A query cache stores queries and corresponding results of the queries, the results of the queries being derived from a primary store. A differential store stores a pointer to data of the primary store which has changed and which affects the result of the queries stored in the query store. A new query may be satisfied by accessing the corresponding query in the query store and determining, by reference to the differential store, whether data relating to the query in the primary store has changed since the query store was compiled and, completing the new query, by accessing the corresponding data in the primary store, if applicable. Data in the differential store may be arranged and partitioned according to labels. The partitioning may be varied according to predetermined rules.