One or more processors initiate cluster feature (CF)-tree based hierarchical clustering on leaf entries of CF-trees included in a plurality of subsets. One or more processors, generate respective partial clustering solutions for the subsets. A partial clustering solution includes a set of regular sub-clusters and candidate outlier sub-clusters. One or more processors generate initial regular clusters by performing hierarchical clustering using the regular sub-clusters. For a candidate outlier sub-cluster, one or more processors determine a closest initial regular cluster, and a distance separating the candidate outlier sub-cluster and the closest initial regular cluster. One or more processors determine which candidate outlier sub-clusters are outlier clusters based on which candidate outlier sub-clusters have a computed distance to their respective closest initial regular cluster that is greater than a corresponding distance threshold associated with their respective closest initial regular cluster.