View-Based Query Answering and Query Containment over Semistructured Data

The basic querying mechanism over semistructured data, namely regular path queries, asks for all pairs of objects that are connected by a path conforming to a regular expression. We consider conjunctive two-way regular path queries (C2RPQ ’s), which extend regular path queries with two features. First, they add the inverse operator, which allows for expressing navigations in the database that traverse the edges both backward and forward. Second, they allow for using conjunctions of atoms, where each atom specifies that a regular path query with inverse holds between two terms, where each term is either a variable or a constant. For such queries we address the problem of view-based query answering, which amounts to computing the result of a query only on the basis of a set of views. More specifically, we present the following results: (1) We exhibit a mutual reduction between query containment and the recognition problem for view-based query answering for C2RPQ ’s, i.e., checking whether a given tuple is in the certain answer to a query. Based on such a result, we can show that the problem of view-based query answering for C2RPQ ’s is EXPSPACE-complete. (2) By exploiting techniques based on alternating two-way automata we show that for the restricted class of tree two-way regular path queries (in which the links between variables form a tree), query containment and view-based query answering are, rather surprisingly, in PSPACE (and hence, PSPACE-complete). (3) We present a technique to obtain view-based query answering algorithms that compute the whole set of tuples in the certain answer, instead of requiring to check each tuple separately. The technique is parametric wrt the query language, and can be applied both to C2RPQ ’s and to tree-queries.