When the relational storage method is adopted in the data warehouse system, the most time-consuming operations in OLAP query are multi-table join and group-by aggregation. Based on the characteristics of the data warehouse itself and the applications run on it, one new multi-table join algorithm -- Mjoin is given in this paper. And the performance of this new Mjoin algorithm is improved greatly, compared with other traditional multi-table join algorithms. In this paper, a new sorting based group-by aggregation algorithm — MuSA is also given, based on the Mjoin algorithm. The speed of sorting is remarkably improved in this new sorting based aggregation algorithm, as the keyword mapping technology is used to compress the sort keywords in the course of sorting.