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Adjoined Dimension Column Clustering ( ADC Clustering ) to improve Data Warehouse Query Performance

Star Schemas were first devised by Red Brick Systems [11] to speed up queries on data loaded from operational databases at intervals of about a day, with no updates between loads. Variant designs (allowing some updates) were adopted by Oracle [8,12], DB2 [1,7], Sybase IQ [14] and recently Vertica [13]. Dimension tables have relatively small numbers of rows compared to the fact table. The POS Transaction fact table of Figure 1 has nine columns, a total of 40 bytes, and disks on an inexpensive PC can contain a fact table of a few hundred gigabytes, or several billion rows, while the largest dimension table is usually Products, with up to a few million rows. (There are only a few thousand rows in Dates, Stores, etc.). As a result, practitioners commonly place a large number of descriptive columns in dimensions; most queries restrict these dimension columns, and combined joins from dimensions determine fact table rows retrieved. In the late 1980s, a query filter factor of about 1/300, selecting rows on about one disk page out of ten, led to a DB2 query plan that retrieved just the selected subset of disk pages; if more than one disk page in ten was selected, a sequential scan that retrieved all table pages gave better performance. Since that time, disk technology has sped up sequential scans about 35 times more than it has sped access to selected disk pages. A query filter factor of about 1/10,000 or less is now required to retrieve just the selected pages, so most Data Warehouse queries scan across hundreds of GB of the fact table. To improve performance, some form of clustering must limit query retrieval range on most queries. A form of clustering by several attributes at once was introduced in 2003 with IBM’s Multi-dimensional Clustering (MDC) technology, explained in Section 2.2. MDC provides a powerful new tool, but does not directly solve the star schema access problem because the attributes typically used in queries on star schemas lie in dimension tables, not in the fact table as assumed by MDC. The fact table normally has just the foreign keys to the dimension tables, typically meaningless surrogate keys.