The nested universal relation data model

摘要

The nested universal relation (UR) model aims to provide logical data independence to the nested relational model by allowing users to view the database as if it were composed of a single nested relation. Moreover, non-technical users may find the nested relational model too complex to interact with, a problem we call herein the usability problem. The nested UR model solves the usability problem by allowing users to interact with the nested database without having to know its detailed structure, which may be complex. In order to formalise the nested UR model we extend the weak instance approach to the (classical) UR model to the nested weak instance approach to the nested UR model. The nested weak instance approach leads naturally to the definition of the underlying data structure for the nested UR model, namely the nested representative instance (NRI) over the nested universal relation scheme. We present two different definitions of the NRI and show that they are equivalent. First, we define the NRI declaratively as the greatest lower bound of the set of nested weak instances with respect to a natural ordering defined on nested relations. Second, we define the NRI constructively as the result of computing the extended chase on the underlying nested database. Finally, we show that the weak instance approach to the UR model is a special case of the nested weak instance approach to the nested UR model, thus allowing us to implement a flexible UR interface by using the nested UR model; this provides us with all the advantages of nested relations over flat relations.