Github user yanboliang commented on a diff in the pull request:

    https://github.com/apache/spark/pull/13129#discussion_r63474760
  
    --- Diff: 
mllib/src/main/scala/org/apache/spark/ml/regression/GeneralizedLinearRegression.scala
 ---
    @@ -239,10 +239,7 @@ class GeneralizedLinearRegression @Since("2.0.0") 
(@Since("2.0.0") override val
         }
         val familyAndLink = new FamilyAndLink(familyObj, linkObj)
     
    -    val numFeatures = dataset.select(col($(featuresCol))).limit(1).rdd
    -      .map { case Row(features: Vector) =>
    -        features.size
    -      }.first()
    +    val numFeatures = 
dataset.select(col($(featuresCol))).first().getAs[Vector](0).size
    --- End diff --
    
    It looks like Spark does not provide encoder for Vector. If I change to use 
```as[Vector]```, the compiler will complain:
    ```
    Error:(244, 61) Unable to find encoder for type stored in a Dataset.  
Primitive types (Int, String, etc) and Product types (case classes) are 
supported by importing spark.implicits._  Support for serializing other types 
will be added in future releases.
        val numFeatures = 
dataset.select(col($(featuresCol))).as[Vector].first().size
                                                                ^
    ``` 


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