Evaluating conjunctive queries and solving constraint satisfaction problems
are fundamental problems in database theory and artificial intelligence,
respectively. These problems are NP-hard, so that several research efforts have
been made in the literature for identifying tractable classes, known as islands
of tractability, as well as for devising clever heuristics for solving
efficiently real-world instances. Many heuristic approaches are based on
enforcing on the given instance a property called local consistency, where (in
database terms) each tuple in every query atom matches at least one tuple in
every other query atom. Interestingly, it turns out that, for many well-known
classes of queries, such as for the acyclic queries, enforcing local
consistency is even sufficient to solve the given instance correctly. However,
the precise power of such a procedure was unclear, but for some very restricted
cases. The paper provides full answers to the long-standing questions about the
precise power of algorithms based on enforcing local consistency. The classes
of instances where enforcing local consistency turns out to be a correct
query-answering procedure are however not efficiently recognizable. In fact,
the paper finally focuses on certain subclasses defined in terms of the novel
notion of greedy tree projections. These latter classes are shown to be
efficiently recognizable and strictly larger than most islands of tractability
known so far, both in the general case of tree projections and for specific
structural decomposition methods