Memory residence can buy both functionality and performance for a database management system. In this paper, we present a description and a benchmark of an experimental implementation of a Main Memory Database System (MMDBS) that was designed to support complex interactive queries. We describe and evaluate the main memory database structures and query processing algorithms implemented in this prototype. Our measurements and analysis, focused on aggregates and joins, include both memory requirements and response time, since there is a clear trade-off between space and time in the design of a MMDBS. In contrast to conventional Disk-based Database Systems (DDBS's), we found that an MMDBS can efficiently execute complex relational queries. We identify strategies that exploit memory residence effectively. We also identified a number of performance problems related to query optimization in main memory and memory management for MMDBS's.