Information Systems Reengineering, Integration and Normalization - Information Systems Reengineering, Integration and Normalization - page 12

simply unsuitable. The pointer approach is much more efficient than general table
operations if relationships between sets of data can be predetermined. So, if the
database contains a large number of records or performance requirements, or both,
or if the transaction volume is high and the ad hoc queries are not common, then the
hierarchical or network models are more efficient than the relational model.
Relational databases have over the past decade become an accepted solution to
the issue of storing and retrieving data. Based upon the mathematical concept of a
relation, these systems use tables (relations) and fixed-size fields (domains) to rep-
resent the information and it's interrelationships. The mathematical rigor and sim-
plicity of these systems have been their major attraction. However, there are many
drawbacks to such database systems. The semantics of relational databases are often
hidden within the many relationships and cannot be extracted without users'help.
Also relations stored in the database must at least be in the normal form, preventing
the representation of multiple or set attributes. Furthermore, relational data models
accept entities in a certain form, and structural changes to an entity require changes
to all the instances of that entity in the database. Thus, it is not possible to change a
single instance without affecting the whole database.
Object-oriented databases offer solutions to many of these problems. Based on
the notions of abstraction and generalization, object-oriented models capture the se-
mantics and complexity of the data. Fundamentals to the object-oriented approach
are the concepts of class, instance, and inheritance. An instance is an occurrence of
a class, where a class is a description of an entity. Classes may inherit the attributes
of one or more superclass (es) and thus capture some of the semantics of an en-
tity. Also object-oriented database supports complex data types. An object-oriented
model is thus more reusable and flexible in schema evolution and data storage.
Nowadays, XML databases are the trend of technology, and have become the data
standard on the Internet for e-commerce, and are popular everyday.

1.3.2 Database Conversion

The complexity of converting an existing system to a new database system may
cause a project to become unmanageable. Most people assume that there is an ap-
plication system ready to be converted to the new environment. The assumption
presumes that most application systems are technically up-to-date, logically sound,
and properly organized. A careful review of the majority of application systems,
however, will prove otherwise. A successful system conversion depends on a good
understanding of management and technical requirements.
A systems manager should consider redesigning the application system if it be-
comes unmaintainable. The redesign should be based on the database concept rather
than wasting precious resources by wandering around a conversion process. There
is no absolute certainty about planning and controlling reengineering projects be-
cause there are no foolproof methods to abide by. However, there are three conven-
tional approaches to system conversion (Yau and Fong 1989 ).