Twin grid files: space optimizing access schemes

Storage access schemes for points, supporting spatial searching, usually suffer from an undesirably low storage space utilization. We show how a given set of points can be distributed among two grid files in such a way that storage space utilization is optimal. The optimal twin grid file can be built practically as fast as a standard grid file, i.e., the storage space optimality is obtained at almost no extra cost. We compare the performances of the standard grid file, the optimal static twin grid file, and an efficient dynamic twin grid file, where insertions and deletions trigger the redistribution of points among the two grid files. Twin grid files utilize storage space at roughly 90%, as compared with the 69% of the standard grid file. Typical range queries - the most important spatial search operations - can be answered in twin grid files at least as fast as in the standard grid file.