1. A method for processing data relating to transactions comprising: with a processor: generating a computer data structure representing a prefix tree based on transaction information describing a number of transactions, each transaction comprising a respective set of items, nodes of the prefix tree representing respective items, and branches of the prefix tree representing respective frequent itemsets occurring in the transactions, the generation resulting in prefix tree information, assigning a transaction group identifier for each prefix tree node representing an item; associating the transaction group identifier of each leaf node with identifiers of transactions comprising items represented by nodes of a prefix tree branch ending at the leaf node, associating the transaction group identifier of each non-leaf node with transaction group identifiers of direct child nodes of the non-leaf node and with identifiers of transactions, comprising items represented by nodes of a prefix tree branch ending at the non-leaf node, the associations resulting in transaction group information describing associations between the transaction group identifiers and the identifiers of the transactions, and storing, in a memory, the prefix tree information, information about assigning the transaction group identifiers to the prefix tree nodes, and the transaction group information.