This paper introduces techniques for reducing data dissemination costs
of query subscriptions. The reduction is achieved by merging queries
with overlapping, but not necessarily equal, answers. The paper
formalizes the query-merging problem and introduces a general cost model
for it. We prove that the problem is NP-hard and propose exhaustive
algorithms and three heuristic algorithms: the Pair Merging Algorithm,
the Directed Search Algorithm and the Clustering Algorithm. We develop a
simulator for evaluating the different heuristics and show that the
performance of our heuristics is close to optimal.