14. A system that is capable of combining rankers to perform a search operation, the system comprising: one or more processors; one or more processor-accessible tangible media storing instructions executable via the one or more processors to implement: a data log to maintain log data that includes instances of respective query-identifier pairs and user interaction information from which associated respective relevance scores may be derived, each query-identifier pair recorded to reflect a submission of a corresponding query by a user; a search unit to perform a search for a particular query to produce a set of search results; and a ranker combining unit to rank the set of search results by relevance score by combining scores from a document-based ranker and a log-based ranker into a combined score for each query-identifier pair using a weighting factor that is adapted as a function of a count of instances of a respective query-identifier pair that includes the particular query in the log data as the count changes, the weighting factor being adapted by at least decreasing a contribution of the document-based ranker to the combined score of the respective query-identifier pair as the count of the instances of the respective query-identifier pair in the log data increases.
14. A system that is capable of combining rankers to perform a search operation, the system comprising: one or more processors; one or more processor-accessible tangible media storing instructions executable via the one or more processors to implement: a data log to maintain log data that includes instances of respective query-identifier pairs and user interaction information from which associated respective relevance scores may be derived, each query-identifier pair recorded to reflect a submission of a corresponding query by a user; a search unit to perform a search for a particular query to produce a set of search results; and a ranker combining unit to rank the set of search results by relevance score by combining scores from a document-based ranker and a log-based ranker into a combined score for each query-identifier pair using a weighting factor that is adapted as a function of a count of instances of a respective query-identifier pair that includes the particular query in the log data as the count changes, the weighting factor being adapted by at least decreasing a contribution of the document-based ranker to the combined score of the respective query-identifier pair as the count of the instances of the respective query-identifier pair in the log data increases. 18. The system as recited in claim 14 , wherein each of the relevance scores comprises a probability value associated with each respective query-identifier pair.