1. A method, comprising: for a set of encoded Resource Description Format entities, attributes, values and encoded categories, forming a category index of encoded categories, each encoded category in the category index comprising a list of corresponding encoded entities; determining for each encoded attribute, those encoded entities that have encoded values for the encoded attribute; responsive to the determination, forming an attribute index of encoded attributes and their corresponding encoded entities and encoded values; and storing the category index and the attribute index to form a knowledge database.