Relational Joins for Data on Tertiary Storage

Despite the steady decrease in secondary storage prices, the data storage requirements of many organizations cannot be met economically using secondary storage alone. Tertiary storage offers a lower-cost alternative but is viewed as a second-class citizen in many systems. For instance, the typical solution in bringing tertiary-resident data under the control of a DBMS is to use operating system facilities to copy the data to secondary storage, and then to perform query optimization and execution as if the data had been in secondary storage all along. This approach fails to recognize the opportunities for saving execution time and storage space if the data were accessed directly on tertiary devices and in parallel with other I/Os. In this paper we explore how to join two DBMS relations stored on magnetic tapes. Both relations are assumed to be larger than available disk space. We show how Grace Hash Join can be modified to handle a range of tape relation sizes. The modified algorithms access data directly on tapes and exploit parallelism between disk and tape I/Os. We also provide performance results of an experimental implementation of the algorithms.