Norwegian University of Science and Technology Technical Report IDI-TR-1/2003 Algorithms for Granularity Reduction in Temporal Document Databases

With rapidly decreasing storage costs temporal document databases is now a viable solution in many contexts. However, storing an ever growing database can still be too costly, and as a consequence it is desirable to be able to physically delete old versions. Traditionally, this has been performed by an operation called vacuuming, where the oldest versions are physically deleted (or migrated from secondary storage to cheaper tertiary storage). However, in temporal document databases it is more appropriate to remove intermediate versions instead of removing the oldest versions. We call this operation granularity reduction. In this paper we describe six approaches to granularity reduction, and discuss advantages and disadvantages of these approaches. Three of the approaches have been implemented into the V2 temporal document database system, and in this context we discuss the cost of applying the approaches.