To improve OLAP performance, set the optimization_workload database option to OLAP to instruct the optimizer to consider using the Clustered Group By Hash operator in the possibilities it investigates. You can also tune indexes for OLAP workloads using the FOR OLAP WORKLOAD option when defining the index. Using this option causes the database server to perform certain optimizations which include maintaining a statistic used by the Clustered Group By Hash operator regarding the maximum page distance between two rows within the same key.

 See also