Time–HOBI: Index for optimizing star queries

One of the important research and technological problems in data warehouse query optimization concerns star queries. So far, most of the research focused on optimizing such queries by means of join indexes, bitmap join indexes, or various multidimensional indexes. These structures neither support navigation well along dimension hierarchies nor optimize joins with the Time dimension, which in practice is used in most of the star queries. In this paper we propose an index, called Time–HOBI, for optimizing the star queries that compute aggregates along dimension hierarchies. Time–HOBI, created on a dimension hierarchy, is composed of (1) a Hierarchically Organized Bitmap Index (HOBI), where one bitmap index is maintained for one dimension level, and (2) a Time Index (TI) that implicitly encodes time in every dimension. HOBI allows to quickly search for fact rows satisfying predicates defined on different levels of dimension hierarchies. With the support of TI joining a fact table with the Time dimension is avoided. Thus, Time–HOBI supports a broad class of star queries. In this paper we explain how query execution plans for star queries can profit from Time–HOBI. We show, based on experiments, the efficiency of Time–HOBI for different classes of queries, as compared to HOBI and a traditional bitmap index. Based on the experiments, we also demonstrate how sensitive Time–HOBI is to variable selectivity of queries. We also analyze the maintenance time of Time–HOBI as compared to HOBI and a traditional bitmap index. The experiments used in the paper have been conducted on a real dataset, coming from the biggest East-European Internet auction platform Allegro.pl. The experiments show that Time–HOBI can be successfully applied to the optimization of star queries as it offers promising performance improvement.

► We propose the Time-HOBI index for optimizing star queries.
► Time-HOBI efficiently supports computations along a dimension hierarchy.
► Time-HOBI eliminates a join of a fact table with the Time dimension.
► Time-HOBI combines an index on a dimension hierarchy and on the Time dimension.
► Star query execution plans can be substantially simplified by means of Time-HOBI.