Browsing by Subject "Object algebra"
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Item Open Access Object-oriented query language facilitating construction of new objects(Elsevier, 1993) Alhajj, R.; Arkun, M. E.In object-oriented database systems, messages can be used to manipulate the database; however, a query language is still a required component of any kind of database system. In the paper, we describe a query language for object-oriented databases where both objects as well as behaviour defined in them are handled. Not only existing objects are manipulated; the introduction of new relationships and new objects constructed out of existing ones is also facilitated. The operations supported in the described query language subsumes those of the relational algebra aiming at a more powerful query language than the relational algebra. Among the additional operators, there is an operator that handles the application of an aggregate function on objects in an operand while still having the result possessing the characteristics of an operand. The result of a query as well as the operands are considered to have a pair of sets, a set of objects and a set of message expressions; where a message expression is a sequence of messages. A message expression handles both stored and derived values and hence provides a full computational power without having an embedded query language with impedance mismatch. Therefore the closure property is maintained by having the result of a query possessing the characteristics of an operand. Furthermore, we define a set of objects and derive a set of message expressions for every class; hence any class can be an operand. Moreover, the result of a query has the characteristics of a class and its superclass/subclass relationships with the operands are established to make it persistent. © 1993.Item Open Access A query model and an object algebra for object-oriented databases(Bilkent University, 1993) Al- Hajj, RedaA query model is an important component of any database system. In this sense, the relational model has a well defined underlying query model. On the other hand, a well defined query model for object-oriented databases has not been accepted yet. This is one of the common complaints against object-oriented databases. So defining a formal object algebra is one of the most challenging steps in developing a theory for object-oriented databases. In object-oriented data models, although messages serve to manipulate the database, a query model is still required to effectively deal with more complex situations and to facilitate associative access. In this thesis, a query model for object-oriented databases is described, where both the structure and the behavior of objects are handled. Not only the manipulation of existing objects, but also the creation of new objects and the introduction of new relationships are supported in the model. Equivalents to the five basic operations of the relational model as ivell as other additional operations such as one level project, nest and aggregate function application are defined. Hence, the proposed object algebra subsumes the relational algebra. Linear recursion is also supported without requiring any additional operator to serve the purpose. Both the operands as well as the results of these operations are characterized as having a pair of sets -a set of objects and a set of message expressions (sequences of messages) applicable to them. The closure property is shown to be preserved in a natural way by the results of operations possessing the same characteristics as the operands in a query. It is shown that every class possesses the properties of an operand by defining a set of objects and deriving a set of message expressions for it. Furthermore, it is shown that the output of a query has the characteristics of a class. Thus, it is also shown how the super/subclass relationships of the result of a query with its operands can be established and how the result can be placed persistently in the lattice (schema) as a class. Such a class is naturally and properly placed in the lattice by maximizing reusability due to inheritance. Also equivalent object algebra expressions are presented and the associativity of the cross-product operation which is an important property in query optimization is proved. Lastly, as it was recognized that schema evolution is an important requirement to be satisfied by object-oriented databases, hence the handling of schema evolution functions through the proposed object algebra operations is also developed as another contribution of the thesis.Item Open Access Query model for object-oriented databases(IEEE, 1993-04) Alhajj, Reda; Arkun, M. ErolA query language should be a part of any database system. While the relational model has a well defined underlying query model, the object-oriented database systems have been criticized for not having such a query model. One of the most challenging steps in the development of a theory for object-oriented databases is the definition of an object algebra. A formal object-oriented query model is described here in terms of an object algebra, at least as powerful as the relational algebra, by extending the latter in a consistent manner. Both the structure and the behavior of objects are handled. An operand and the output from a query in the object algebra are defined to have a pair of sets, a set of objects and a set of message expressions where a message expression is a valid sequence of messages. Hence the closure property is maintained in a natural way. In addition, it is proved that the output from a query has the characteristics of a class; hence the inheritance (sub/superclass) relationship between the operand(s) and the output from a query is derived. This way, the result of a query can be persistently placed in its proper place in the lattice.