Chained computations
Monday, March 01, 2010
3:47 PM
Suppose you want to search for documents containing three words. 
Map outputs key=URL, value=word
Reduce by ignoring all URLs that don't have all three.  
Now suppose you want a fourth word too
Start with URLs that contain all three
Map outputs key=URL, value=4th word where we only consider URLs that contain other three. 
Result: no repeated work. 