From a corpus of segments, a hierarchical index is derived that indexes high frequency events of a selected event type occurring in segments of the corpus at a frequency higher than or equal to a threshold frequency, and also indexes at least some low frequency events that occur in segments of the corpus at a frequency lower than the threshold frequency. The hierarchy relates events by an order structure in which coarser events subsume finer events. A query is processed respective to a queried event. The processing references information stored in the index relating to either (i) the queried event if the queried event is indexed or (ii) a coarser event that is indexed and that subsumes the queried event if the queried event is not indexed.