Multi-dimensional clustering (MDC) enables a table to be physically clustered on more than one key, or dimension, simultaneously. Prior to Version 8, DB2 supported only single-dimensional clustering of data using clustering indexes. When a clustering index is defined on a table, DB2 attempts to maintain the physical order of the data on pages, based on the key order of the clustering index, as records are inserted into and updated in the table. This can significantly improve the performance of queries that have predicates containing the key(s) of the clustering index because, with good clustering, only a portion of the physical table needs to be accessed. In addition, when the pages are stored sequentially on disk, more efficient prefetching can be performed.
With MDC, these same benefits are extended to more than one dimension, or clustering key. In the case of query performance, range queries involving any one or combination of the specified dimensions of the table will benefit from the underlying clustering. These queries will need to access only those pages having records with the specified dimension values, and these qualifying pages will be grouped together in extents.
A table with a clustering index can become unclustered over time, as available space is filled in the table; however, an MDC table is able to maintain its clustering over the specified dimensions automatically and continuously, eliminating the need to reorganize the table in order to restore the physical order of the data.