Answering regular path queries using views

Query answering using views amounts to computing the answer to a query having information only on the extension of a set of views. This problem is relevant in several fields, such as information integration, data warehousing, query optimization, mobile computing, and maintaining physical data independence. We address query answering using views in a context where queries and views are regular path queries, i.e., regular expressions that denote the pairs of objects in the database connected by a matching path. Regular path queries are the basic query mechanism when the database is conceived as a graph, such as in semistructured data and data on the Web. We study algorithms for answering regular path queries using views under different assumptions, namely, closed and open domain, and sound, complete, and exact information on view extensions. We characterize data, expression, and combined complexity of the problem, showing that the proposed algorithms are essentially optimal. Our results are the first to exhibit decidability in cases where the language for expressing the query and the views allows for recursion.