Currently the indexer in Nutchgora consists of both mappers and reduces. But the reduce code does not actually iterate over any (grouped/sorted) values. It simply indexes individual key/value (String/Webpage) pairs. Therefore by moving this indexing code to the mapper we can eliminate the reduce step therefore making the indexing job much faster. (No more unnecessary spilling to disk/network and no cpu wasted to sorting).
Note this is not (directly) applicable to trunk because trunk uses a quite different approach. Different types of input are combined to a single value in the reducer. Although I think it is possible to implement a similar optimization I am not sure how to do this. So if anyone wants this for trunk too feel free to implement a similar patch.