15. A computer-readable storage device storing computer-executable instructions that, when executed by one or more computer systems, configure the one or more computer systems to perform operations comprising: presenting an interface for a search engine; receiving a search string from a user; searching a first data set using a first search index based at least in part on a search query that includes the search string; obtaining first search results including a subset of the first data set based at least in part on said searching the first data set using the first search index; identifying a second search index from among a plurality of predetermined indices based at least in part on the first search results; searching the first search results using the second search index; obtaining second search results from among the subset of the first data set based at least in part on the second search index; ranking at least a part of the second search results with respect to each other based at least in part on a ranking algorithm of the second search index; modifying the second search index based at least in part on the ranked part of the second search results and data that is obtained independent of the second search results; and transmitting the second search results to the user.
15. A computer-readable storage device storing computer-executable instructions that, when executed by one or more computer systems, configure the one or more computer systems to perform operations comprising: presenting an interface for a search engine; receiving a search string from a user; searching a first data set using a first search index based at least in part on a search query that includes the search string; obtaining first search results including a subset of the first data set based at least in part on said searching the first data set using the first search index; identifying a second search index from among a plurality of predetermined indices based at least in part on the first search results; searching the first search results using the second search index; obtaining second search results from among the subset of the first data set based at least in part on the second search index; ranking at least a part of the second search results with respect to each other based at least in part on a ranking algorithm of the second search index; modifying the second search index based at least in part on the ranked part of the second search results and data that is obtained independent of the second search results; and transmitting the second search results to the user. 19. The computer-readable storage device of claim 15 , wherein identifying the second search index is further based at least in part on results of previous searches using at least one of the first search index or one of the plurality of predetermined indices.