Expressiveness of Efficient Semi-Deterministic Choice Constructs

Recently, Abiteboul and Kanellakis introduced the notion of determinate query to describe database queries having the ability to create new domain elements. As there are no natural determinate-complete query languages known, more restrictive (the constructive queries) and more general (the semi-deterministic queries) notions of query were considered. Here, we show that the advantage of the second approach over the first is not so much in increased expressiveness, but in the ability of expressing queries more efficiently. Concretely, we provide a precise characterization of a minimal subclass of the semi-deterministic queries containing the constructive queries in which the polynomial-time counting queries can be expressed efficiently. This subclass is defined via a semi-deterministic query language, which is obtained by introducing a particular choice operation, called swap-choice. The main claim of this paper is further substantiated by showing that the introduction of swap-choice,..