Adding soft link to plan to solve input file dependency
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Key: PIG-1605
URL: https://issues.apache.org/jira/browse/PIG-1605
Project: Pig
Issue Type: Bug
Components: impl
Affects Versions: 0.8.0
Reporter: Daniel Dai
Assignee: Daniel Dai
Fix For: 0.8.0
In scalar implementation, we need to deal with implicit dependencies.
[PIG-1603|https://issues.apache.org/jira/browse/PIG-1603] is trying to solve
the problem by adding a LOScalar operator. Here is a different approach. We
will add a soft link to the plan, and soft link is only visible to the walkers.
All other part of the logical plan does not know the existence of the soft
link. The benefits are:
1. Logical plan do not need to deal with LOScalar, this makes logical plan
cleaner
2. Conceptually scalar dependency is different. Regular link represent a data
flow in pipeline. In scalar, the dependency means an operator depends on a file
generated by the other operator. It's different type of data dependency.
3. Soft link can solve other dependency problem in the future. If we introduce
another UDF dependent on a file generated by another operator, we can use this
mechanism to solve it.
Currently, there are two cases we can use soft link:
1. scalar dependency, where ReadScalar UDF will use a file generate by a LOStore
2. store-load dependency, where we will load a file which is generated by a
store in the same script. This happens in multi-store case. Currently we solve
it by regular link. It is better to use a soft link.
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