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  C = JOIN A by url, B by url PARALLEL 50. 
- Even if you do not specify the parallel clause, the framework uses a default 
number of reducers, in the order of 0.9*(number of nodes allocated by user 
-1)*n where n is the number of maximum reduce slots, for running your M/R jobs 
which result from statements such as GROUP, COGROUP, JOIN, and ORDER BY. For 
example, when allocating 3 machines you get about 0.9*2*4 = 7 reducers for 
operating on your parallel jobs. 
+ Besides PARALLEL clause, you can also use "set default_parallel" statement in 
Pig script, or set "mapred.reduce.tasks" system property to specify default 
parallel to use. If none of these values are set, Pig will only use 1 reducers. 
(In Pig 0.8, we change the default reducer from 1 to a number calculated by a 
simple heuristic for foolproof purpose)
  '''Q: Can I do a numerical comparison while filtering?'''

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