Increasing Buffer-Locality for Multiple Relational Table Scans through Grouping and Throttling

Decision support (DSS) workloads generally contain multiple large concurrent scan operations. These are often executed as relational table scans which can take up a lot of I/O bandwidth. This is especially true for ad-hoc queries where the workload is not known in advance. Common database management systems have only limited ability to reuse memory buffer content across multiple running queries due to their treatment of queries in isolation. Previous attempts to coordinate scans for better buffer reuse were less than satisfactory due to drifting between scans and the required radical DBMS architecture changes. In this paper, we describe a new mechanism to keep similar table scans closer together during scanning. This is achieved via dynamic grouping and regrouping of scans based on their runtime behavior and via adaptive throttling of scan speeds based on scan group characteristics. The required memory footprint is very small and the effort required to extend existing database management systems is minimal, as shown in our DB2 UDB prototype. Our experiments show significant gains in end-to-end response times as well as average response times for TPC-H workloads.