23. The system of claim 14 , wherein: the set of search results has a preliminary search result order; and ordering the portion of the search results in accordance with the identified predicted performance function includes: comparing predicted click through rates of a plurality search results in multiple different search result orders; selecting a respective search result order that increases a predicted click through rate of a plurality of the results over the preliminary search result order; and ordering the portion of the search results in the respective search result order.
14. A system, comprising: one or more processors; memory; one or more programs stored in the memory to be executed by the one or more processors, the one or more programs comprising: instructions for determining a correlation, for each search context of a plurality of search contexts, for each scoring primitive of a plurality of scoring primitives, and for a set of previously executed search queries that are consistent with the search context, between the scoring primitive and actual user selections of results of the previously executed search queries by a plurality of users; instructions for performing machine learning, for each search context, on the correlations to identify a predicted performance function comprising a weighted subset of the scoring primitives that meet predefined predictive quality criteria, wherein the identified predicted performance function is associated with the search context; and instructions for receiving and executing a user submitted search query, submitted by a user, to produce a set of search results, including associating the user submitted search query with a respective search context of the plurality of search contexts, and ordering at least a portion of the search results in accordance with the identified predicted performance function for the search context associated with the user submitted search query.