Partial Natural Outerjoin- An Operation for Interoperability in a Multidatabase Environment

Natural outerjoin has been considered as an important operation in schema integration. It can be used to define various views in cooperation with other operations. Due to the existence of inconsistent data and null values in base relations of multiple databases, the traditional natural outerjoin cannot be directly applied to schema integration in a multidatabase environment. In this paper, the effects of execution orders, inconsistent data, and null values on the resultant semantics of natural outerjoins are explored. Because an arbitrary execution order of natural outerjoins may cause the resultant semantics to be ambiguous, care needs to be taken in specification of the execution order. We investigate how to determine the execution order of natural outerjoins such that the result is desirable. Moreover, an extension of traditional natural outerjoin, called partial natural outerjoin, is proposed to handle null values and inconsistent data. When a user issues a query against the global view, the query is modified to obtain one which may contain partial natural outerjoins, selections, and projections, based on the definition of the global view. A set of equivalence transformation rules is developed to transform a modified query into one with simpler operations, which lowers the query processing cost. Moreover, the semijoin technique is applied in query processing. Therefore, the cost of data transmission for processing a query can be further reduced, especially in a wide area network environment