An Algebraic Calculus of Database Preferences

Abstract

Preference algebra, an extension of the algebra of database relations, is a well-studied field in the area of personalized databases. It allows modelling user wishes by preference terms; they represent strict partial orders telling which database objects the user prefers over other ones. There are a number of constructors that allow combining simple preferences into quite complex, nested ones. A preference term is then used as a database query, and the results are the maximal objects according to the order it denotes. Depending on the size of the database, this can be computationally expensive. For optimisation, preference queries and the corresponding terms are transformed using a number of algebraic laws. So far, the correctness proofs for such laws have been performed by hand and in a point-wise fashion. We enrich the standard theory of relational databases to an algebraic framework that allows completely point-free reasoning about complex preferences. This black-box view is amenable to a treatment in first-order logic and hence to fully automated proofs using off-the-shelf verification tools. We exemplify the use of the calculus with some non-trivial laws, notably concerning so-called preference prefilters which perform a preselection to speed up the computation of the maximal objects proper.