Most previous work in the area of main the memory database systems has focused on the problem of developing query processing techniques that work well with a very large buffer pool in this paper we address query processing issues for memory president relational databases an environment with a very different set of costs and priorities we present an architecture for a main memory DBMS, discussing the ways in which a memory resident database differs from a disk-based database we then address the problem of processing relational queries in this architecture considering alternative algorithms for selection, projection and join operations and studying their performance we show that a new index structure the t tree works well for selection and join processing in memory residents databases we also show that hashing methods work well for processing projections and joins and that an old join method sort-merge still has a place in main memory.