1. A computer-implemented method of identifying web page impressions that satisfy a query comprising: receiving, from a client device, a query having a number of criteria greater than a predetermined number, wherein the number of criteria and the predetermined number are both greater than zero and less than infinity, and the query specifies one or more attributes of web page impressions; generating a plurality of sub-queries from the query, including generating a first sub-query associated with first criteria and generating a second sub-query associated with second criteria, wherein each criterion of the query is represented in at least one of the plurality of sub-queries, and the first criteria and the second criteria are mutually exclusive; obtaining a response for each of the sub-queries, from a database of web page impressions, wherein each response comprises identifiers of web page impressions satisfying criteria of the associated sub-query; generating a response to the query by comparing the responses to the sub-queries; and providing an answer to the client device based at least in part on said generating a response to the query by comparing the responses to the sub-queries.