How to select a subset of fields from an array column in Spark?

Question

Let say I have a DataFrame as follow :

case class SubClass(id:String, size:Int,useless:String)
case class MotherClass(subClasss: Array[SubClass])
val df = s         


        

Answer 1

Spark >= 2.4:

It is possible to use arrays_zip with cast:

import org.apache.spark.sql.functions.arrays_zip

df.select(arrays_zip(
  $"subClasss.id", $"subClasss.size"
).cast("array<struct<id:string,size:int>>"))

where cast is required to rename nested fields - without it Spark uses automatically generated names 0, 1, ... n.

Spark < 2.4:

You can use an UDF like this:

import org.apache.spark.sql.Row

case class Record(id: String, size: Int)

val dropUseless = udf((xs: Seq[Row]) =>  xs.map{
  case Row(id: String, size: Int, _) => Record(id, size)
})

df.select(dropUseless($"subClasss"))