Optimizer should allow search for global patterns

                 Key: PIG-873
                 URL: https://issues.apache.org/jira/browse/PIG-873
             Project: Pig
          Issue Type: Improvement
    Affects Versions: 0.3.1
            Reporter: Santhosh Srinivasan
             Fix For: 0.4.0

Currently, the optimizer works on the following mechanism:

1. Specify the pattern to be searched
2. For each occurrence of the pattern, check and then apply a transformation

With this approach, the search for a pattern is localized. An example will 
illustrate the problem.

If the pattern to be searched for is foreach (with flatten) connected to any 
operator and if the graph has more than one foreach (with flatten) connected to 
an operator (cross, join, union, etc), then each instance of foreach connected 
to the operator is returned as a match. While this is fine for a localized view 
(per match), at a global view the pattern to be searched for is any number of 
foreach connected to an operator.

The implication of not having a globalized view is more rules. There will be 
one rule for one foreach connected to an opeator, one rule for two foreachs 
connected to an operators, etc.

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