Queries Independent of Updates

This paper considers the problem of detecting independence of a queries expressed by datalog programs from updates. We provide new insight into the independence problem by reducing it to the equivalence problem for datalog programs (both for the case of insertion and deletion updates). Equivalence, as well as independence, is undecidable in general. However, algorithms for detecting subclasses of equivalence provide sufficient (and sometimes also necessary) conditions for independence. We consider two such subclasses. The first, query-reachability, generalizes previous work on independence [BCL89, El90], which dealt with nonrecursive programs with a single occurrence of the updated predicate. Using recent results on queryreachability [LS92, LMSS93], we generalize these earlier independence tests to arbitrary recursive datalog queries with dense-order constraints and negated EDB subgoals. The second subclass is uniform equivalence (introduced in [Sa88]). We extend the results of [Sa88] to datalog programs that include dense-order constraints and stratified negation. Based on these extensions, we present new cases in which independence is decidable and give algorithms that are sound for the general case. Aside for their use in detecting independence, the algorithms for detecting uniform equivalence are also important for optimizing datalog programs.