Query Optimization in Distributed Database Systems

The distributed database technology is comparatively recent development within the overall database field. A distributed database is, typically a database that is not stored in its entirety, at a single physical location, but rather is spread across a network of computers that geographically dispersed and connected via communications links. In a distributed relational database system, the processing of a query involves data transmission among different sites via a computer network. One of the aims of distributed database system is to maintain better control on organization of data. The data is distributed at different sites and the distribution is based on the access pattern and costs. The distributed database (DDB) technology emerged as a merger of two technologies (i) Database Technology (ii) Network and Data Communication Technology. The fragment and replicate strategy permits parallel processing of a query. However, if no relation referenced by a query is fragmented, it is necessary to decide which relation is to be partitioned into fragments, which copy of the relation should be used, how the relation is to be partitioned and where the fragments are to be sent for processing.