We consider the problems of finding and determining certain query answers and of determining containment between queries; each problem is formulated in presence of materialized views and dependencies under the closed-world assumption. We show a tight relationship between the problems in this setting. Further, we introduce algorithms for solving each problem for those inputs where all the queries and views are conjunctive, and the dependencies are embedded weakly acyclic. We also determine the complexity of each problem under the security-relevant complexity measure introduced by Zhang and Mendelzon in 2005. The problems studied in this paper are fundamental in ensuring correct specification of database access-control policies, in particular in case of fine-grained access control. Our approaches can also be applied in the areas of inference control, secure data publishing, and database auditing.
The work @cite_1 by Zhang and Mendelzon introduced and solved the problem of conditional containment'' between two CQ queries in presence of materialized CQ views, under CWA and in the absence of dependencies. @cite_1 also introduced a security-relevant complexity metric, under which their problem is @math complete. ( @cite_1 also provides an excellent overview of the connections of the query-containment problem of @cite_1 to database-theory literature.) In our work, we add dependencies to the formulation of the problem of @cite_1 , and extend the approach of @cite_1 , both to solve the resulting problem in the CQ weakly acyclic setting and to analyze the complexity of the problem. We also uncover a tight relationship of the problem with the problems of finding and determining certain query answers, under CWA in presence of view materializations and of dependencies.
@cite_1
2013409229 1967827263 1508622259 2108930340
Query containment under constraints is the problem of checking whether for every database satisfying a given set of constraints, the result of one query is a subset of the result of another query. Recent research points out that this is a central problem in several database applications, and we address it within a setting where constraints are specified in the form of special inclusion dependencies over complex expressions, built by using intersection and difference of relations, special forms of quantification, regular expressions over binary relations, and cardinality constraints. These types of constraints capture a great variety of data models, including the relational, the entity-relational, and the object-oriented model. We study the problem of checking whether q is contained in q′ with respect to the constraints specified in a schema S, where q and q′ are nonrecursive Datalog programs whose atoms are complex expressions. We present the following results on query containment. For the case where q does not contain regular expressions, we provide a method for deciding query containment, and analyze its computational complexity. We do the same for the case where neither S nor q, q′ contain number restrictions. To the best of our knowledge, this yields the first decidability result on containment of conjunctive queries with regular expressions. Finally, we prove that the problem is undecidable for the case where we admit inequalities in q′. Conjunctive queries (CQs) fail to provide an answer when the pattern described by the query does not exactly match the data. CQs might thus be too restrictive as a querying mechanism when data is semistructured or incomplete. The semantic web therefore provides a formalism - known as well-designed pattern trees (WDPTs) - that tackles this problem: WDPTs allow us to match patterns over the data, if available, but do not fail to give an answer otherwise. Here we abstract away the specifics of semantic web applications and study WDPTs over arbitrary relational schemas. Our language properly subsumes the class of CQs. Hence, WDPT evaluation is intractable. We identify structural proper ties of WDPTs that lead to tractability of various variants of the evaluation problem. For checking if a WDPT is equivalent to one in our tractable class, we prove 2EXPTIME-membership. As a corollary, we obtain fixed-parameter tractability of (variants of) the evaluation problem. Our techniques also allow us to develop a theory of approximations for WDPTs. A recent proposal for database access control consists of defining “authorization views” that specify the accessible data, and declaring a query valid if it can be completely rewritten using the views. Unlike traditional work in query rewriting using views, the rewritten query needs to be equivalent to the original query only over the set of database states that agree with a given set of materializations for the authorization views. With this motivation, we study conditional query containment, i.e. , containment over states that agree on a set of materialized views. We give an algorithm to test conditional containment of conjunctive queries with respect to a set of materialized conjunctive views. We show the problem is @math -complete. Based on the algorithm, we give a test for a query to be conditionally authorized given a set of materialized authorization views. The authors address the issue of reasoning with two classes of commonly used semantic integrity constraints in database and knowledge-base systems: implication constraints and referential constraints. They first consider a central problem in this respect, the IRC-refuting problem, which is to decide whether a conjunctive query always produces an empty relation on (finite) database instances satisfying a given set of implication and referential constraints. Since the general problem is undecidable, they only consider acyclic referential constraints. Under this assumption, they prove that the IRC-refuting problem is decidable, and give a novel necessary and sufficient condition for it. Under the same assumption, they also study several other problems encountered in semantic query optimization, such as the semantics-based query containment problem, redundant join problem, and redundant selection-condition problem, and show that they are polynomially equivalent or reducible to the IRC-refuting problem. Moreover, they give results on reducing the complexity for some special cases of the IRC-refuting problem.