  Consider the situation where a query is to be answered using Web sources that
restrict the accesses that can be made on backend relational data by requiring
some attributes to be given as input of the service. The accesses provide
lookups on the collection of attributes values that match the binding. They can
differ in whether or not they require arguments to be generated from prior
accesses. Prior work has focused on the question of whether a query can be
answered using a set of data sources, and in developing static access plans
(e.g., Datalog programs) that implement query answering. We are interested in
dynamic aspects of the query answering problem: given partial information about
the data, which accesses could provide relevant data for answering a given
query? We consider immediate and long-term notions of "relevant accesses", and
ascertain the complexity of query relevance, for both conjunctive queries and
arbitrary positive queries. In the process, we relate dynamic relevance of an
access to query containment under access limitations and characterize the
complexity of this problem; we produce several complexity results about
containment that are of interest by themselves.
