An "intelligent front-end" or "logic assistant" is an interactive program devised to assist the users of an information retrieval system in the formulation of their queries. In order to provide knowledge usable in such a program, we study a problem of queries optimization with an average efficiency criterion. We formulate it as a new combinatorial optimization problem, which we call 0-1 hyperbolic sum, and provide an exact branch-and-bound algorithm and two heuristics (of simulated annealing and tabu search type) to solve it. Computational experience illustrating the effectiveness of the tabu search technique is reported.