Showing papers by "Nikos Mamoulis published in 2002"

View selection using randomized search

Abstract: An important issue in data warehouse development is the selection of a set of views to materialize in order to accelerate On-line analytical processing queries, given certain space and maintenance time constraints. Existing methods provide good results but their high execution cost limits their applicability for large problems. In this paper, we explore the application of randomized, local search algorithms to the view selection problem. The efficiency of the proposed techniques is evaluated using synthetic datasets, which cover a wide range of data and query distributions. The results show that randomized search methods provide near-optimal solutions in limited time, being robust to data and query skew. Furthermore, they can be easily adapted for various versions of the problem, including the simultaneous existence of size and time constraints, and view selection in dynamic environments. The proposed heuristics scale well with the problem size, and are therefore particularly useful for real life warehouses, which need to be analyzed by numerous business perspectives.

Efficient k-NN search on vertically decomposed data

Abstract: Applications like multimedia retrieval require efficient support for similarity search on large data collections. Yet, nearest neighbor search is a difficult problem in high dimensional spaces, rendering efficient applications hard to realize: index structures degrade rapidly with increasing dimensionality, while sequential search is not an attractive solution for repositories with millions of objects. This paper approaches the problem from a different angle. A solution is sought in an unconventional storage scheme, that opens up a new range of techniques for processing k-NN queries, especially suited for high dimensional spaces. The suggested (physical) database design accommodates well a novel variant of branch-and-bound search, that reduces the high dimensional space quickly to a small candidate set. The paper provides insight in applying this idea to k-NN search using two similarity metrics commonly encountered in image database applications, and discusses techniques for its implementation in relational database systems. The effectiveness of the proposed method is evaluated empirically on both real and synthetic data sets, reporting the significant improvements in response time yielded.

Indexing and Retrieval of Historical Aggregate Information about Moving Objects

Abstract: Spatio-temporal databases store information about the positions of individual objects over time. In many applications however, such as traffic supervision or mobile communication systems, only summarized data, like the average number of cars in an area for a specific period, or phones serviced by a cell each day, is required. Although this information can be obtained from operational databases, its computation is expensive, rendering online processing inapplicable. A vital solution is the construction of a spatiotemporal data warehouse. In this paper, we describe a framework for supporting OLAP operations over spatiotemporal data. We argue that the spatial and temporal dimensions should be modeled as a combined dimension on the data cube and present data structures, which integrate spatiotemporal indexing with pre-aggregation. While the well-known materialization techniques require a-priori knowledge of the grouping hierarchy, we develop methods that utilize the proposed structures for efficient execution of ad-hoc group-bys. Our techniques can be used for both static and dynamic dimensions.