Complete Answers to Queries in the Presence of LimitedAccess Patterns ( Extended

We consider the problem of computing the complete answer to a query when there is limited access to relations, i.e., when binding patterns require values to be speciied for certain attributes in order to retrieve data from a relation. This problem is common in information-integration systems , where heterogeneous sources have diverse and limited query capabilities. A query is stable if for any instance of the relations mentioned in the query, the complete answer to the query can be computed, using only the binding patterns permitted for queries at the various sources. We rst study conjunctive queries, and we show that a conjunctive query is stable if and only if its minimal equivalent query Q m has an order of all the subgoals in Q m , such that each subgoal in the order can be queried with a legal binding pattern. We propose two algorithms for testing stability of conjunctive queries, and we prove this problem is NP-complete. For a nonstable conjunctive query, whether its complete answer can be computed is data dependent. We develop a decision tree that guides the query planning process to compute the complete answer to a conjunctive query, if it can be computed at all. We also study stability of unions of conjunctive queries, and conjunctive queries with arithmetic comparisons. In both cases we propose algorithms for testing stability of queries. Finally, we study datalog queries, and prove that if a set of rules and a query goal have a feasible rule/goal graph, then the query is stable. We also prove that stability of datalog programs is undecidable.