Datalog Rewritings of Regular Path Queries using Views

TL;DR

This paper demonstrates that for graph database queries specified by Regular Path Queries, if a view determines a query monotonically, then the query can be rewritten as a Datalog program evaluated over the view, enabling efficient query answering.

Contribution

It establishes that monotone determinacy of RPQ views guarantees a Datalog rewriting of the query, with polynomial-time evaluation and decidability of the rewriting process.

Findings

Existence of Datalog rewritings for monotone RPQ view-determined queries.

Polynomial-time evaluation of the Datalog rewriting.

Decidability of whether an RPQ can be rewritten as Datalog using RPQ views.

Abstract

We consider query answering using views on graph databases, i.e. databases structured as edge-labeled graphs. We mainly consider views and queries specified by Regular Path Queries (RPQ). These are queries selecting pairs of nodes in a graph database that are connected via a path whose sequence of edge labels belongs to some regular language. We say that a view V determines a query Q if for all graph databases D, the view image V(D) always contains enough information to answer Q on D. In other words, there is a well defined function from V(D) to Q(D). Our main result shows that when this function is monotone, there exists a rewriting of Q as a Datalog query over the view instance V(D). In particular the rewriting query can be evaluated in time polynomial in the size of V(D). Moreover this implies that it is decidable whether an RPQ query can be rewritten in Datalog using RPQ views.