7. A non-transitory computer storage medium encoded with a computer program, the program comprising instructions that when executed by one or more data processing apparatus cause the one or more data processing apparatus to perform operations comprising: determining that a given user device previously submitted a search query; determining that the given user device requested presentation of two or more pages of search results responsive to the previously submitted search query; determining that no user interaction occurred with the search results presented on the two or more pages of search results presented on the given user device responsive to the previously submitted search query; in response to determining that both the two or more pages of search results were requested and that no user interaction occurred with the search results presented on the two or more pages of search results presented on the given user device, determining that a user dissatisfaction condition has occurred; in response to determining the user dissatisfaction condition has occurred, determining to perform a remedial search operation that includes identifying, as a remedial search query for the remedial search operation, a related query for the previously submitted search query that is different from the previously submitted search query; receiving a current search query from the given user device, the current search query being received subsequent to the previously submitted search query; determining that a topic of the current search query has been classified as being similar to a topic of the previously submitted search query; and in response to determining that the topic of the current search query has been classified as being similar to a topic of the previously submitted search query and in response to determining to perform the remedial search operation, performing the remedial search operation by providing, to the given user device and for presentation on a search results page that includes search results for the current search query, a link that, when selected, initiates a search using the identified remedial search query.
8. The computer storage medium of claim 7 , wherein the instructions cause the one or more data processing apparatus to perform operations further comprising: identifying, from a data store, a plurality of previous search queries that were received from the given user device; identifying, from among the previous search queries, a first search query that is related to a first topic; determining that a second search query, from among the previous search queries, was received after the first search query and is related to a second topic that differs from the first topic; and defining, as a beginning of a search session for the second search query, a time after receipt of the first search query and up to a time at which the second search query was received.