1. A method in a computing system for constructing an index usable to identify expanded queries for a set of seed queries, comprising: receiving a query history identifying, for each of a plurality of queries received from users, text of the query, a user submitting the query, and a time at which the query was submitted; for each unique query identified by the query history: for each user: for every submission of the unique query by the user: adding to an entry of the index for the unique query an instance of any query submitted by the user within a predetermined time period of the submission of the unique query by the user; receiving a set of seed queries; for each of the seed queries of the set: identifying an entry of the index corresponding to the seed query; submitting to a search engine a request to match the query instances added to the identified entry against the query instances added to the entries of the index other than the identified entry; receiving from the search engine a score for each entry of the index other than the identified entry indicating the extent to which the query instances added to the identified entry matches the query instances added to the entry; and selecting as expanded queries the queries corresponding to the entries of the index other than the identified entry having the highest scores.