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https://issues.apache.org/jira/browse/SPARK-5114?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Joseph K. Bradley updated SPARK-5114:
-------------------------------------
    Description: 
Pipelines can currently contain Estimators and Transformers.

Question for debate: Should Pipelines be able to contain Evaluators?

Pros:
* Schema check: Evaluators take input datasets with particular schema, which 
should perhaps be checked before running a Pipeline.
* Intermediate results:
** If a Transformer removes a column (which is not done by built-in 
Transformers currently but might be reasonable in the future), then the user 
can never evaluate that column.  (However, users could keep all columns around.)
** If users have to evaluate after running a Pipeline, then each evaluated 
column may have to be re-materialized.

Cons:
* API: Evaluators do not transform datasets.   They produce a scalar (or a few 
values), which makes it hard to say how they fit into a Pipeline or a 
PipelineModel.


  was:
Pipelines can currently contain Estimators and Transformers.

Question for debate: Should Pipelines be able to contain Evaluators?

Pros:
* Schema check: Evaluators take input datasets with particular schema, which 
should perhaps be checked before running a Pipeline.
* Intermediate results:
** If a Transformer removes a column (which is not done by built-in 
Transformers currently but might be reasonable in the future), then the user 
can never evaluate that column.  (However, users could keep all columns around.)
** If users have to evaluate after running a Pipeline, then each evaluated 
column may have to be re-materialized.

Cons:
* Evaluators do not transform datasets.   They produce a scalar (or a few 
values), which makes it hard to say how they fit into a Pipeline or a 
PipelineModel.



> Should Evaluator be a PipelineStage
> -----------------------------------
>
>                 Key: SPARK-5114
>                 URL: https://issues.apache.org/jira/browse/SPARK-5114
>             Project: Spark
>          Issue Type: Question
>          Components: ML
>    Affects Versions: 1.2.0
>            Reporter: Joseph K. Bradley
>
> Pipelines can currently contain Estimators and Transformers.
> Question for debate: Should Pipelines be able to contain Evaluators?
> Pros:
> * Schema check: Evaluators take input datasets with particular schema, which 
> should perhaps be checked before running a Pipeline.
> * Intermediate results:
> ** If a Transformer removes a column (which is not done by built-in 
> Transformers currently but might be reasonable in the future), then the user 
> can never evaluate that column.  (However, users could keep all columns 
> around.)
> ** If users have to evaluate after running a Pipeline, then each evaluated 
> column may have to be re-materialized.
> Cons:
> * API: Evaluators do not transform datasets.   They produce a scalar (or a 
> few values), which makes it hard to say how they fit into a Pipeline or a 
> PipelineModel.



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