Query Evaluation via Tree-Decompositions.

A number of efficient methods for evaluating first-order and monadicsecond order queries on finite relational structures are based on tree-decompositions of structures or queries. We systematically study these methods. In the first-part of the paper we consider tree-like structures. We generalize a theorem of Courcelle [7] by showing that on such structures a monadic second-order formula (with free first-order and second-order variables) can be evaluated in time linear in the structure size plus the size of the output. In the second part we study treelike formulas. We generalize the notions of acyclicity and bounded tree-width from conjunctive queries to arbitrary first-order formulas in a straightforward way and analyze the complexity of evaluating formulas of these fragments. Moreover, we show that the acyclic and bounded tree-width fragments have the same expressive power as the well-known guarded fragment and the finite-variable fragments of first-order logic, respectively.