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From: twine@uqcspe.OZ (Steven Twine)
Newsgroups: comp.databases
Subject: The Place of OO DBMS in the ANSI/SPARC architecture
Message-ID: <1983@uqcspe.OZ>
Date: 31 Aug 88 23:29:31 GMT
References: <3560019@wdl1.UUCP> <37@calmasd.GE.COM> <408@cullsj.UUCP>
Reply-To: twine@uqcspe.oz (Steven Twine)
Organization: Computer Science, Queensland Uni, Australia
Lines: 71

Before everybody jumps into the wading pool with their
two cent's worth about OO databases, I'd like people to be
very clear about *what* purpose these OO DBMS will be used
for.  There seems to be a general tendency to say that OO
DBMS are *a priori* good, without saying what they are good
at.  

One useful way of viewing matters is the ANSI/SPARC three schema 
architecture (Hands up everybody who knows this; hands up everybody who's 
forgotten about it..) This architecture includes three schemas: an external 
schema, a conceptual schema and an internal schema.

The external schema specifies a particular way to view the
data in the database.  There may be different external
schemas for different users (or user groups).

The conceptual schema specifies the "semantics" of the data.
In particular, it must specify all constraints on valid
database states and transitions.  There is only one conceptual schema.

The internal schema specifies how the database information
is physically stored and manipulated.  There is only one internal schema.

Now, the question comes: Are OO DB techniques useful at the
external level, conceptual level, or internal level (perhaps
more than one).  

Some people say OO DB are good because they are more
efficient that (say) relational DB for certain applications
(eg CAD).  This is an argument at the internal level.

Some people say that OO DB are good because they "express
more semantics".  This is an argument at the conceptual
level.

Some people say that OO DB are good because they present an
intuitive interface to the user (eg smalltalk-based DBMS).
This is an argument at the external level.

If we can clearly distinguish these arguments, and see that
OO DB may be good at one level, but not at another, then we
might have a chance for a productive discussion rather than
a lot of unsubstantiated claims and "waste heat".

My personal view is: they may be good at the internal level
(they support clustering, by definition); they are not good
at the conceptual level (they require arbitrary grouping of
facts, and there are generally too many ways to represent an
arbitrary fact -- btw, these are similar to Bill Kent's
arguments against record-based models); they may be good,
for certain classes of users, at the external level.  I
could elaborate on this if there is interest.

As an example, most of Yogesh Gupta's points for OO DBMS are
at the internal level (authorization, persistent objects
etc).  This is irrelevant to the use of OO DB at the
conceptual or external level.  Most OO DBMS do not
distinguish these levels: this is bad, because it means that
performance and user convenience factors influence the
design of the database (with negative effects on
evolvability and maintenance).

References for further reading:
D. Tsichritzis, A. Klug (eds)
  The ANSI/X3/SPARC DBMS Framework.  Report of the Study
  Group on Database Management Systems.  AFIPS Press,
  Montvale, NJ, 1977.

J. J. van Griethuysen (ed)
ISO TC97/SC5/WG3 Report on Concepts and Terminology for the
Conceptual Schema and the Information Base.