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https://issues.apache.org/jira/browse/SPARK-8515?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16206111#comment-16206111
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Timothy Hunter commented on SPARK-8515:
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Before we commit to an implementation, we should think about the goal of adding 
metadata in ML, because it comes with its own costs. For instance, there has 
been a number of bug reports around them. See for example SPARK-2008, 
SPARK-14862.

I see a couple of use cases for metadata:
 - feature indexing -> that case should require just longs (or strings) for 
each dimension of a feature vector
 - expressing categorical info -> the Estimator -> Model -> Transformer pattern 
is more appropriate, I believe
 - vector dimensions -> I think that in all cases, the underlying code should 
be able to proceed without this information, although this is debatable

> Improve ML attribute API
> ------------------------
>
>                 Key: SPARK-8515
>                 URL: https://issues.apache.org/jira/browse/SPARK-8515
>             Project: Spark
>          Issue Type: Improvement
>          Components: ML
>    Affects Versions: 1.4.0
>            Reporter: Xiangrui Meng
>              Labels: advanced
>         Attachments: SPARK-8515.pdf
>
>
> In 1.4.0, we introduced ML attribute API to embed feature/label attribute 
> info inside DataFrame's schema. However, the API is not very friendly to use. 
> We should re-visit this API and see how we can improve it.



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