A data item is retrieved from a data store in a data processing system. The data processing system comprises an index, which at least partially indexes the data store, and also a partiality definition store. The partiality definition store indicates a part of the data store which is referenced by the partial index. A request is received for a data item from the data store and responsive to determining that the requested data item is not referenced by the partial index, the data store is searched by using the partiality definition store to avoid searching a part of the data store which is known to be indexed.