1. A system comprising: one or more computers including one or more storage devices storing instructions that when executed by the one or more computers cause the one or more computers to perform operations comprising: receiving extraction patterns from a repository of extraction patterns; applying the extraction patterns to document text of one or more documents to derive a plurality of candidate class-instance pairs from the document text, wherein each candidate class-instance pair comprises a candidate class name and a candidate instance name derived by matching at least one of the extraction patterns to document text including the candidate class name and the candidate instance name; determining a frequency score and a diversity score for each candidate class-instance pair, wherein the frequency score relates to a frequency with which the class-instance pair was derived and the diversity score relates to a number of distinct extraction patterns that matched document text including the candidate class name and the candidate instance name of the candidate class-instance pair; and determining a pair score for each candidate class-instance pair in the plurality of candidate class-instance pairs from the frequency score and the diversity score, wherein determining the pair score comprises multiplying the frequency score by the diversity score.
6. The system of claim 1 , wherein deriving a candidate class-instance pair from the document text comprises identifying a candidate class name and a plurality of candidate instance names from the document text, selecting one of the candidate instance names using query data for previously submitted search queries, and deriving a class-instance pair corresponding to the candidate class name and the selected candidate instance name.