Github user MLnick commented on the pull request:

    https://github.com/apache/spark/pull/11601#issuecomment-207279502
  
    @hhbyyh @jkbradley I'm thinking that an `Imputer` is a useful feature 
transformer to have. However, perhaps we limit this first implementation to 
either (a) single numerical column, or (b) multiple numerical columns. i.e. we 
don't support vector column at this time.
    
    Supporting vectors is a bit complex as evidenced by the code here, and will 
require changes to `colStats` as well as some deeper changes to 
`approxQuantile` if we wish to support array/vectors - and even then, are we 
able to support sparse vectors, etc? (and I do think we want to use 
`approxQuantile` since computing exactly is expensive). Overall it adds a lot 
of complexity, and I'm not convinced it adds that much usefulness.
    
    What do you think?


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