Efficiently Ordering Subgoals with Access Constraints

Guizhen Yang    Michael Kifer    Vinay K. Chaudhri


In this paper, we study the problem of ordering subgoals under binding pattern restrictions for queries posed as nonrecursive Datalog programs. We prove that despite their limited expressive power, the problem is computationally hard --- PSPACE-complete in the size of the nonrecursive Datalog program even for fairly restricted cases. As a practical solution to this problem, we develop an asymptotically optimal algorithm that runs in time linear in the size of the query plan. We also study extensions of our algorithm that efficiently solve other query planning problems under binding pattern restrictions. These problems include conjunctive queries with nested grouping constraints, distributed conjunctive queries, and first-order queries.