Jerome Garcia writes:
>>  Also, I do not have any experience in access of object 
>>      oriented databases so I can not say whether it is useful to include 
>>      them when considering the problem.

It certainly has appeared useful to a lot of commercial programmers; the
OODB market is certainly not as large as the relational market, but it
is quite healthy.

The major reason for using an OO db as opposed to relational, is that
relational requires you to develop some sort of mapping of your objects
to relational tables.  The more complex your object model, the more
complex that mapping is likely to be.

Object-Relational works well when you are marrying a new object model to
an existing database, because you can design the object model to fit the
relational schema.  It works less well when you are marrying an existing
object model to an existing database, since you typically have to create
a lot of "glue" to put between them.  In the case of marrying a new
object model to a new relational schema, the reasons for using
relational at all are usually based on the experience and skills of the
designers and the availability of in-house databases.  New applications
can gain a lot in simplicity of design, speed of implementation, and
ease of maintenance from using OODBs, since they don't have to deal with
the object-relational impedance mismatch.

Open disclosure: I work for GemStone Systems, and have both a monetary
and professional interest in its success.  GemStone sells both Smalltalk
and Java-based active object servers (OODB's with language
execution engines, built-in distributed object support, and a lot of
application programming services).
-----------------------------------------------------------------------------
Engineering is never having to say you're finished.
-----------------------------------------------------------------------------
Bruce Cohen,                               |  email: cohenb at gemstone.com
GemStone Systems, Inc.                     |  phone: (503)533-3602
20575 NW Von Neumann Drive                 |  fax:   (503)629-8556
Beaverton, OR USA 97006                    |  web:   http://www.gemstone.com