1. A method comprising: monitoring a plurality of numbers of distinct values (NDVs) for a plurality of candidate distribution keys, wherein each candidate distribution key of said plurality of candidate distribution keys comprises one or more group-by keys of an ordered list of group-by columns specified by a database statement that requests performing one or more rollup operations relating to the ordered list of group-by keys; selecting a distribution key from said plurality of candidate distribution keys based at least in part on results of monitoring the plurality of NDVs; distributing a set of rows, based at least in part on the selected distribution key, between first parallel executing processes and second parallel executing processes, wherein the one or more rollup operations are performed by the first parallel executing processes and the second parallel executing processes against the set of rows; wherein the method is performed by one or more computing devices.