1. A computer-implemented method comprising: accessing data that identifies one or more query term optionalization rules that are specific to a particular query term, the one or more query term optionalization rules indicating whether the particular query term should be made optional in revisions of search queries that include the particular query term; accessing query log data that identifies, for an initial search query that includes the particular query term: one or more revised search queries that a search engine system generated in response to the initial search query using the one or more query term optionalization rules that are specific to the particular query term, wherein the particular query term is optional in the one or more revised search queries, search results that were identified using the initial search query, search results that were identified using the one or more revised search queries, and a particular search result selected by a user from among a set of search results that includes (i) one or more of the search results that were identified using the initial search query and (ii) one or more of the search results that were identified using the one or more revised search queries; determining, using the query log data, that the particular search result is one of the search results that were identified using the one or more revised search queries; in response to determining that the particular search result is one of the search results that were identified using the one or more revised search queries, adjusting a click count for the one or more query term optionalization rules that are specific to the particular query term; and after adjusting the click count for the one or more query term optionalization rules that are specific to the particular query term, determining, based at least on the click count for the one or more query term optionalization rules that are specific to the particular query term, whether to revise a subsequently-received search query that includes the particular query term using the one or more query term optionalization rules that are specific to the particular query term.