Expressive Logical Combinators for Free
Abstract
A popular technique in the static analysis for query languages relies on the construction of compilers that effectively translate queries into logical formulas. These formulas are then solved for satisfiability using an off-the-shelf satisfiability solver. A critical aspect in this approach is the size of the obtained logical formula, since it constitutes a factor that affects the combined complexity of the global approach. We show that a whole class of logical combinators (or ''macros'') can be used as an intermediate language between the query language and the logical language. Those logical combinators provide an exponential gain in succinctness over the corresponding explicit logical representation, yet preserving the typical exponential time complexity of the subsequent logical decision procedure. This opens the way for solving a wide range of problems such as satisfiability and containment for expressive query languages in exponential-time even though their direct formulation into the underlying logic results in an exponential blowup of the formula size, yielding an incorrectly presumed two-exponential time complexity. We illustrate this from a very practical point of view on a few examples such as numerical occurrence constraints and tree frontier properties which are concrete problems found in the XML world.
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