Default parallel for Pig

                 Key: PIG-895
             Project: Pig
          Issue Type: New Feature
          Components: impl
    Affects Versions: 0.3.0
            Reporter: Daniel Dai
             Fix For: 0.4.0

For hadoop 20, if user don't specify the number of reducers, hadoop will use 1 
reducer as the default value. It is different from previous of hadoop, in which 
default reducer number is usually good. 1 reducer is not what user want for 
sure. Although user can use "parallel" keyword to specify number of reducers 
for each statement, it is wordy. We need a convenient way for users to express 
a desired number of reducers. Here is my propose:

1. Add one property "default_parallel" to Pig. User can set default_parallel in 
script. Eg:
   set default_parallel '10';

2. default_parallel is a hint to Pig. Pig is free to optimize the number of 
reducers (unlike parallel keyword). Currently, since we do not have a mechanism 
to determine the optimal number of reducers, default_parallel will be always 
granted, unless it is override by "parallel" keyword.

3. If user put multiple default_parallel inside script, the last entry will be 

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