We are working on the Pig Logical Optimizer, and running into some
difficulty navigating the plan.

If we run explain on a query with a CoGroup, we get something like:

|    |
|    |-- Project [0]
|------ ForEach
            |  <etc>

What we want to do is determine that this particular Cogroup operates on a
projection of field 0.

If we create a new LogicalTransformer that is applied to Cogroup operators,
and call

mPlan.getPredecessors(ourCogroupOperator) , we only get the ForEach.
Calling getSuccessors results in a null being returned (Cogroup is indeed
the root).

How do we find the Project operator above? What is its relationship,
plan-wise, with the Cogroup operator?

Thanks a lot,

Dmitriy Ryaboy, Ashutosh Chauhan, Tejal Desai

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