19. The system of claim 17 , wherein the candidate query is further selected in response to determining that a quality score for the revised query is greater than a quality score threshold and that the navigation score for the revised query is greater than a navigation score threshold, and selecting the candidate query comprises: selecting a query that is prior to the revised query in the query sequence; determining a quality score for the selected query; determining navigation score for the selected query; and determining that the selected query is a candidate query if the quality score of the selected query is less than the quality score threshold and the navigation score of the selected query is less than the navigation score threshold.
19. The system of claim 17 , wherein the candidate query is further selected in response to determining that a quality score for the revised query is greater than a quality score threshold and that the navigation score for the revised query is greater than a navigation score threshold, and selecting the candidate query comprises: selecting a query that is prior to the revised query in the query sequence; determining a quality score for the selected query; determining navigation score for the selected query; and determining that the selected query is a candidate query if the quality score of the selected query is less than the quality score threshold and the navigation score of the selected query is less than the navigation score threshold. 21. The system of claim 19 , wherein the quality score for the revised query is determined by: determining resource metrics for each resource identified by a search engine in response to the revised query, the resource metrics including one or more of an authority score and a search result click-through rate; and determining revision metrics for the revised query, the revision metrics including one or more of a revision frequency and a revision time; wherein: the authority score is a score of a node in a link graph and that is representative of the resource; the revision frequency is frequency at which the revised query is further revised in search sessions; the revision time is a time measured relative to the entry of the candidate query; and the click-through rate is a rate at which one or more search results for the revised query in response to the search operation are selected.