|The extended relational database|
|Many people are reluctant to give up their
relational databases altogether and yet realise that this model
cannot easily support some of the more complex modem applications.
Relational model advocates have responded to the challenge of 00 by adding some 00 characteristics to the relational model to produce what is sometimes called the extended relational model'.
C.J.Date, who is a confirmed 'relational' man, concedes that 00 does have some good ideas. In particular,
However, he also thinks that OO has some
bad ideas, such as its strong procedural (3GL) orientation, the
complex treatment of relationships and its lack of declarative
For further discussions of these points, see chapter 25 in An Introduction to database Systems'.
It is generally accepted that the relational model needs enhancing because the addition of semantic and OO concepts are necessary to support the new generation of applications. However, it is recognised that the benefits of the relational model are too valuable to be discarded and this model needs to be preserved to protect the investment in relational technology and to provide downward compatibility.
Bearing these points in mind, several projects have attempted to marry the 'good' features of 00 with the relational model. These features aim to provide support for:
The Starburst RDBMS (from IBM) has already been mentioned in connection with Active Databases. This system has also been 'extended' and permits the definition of user-defined types and functions, support for complex objects. encapsulation and inheritance.
The Postgres RDBMS was developed
by Michael Stonebraker and colleagues at the
POSTGRES allows the definition of classes
and supports both single and multiple inheritance.
Neither Starburst nor Postgres use Date's idea that object class = domain.