16. The computer storage medium of claim 15 , the operations further comprising: updating a cluster index with information from the summarized cluster.
16. The computer storage medium of claim 15 , the operations further comprising: updating a cluster index with information from the summarized cluster. 17. The computer storage medium of claim 16 , wherein identifying one or more candidate clusters of observations responsive to the generated query comprises: identifying one or more candidate clusters of observations responsive to the generated query using the cluster index.