  This paper contains analysis of concept of a class within different
object-oriented knowledge representation models. The main attention is paid to
structure of the class and its efficiency in the context of data storage, using
object-relational mapping. The main achievement of the paper is extension of
concept of homogeneous class of objects by introducing concepts of single-core
and multi-core inhomogeneous classes of objects, which allow simultaneous
defining of a few different types within one class of objects, avoiding
duplication of properties and methods in representation of types, decreasing
sizes of program codes and providing more efficient information storage in the
databases. In addition, the paper contains results of experiment, which show
that data storage in relational database, using proposed extensions of the
class, in some cases is more efficient in contrast to usage of homogeneous
classes of objects.
