Optimal Planar Range Skyline Reporting with Linear Space in External Memory

Let P be a set of n points in R. Given a rectangle Q = [α1, α2] × [β1, β2], a range skyline query returns the maxima of the points in P ∩Q. An important variant is the so-called topopen queries, where Q is a 3-sided rectangle whose upper edge is grounded at y = ∞ (that is, β2 = ∞). These queries are crucial in numerous database applications. In internal memory, extensive research has been devoted to designing data structures that can answer such queries efficiently. In contrast, currently there is no clear understanding about their exact complexities in external memory. This paper presents several structures of linear size for answering the above queries with the optimal I/O cost. We show that a top-open query can be solved in O(logB n+ k/B) I/Os, where B is the block size and k is the number of points in the query result. The query cost can be made O(log logB U + k/B) when the data points lie in a U × U grid for some integer U ≥ n, and further lowered to O(1 + k/B) if U = O(n). The same efficiency also applies to 3sided queries where Q is a right-open rectangle. However, the hardness of the problem increases if Q is a leftor bottom-open 3-sided rectangle. We prove that any linear-size structure must perform Ω((n/B) + k/B) I/Os to solve such a query in the worst case, where ǫ > 0 can be an arbitrarily small constant. In fact, leftand right-open queries are just as difficult as general (4-sided) queries, for which we give a linear-size structure with query time O((n/B) + k/B). Interestingly, this indicates that 4-sided range skyline queries have exactly the same hardness as 4-sided range reporting (where the goal is to report simply the whole P ∩Q). That is, the skyline requirement does not alter the problem difficulty at all.