A computer-readable storage medium storing a dataset sorting program is provided to sort records in a dataset into a plurality of destination groups according to a given key item specification. An item value extractor creates an item value list for every record. Then a frequent tree builder builds a frequent tree from the item value lists by finding patterns of item values that appear more often than a threshold specified by a given growth rate parameter. Each item value pattern is a leading part of an item value list with a variable length. A destination group mapper associates each node of the frequent tree with one of the plurality of destination groups. A record sorter traces the frequent tree according to the item value list of each given record, and upon reaching a particular node, puts the record into the destination group associated with that node.