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 first node containing a bitmap value that represents a subset of a set of objects; 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 determined by a logical operation on bits of the bitmap values contained in the plurality of first nodes that are descendants of the second node.