Data warehouse architectural choices and optimization techniques are critical to decision support query performance. To facilitate these choices, the performance of the designed data warehouse must be assessed, usually with benchmarks. These tools can either help system users comparing the performances of different systems, or help system engineers testing the effect of various design choices. While the Transaction Processing Performance Council's standard benchmarks address the first point, they are not tunable enough to address the second one and fail to model different data warehouse schemas. By contrast, our Data Warehouse Engineering Benchmark (DWEB) allows generating various ad-hoc synthetic data warehouses and workloads. DWEB is implemented as a Java free software that can be interfaced with most existing relational database management systems. The full specifications of DWEB, as well as experiments we performed to illustrate how our benchmark may be used, are provided in this paper.