Merge join should restrict join key expressions to simple projects

                 Key: PIG-981
             Project: Pig
          Issue Type: Bug
    Affects Versions: 0.4.0
            Reporter: Pradeep Kamath
            Assignee: Pradeep Kamath

Currently merge join allows join key expressions to be arbitrary expressions 
with the assumption that the expressions keep the sort order. Since currently 
only ascending sort order is supported, the code checks at run times for sort 
order and catches the case where sort order is broken because the join key 
expression is not order preserving. However there is a reason we should 
restrict the join keys to projection of columns only:
 PIG-953 will enable pig to perform merge join  to work with loaders and store 
functions which can internally index sorted data. These store functions can 
only create an index (and hence lookup on the index) on raw data columns (and 
not expressions on the columns).
Hopefully this does not downgrade the usability of merge join much since if the 
expressions can always be applied post join on the join columns and since the 
expressions are order preserving they do not affect the outcome of the join. 

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