to the tables in the SYSTEM schema. These tables contain metadata that describes the schema objects (tables, indexes, views, and the like) and database objects (tablespaces and the like).

If you know the definitions for the SYSTEM schema, then you can bootstrap your way up to knowing (except for semantics) the content of any Oracle database. some of this is done automatically by tools that can reverse engineer an existing Oracle database back to a conceptual data model, and then forward engineer for implementation on a different database.

For many years, the SQL standard described the language itself, but didn't have a tight definition for what the metadata ought to look like. Then, in SQL-92 (I think), they added standard metadata definitions to the standard. (If I'm wrong, Joe Celko will surely jump in). Most vendors like Oracle did not go back and redefine their own system tables. Instead, they just added views that make the metadata appear to conform to the standard.

HTH. Received on Tue Oct 05 2004 - 23:26:28 CEST