In this paper we present a novel method for preprocessing complex non-ground rules in answer set programming (ASP). Using a well-known result from the area of conjunctive query evaluation, we apply hypertree decomposition to ASP rules in order to make the structure of rules more explicit to grounders. In particular, the decomposition of rules reduces the number of variables per rule, while on the other hand, additional predicates are required to link the decomposed rules together. As we show in

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... this paper, this technique can reduce the size of the grounding significantly and thus improves the performance of ASP systems in certain cases. Using a prototype implementation and the benchmark suites of the Answer Set Programming Competition 2011, we perform extensive tests of our decomposition approach that clearly show the improvements in grounding time and size. 1 Introduction Starting from the pioneering work of Gelfond and Lifschitz [16, 17], the declarative problem solving paradigm of answer set programming (short: ASP, see e.g. [2]) has become a central formalism in artificial intelligence and knowledge representation. This is due to its simple, yet expressive modelling language, which is implemented by systems showing a steadily increasing performance. Such systems follow a two-step approach for evaluating a program: The so-called grounder instantiates rules by replacing the various variables with applicable constants. This yields a propositional logic program (consisting of propositional or "ground" rules) that is equivalent for the given domain. This program is then finally fed into the actual solver. In systems like lparse/smodels [23] or gringo/clasp [12] this separation is quite strict whereas DLV [20] followed a more integrated approach. Although today's ASP systems have reached an impressive state of sophistication, we believe that there is still room for improvement, in particular on the level of grounding. In fact, since checking whether a non-ground rule fires is already NP-complete [9] in general (as easily shown by analogy to the conjunctive query evaluation problem, which is also NP-complete, cf. [1]), grounders have to list all possibly applicable instantiations of non-ground rules which are, by the NP-completeness of the aforementioned problem, exponentially many in the worst case. However, often the rules exhibit a particular structure which, in theory, could be used to avoid or at least reduce this blow-up. Several preprocessing and optimization techniques * This work was supported by special fund "Innovative Projekte 9006.09/008" of TU Vienna.