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The following page has been changed by AshutoshChauhan:
http://wiki.apache.org/pig/PigMergeJoin

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  getNext() in POMergeJoin should be updated so that it doesn't make this 
assumption.
  
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+ ''' Performance ''' : Currently POMergeJoin buffers all the tuples belonging 
to same key on left side before looking at right key. If keys don't match it 
throws away that buffer and moves on. Better way is to first look at both keys 
and then determine if there is any need of doing buffering. This will not save 
on CPU time because amount of work is nearly equal in either case, though it 
may affect memory footprint because in case of skewed key which don't match we 
will unnecessarily be consuming memory.   
  

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