In this paper, we describe heuristic rules for choos ing an appropriate access path for a given query in main memory databases using a functional memory system. We have asserted that conventional cache- based memory access is not suitable for query processing in main memory databases. To cope with this problem, we have proposed three hardware-supporte d memory access scheme s. If we select an appropriate memory acce ss scheme from the three proposed sche mes and the cache-based access, the execution time of query processing can be reduced. In this paper, we derive heuristic rules on which access path should be selected and show its impact on execution time through experimentation.