3. The method of claim 1 , wherein features of the received and expanded queries comprise one or more of unigrams, phrases, and semantic classes.
1. A computer-implemented method for expansion of rare queries to improve advertisement results, the method comprising: receiving a query from a user by a search engine; determining that the query does not match an entry in an ad query lookup table stored in data storage of the search engine; retrieving one or more expanded queries located within a query feature index whose features relate to one or more features of the received query, wherein the query feature index is stored in a database of the data storage and comprises a plurality of expanded queries; wherein retrieving comprises: representing the features as vectors; weighting the vectors of the received query based on a number of times corresponding respective features occur in the query and on an inverse document frequency for corresponding respective features in an ad corpus, to more heavily weight the rare queries; and using a vector space-based retrieval approach for retrieving the expanded queries; generating, in real time and by the search engine, an ad query comprising an expanded version of the received query based on features of the retrieved expanded queries; and selecting one or more advertisements based on the generated ad query, wherein the one or more advertisements are displayed to the user in response to the query received from the user.