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19th International Conference on Data Engineering (ICDE'03)   p. 329
Combining Hierarchy Encoding and Pre-Grouping: Intelligent Grouping in Star Join Processing
Roland Pieringer, Transaction Software GmbH, Germany
Klaus Elhardt, Transaction Software GmbH, Germany
Frank Ramsak, Bayerisches Forschungszentrum für Wissensbasierte Systeme, Germany
Volker Markl, IBM Almaden Research Center, San Jose, CA
Robert Fenk, Bayerisches Forschungszentrum für Wissensbasierte Systeme, Germany
Rudolf Bayer, TU-München, Germany
Nikos Karayannidis, National Technical University of Athens, Hellas
Aris Tsois, National Technical University of Athens, Hellas
Timos Sellis, National Technical University of Athens, Hellas

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DOI Bookmark: http://doi.ieeecomputersociety.org/10.1109/ICDE.2003.1260803
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Abstract
Efficient star query processing is crucial for a performant data warehouse (DW) implementation and much work is available on physical optimization (e.g., indexing and schema design) and logical optimization (e.g., pre-aggregated materialized views with query rewriting). One important step in the query processing phase is, however, still a bottleneck: the residual join of results from the fact table with the dimension tables in combination with grouping and aggregation. This phase typically consumes between 50% and 80% of the overall processing time. In typical DW scenarios pre-grouping methods only have a limited effect as the grouping is usually specified on the hierarchy levels of the dimension tables and not on the fact table itself. In this paper, we suggest a combination of hierarchical clustering and pre-grouping as we have implemented in the relational DBMS Transbase. Exploiting hierarchy semantics for the pre-grouping of fact table result tuples is several times faster than conventional query processing. The reason for this is that hierarchical pre-grouping reduces the number of join operations significantly. With this method even queries covering a large part of the fact table can be executed within a time span acceptable for interactive query processing.
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Citation:  Roland Pieringer, Klaus Elhardt, Frank Ramsak, Volker Markl, Robert Fenk, Rudolf Bayer, Nikos Karayannidis, Aris Tsois, Timos Sellis, "Combining Hierarchy Encoding and Pre-Grouping: Intelligent Grouping in Star Join Processing," icde, p. 329,  19th International Conference on Data Engineering (ICDE'03),  2003