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The following page has been changed by RichardDing:
http://wiki.apache.org/pig/PigMultiQueryPerformanceSpecification

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     * The parallelism of the merged splitter job is the maximum of the 
parallelisms of all splittee jobs.
     * The keys from inner plans are partitioned into all the buckets via the 
default hash partitioner.
  
+ This scheme has advantages: 
+ 
+    * Simplicity. No new partition class needed.
+    * Performance. The parallelism of a job specified by users most likely is 
determined by the number of available reducers (machines), so the merged 
parallelism confirms to the user expectation.   
+ 
  To avoid the key collision of different inner plans with this scheme, the 
PigNullableWritable class is modified to take into account of the indexes when 
two keys are compared (hashed). 
    
  [[Anchor(Local_Execution_engine)]]

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