Removing redundant join operations in queries involving views

摘要

Views are understood as a good means to tailor base relations individually to the needs of each user. However, if a user formulates his queries in terms of views he often has no chance to express these queries without joins. In terms of base relations many of these joins would not be necessary, and therefore the advantages of the view concept are payed for with a reduced performance. This study shows that this performance reduction can be avoided by automatically transforming a certain class of queries formulated in terms of views into equivalent queries on their base relations. This transformation is performed on the source level of SQL and uses the functional dependencies of the base relations to remove redundant join operations. Performance measurements in a real application of System/R show that this method is very efficient.