A formal framework for prefetching based on the type-level access pattern in object-relational DBMSs

Abstract

Prefetching is an effective method for minimizing the number of fetches between the client and the server in a database management system. In this paper, we formally define the notion of prefetching. We also formally propose new notions of the type-level access locality and type-level access pattern. The type-level access locality is a pheonomenon that repetitive patterns exist in the attributes referenced. The type-level access pattern is a pattern of attributes that are referenced in accessing the objects. We then develop an efficient capturing and prefetching policy based on this formal framework. Existing prefetching methods are based on object-level or page-level access patterns, which consist of object-ids or page-ids of the objects accessed. However, the drawback of these methods is that they work only when exactly the same objects or pages are accessed repeatedly. In contrast, even though the same objects are not accessed repeatedly, our technique effectively prefetches objects if the same attributes are referenced repeatedly, i.e., if there is type-level access locality. Many navigational applications in Object-Relational Database Management Systems (ORDBMSs) have type-level access locality. Therefore, our technique can be employed in ORDBMSs to effectively reduce the number of fetches, thereby significantly enhancing the performance. We also address issues in implementing the proposed algorithm. We have conducted extensive experiments in a prototype ORDBMS to show effectiveness of our algorithm. Experimental results using the OO7 benchmark, a real GIS application, and an XML application show that our technique reduces the number of fetches by orders of magnitude and improves the elapsed time by several factors over on-demand fetching and context-based prefetching, which is a state-of-the-art prefetching method. These results indicate that our approach provides a new paradigm in prefetching that improves performance of navigational applications significantly and is a practical method that can be implemented in commercial ORDBMSs.