5. A method implemented by a system for combining rankers to perform a search operation, the method comprising acts of: ascertaining a count of a number of instances of a respective query-identifier pair from among multiple query-identifier pairs in log data based on a query that reflects entries recorded in the log data responsive to user interactions with the respective query-identifier pair that are associated with previous submissions of the query by users; performing a search for the query to produce a set of search results; and ranking the set of search results by relevance score with a document-based ranker and a log-based ranker using a weighting factor that is adapted as a function of the number of the instances of the respective query-identifier pair in the log data to decrease a contribution of the document-based ranker to the relevance score relative to the log-based ranker as the count of the number of instances of the respective query-identifier pair increases.
5. A method implemented by a system for combining rankers to perform a search operation, the method comprising acts of: ascertaining a count of a number of instances of a respective query-identifier pair from among multiple query-identifier pairs in log data based on a query that reflects entries recorded in the log data responsive to user interactions with the respective query-identifier pair that are associated with previous submissions of the query by users; performing a search for the query to produce a set of search results; and ranking the set of search results by relevance score with a document-based ranker and a log-based ranker using a weighting factor that is adapted as a function of the number of the instances of the respective query-identifier pair in the log data to decrease a contribution of the document-based ranker to the relevance score relative to the log-based ranker as the count of the number of instances of the respective query-identifier pair increases. 12. The method as recited in claim 5 , wherein the weighting factor comprises a function with an exponential distribution.