9. A system comprising: one or more computers and one or more storage devices storing instructions that are operable, when executed by the one or more computers, to cause the one or more computers to perform operations comprising: receiving, at a computer system, a request to classify a first query; obtaining, by the computer system, session data for the first query, wherein: the session data for the first query identifies a plurality of sessions in which the first query occurs, each session of the plurality of sessions includes (i) one or more queries submitted in succession by a single respective user for the session and (ii) one or more search entities returned in response to executing the one or more queries, and each search entity is assigned one or more classifications; selecting, by the computer system from the session data for the first query, a first plurality of search entities, wherein the first plurality of search entities are the search entities that frequently occur in the plurality of query sessions in response to executing the first query; identifying, by the computer system, one or more potential classifications for the first query; for each potential classification of the one or more potential classifications, determining a first measure of how many of the first plurality of search entities have been assigned the potential classification; determining that the first measure have been assigned the classification satisfies a classification threshold; and in response to determining that the first measure have been assigned the classification satisfies the classification threshold, assigning the potential classification to the first query, determining that a second query occurs in the same first session with the first query; and assigning the potential classification to the second query.
9. A system comprising: one or more computers and one or more storage devices storing instructions that are operable, when executed by the one or more computers, to cause the one or more computers to perform operations comprising: receiving, at a computer system, a request to classify a first query; obtaining, by the computer system, session data for the first query, wherein: the session data for the first query identifies a plurality of sessions in which the first query occurs, each session of the plurality of sessions includes (i) one or more queries submitted in succession by a single respective user for the session and (ii) one or more search entities returned in response to executing the one or more queries, and each search entity is assigned one or more classifications; selecting, by the computer system from the session data for the first query, a first plurality of search entities, wherein the first plurality of search entities are the search entities that frequently occur in the plurality of query sessions in response to executing the first query; identifying, by the computer system, one or more potential classifications for the first query; for each potential classification of the one or more potential classifications, determining a first measure of how many of the first plurality of search entities have been assigned the potential classification; determining that the first measure have been assigned the classification satisfies a classification threshold; and in response to determining that the first measure have been assigned the classification satisfies the classification threshold, assigning the potential classification to the first query, determining that a second query occurs in the same first session with the first query; and assigning the potential classification to the second query. 15. The system of claim 9 , wherein determining the first measure of how many of the first plurality of search entities have been assigned the potential classification comprises weighting a classification assigned to each of the first plurality of search entities according to a measure of proximity of the search entity to the first query in one or more query sessions.