Querying the deep web: Back to the foundations

The Deep Web is the large corpus of data accessible on the Web through forms and presented in dynamically-generated pages, but not indexable as static pages, and therefore invisible to search engines. Deep Web data are usually modelled as relations with so-called access limitations, that is, they can be queried only by selecting certain at-Tributes. In this paper we give some fundamental complexity results on the problem of processing conjunctive (select-project-join) queries on re- lational data with access limitations.