9. A system comprising at least one hardware processor coupled to computer storage to perform operations comprising: receiving a low frequency query from a client machine; analyzing a query log of user query sessions to identify a plurality of ordered sequences of low frequency queries including the low frequency query; generating a set of transition scores for the plurality of ordered sequences of low frequency queries based at least in part on a maximum number of transitions between a first query of an ordered pair of the plurality of ordered sequences of low frequency queries and a second query of the ordered pair to a purchase-related event, wherein an individual transition score comprises an accumulation of a popularity score and an accumulation of a purchase efficiency score; determining a normalized set of transition scores based on a frequency of the first query of each of the plurality of ordered sequences of low frequency queries; building a set of query suggestions for the low frequency query from the normalized set of transition scores; computing a set of similarity metrics for members of the set of query suggestions; determining whether one or more similarity metrics for individual suggestions of the set of query suggestions meet a predetermined similarity; and providing to the client machine a list of the individual suggestions whose similarity metrics meet the predetermined similarity.
9. A system comprising at least one hardware processor coupled to computer storage to perform operations comprising: receiving a low frequency query from a client machine; analyzing a query log of user query sessions to identify a plurality of ordered sequences of low frequency queries including the low frequency query; generating a set of transition scores for the plurality of ordered sequences of low frequency queries based at least in part on a maximum number of transitions between a first query of an ordered pair of the plurality of ordered sequences of low frequency queries and a second query of the ordered pair to a purchase-related event, wherein an individual transition score comprises an accumulation of a popularity score and an accumulation of a purchase efficiency score; determining a normalized set of transition scores based on a frequency of the first query of each of the plurality of ordered sequences of low frequency queries; building a set of query suggestions for the low frequency query from the normalized set of transition scores; computing a set of similarity metrics for members of the set of query suggestions; determining whether one or more similarity metrics for individual suggestions of the set of query suggestions meet a predetermined similarity; and providing to the client machine a list of the individual suggestions whose similarity metrics meet the predetermined similarity. 11. The system of claim 9 wherein the operations further comprise deleting at least one query suggestion based on a predetermined probability that selection of the query suggestion would result in returning a null search results page.