1. An index tree, the index tree being stored in a medium accessible to a processor and being manipulatable by the processor, the index tree comprising: a plurality of first nodes at a level of the tree, each node containing a bitmap value; and a second node at a next higher level of the tree, the second node being a parent of the plurality of first nodes and containing a summary bitmap value that equals the result of an aggregation operation on bits of the bitmap values contained in the plurality of first nodes.