Variable Independence and Aggregation Closure

We discuss the issue of adding aggregation to constraint databases. Previous work has shown that, in general, adding aggregates to constraint databases results in languages that are not closed. We show that by imposing a natural restriction, called variable independence (which is a generalization of the assumptions underlying the classical relational model of data) on the schema, we can guarantee that a restricted version of the language with aggregation is closed. We illustrate our approach in the context of linear constraint databases. 1 Introduction Constraint databases [KKR90] are a natural generalization of the relational model of data by allowing infinite relations that are finitely representable using constraints. Constraint databases find numerous applications in spatial [BJM93, BK95, BLLM95, PVdBVG94, VGVG95] and temporal databases [Cho94]. Generalizing aggregation operators to constraint databases has been identified as one of the most important open research issues in this..