Abstract
A major theme in relational database theory is navigating the tradeoff between expressiveness and tractability for query languages, where the query-containment problem is considered a benchmark of tractability. The query class UCQ, consisting off unions of conjunctive queries, is a fragment of first-order logic that has a decidable query containment problem, but its expressiveness is limited. Extending UCQ with recursion yields Datalog, an expressive query language that has been studied extensively and has recently become popular in application areas such as declarative networking. Unfortunately, Datalog has an undecidable query containment problem. Identifying a fragment of Datalog that is expressive enough for applications but has a decidable query-containment problem has been an open problem for several years.
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