A practical example of Map/Reduce algorithm. Very easily understandable article … with no buzz jargon.
Example target’s the problem of Finding count of comments, group by blog_id, on very large dataset (not feasible for DB SELECT)
In short … Map/reduce is a very hot topic, but you need to realize what it is for. It isn’t some magic formula from Google to make things run faster, it is just Select and GroupBy, run over a distributed network.
Map/Reduce or Hadoop sound best for large data aggregation and summarization
(as name suggest … reduce function – reducing the chunk of data)
Few google index stats … http://practicalquant.blogspot.com/2010/11/inside-googles-infrastructure-mapreduce.html