The problem of physical design of data warehouses should
rekindle interest in the well-known problems of index
selection, data partitioning and the selection of materialized
views. However, while revisiting these problems, it is
important to recognize the special role played by aggregation.
Decision support systems already provide the field of query
optimization with increasing challenges in the traditional
questions of selectivity estimation and cost-based algorithms
that can exploit transformations without exploding the search
space (there are plenty of transformations, but few reliable
cost estimation techniques and few smart cost-based
algorithms/search strategies to exploit them). Partitioning the
functionality of the query engine between the middleware
(e.g., ROLAP layer) and the back end server is also an
interesting problem.