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https://issues.apache.org/jira/browse/PIG-1605?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Daniel Dai updated PIG-1605:
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Description:
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.
By doing this, we can make sure we visit LOStore which generate scalar first,
and then LOForEach which use the scalar. 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.
4. With soft link, we can use scalar come from different sources in the same
statement, which in my mind is not a rare use case. (eg: D = foreach C generate
c0/A.total, c1/B.count;)
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.
was:
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.
By doing this, we can make sure we visit LOStore which generate scalar first,
and then LOForEach which use the scalar. 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.
> Adding soft link to plan to solve input file dependency
> -------------------------------------------------------
>
> 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. By doing this, we can make sure we visit LOStore which generate
> scalar first, and then LOForEach which use the scalar. 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.
> 4. With soft link, we can use scalar come from different sources in the same
> statement, which in my mind is not a rare use case. (eg: D = foreach C
> generate c0/A.total, c1/B.count;)
> 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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