[GitHub] HyukjinKwon opened a new pull request #23349: [SPARK-26403][SQL] Support pivoting using array column for `pivot(column)` API

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HyukjinKwon opened a new pull request #23349: [SPARK-26403][SQL] Support 
pivoting using array column for `pivot(column)` API
URL: https://github.com/apache/spark/pull/23349
 
 
   ## What changes were proposed in this pull request?
   
   This PR fixes `Literal(..: Any)` can accepts 
`collection.mutable.WrappedArray` in order to `pivot(Column)` can accepts array 
column as well.
   
   We can unwrap the array and use it for type dispatch.
   
   ```scala
   val df = Seq(
     (2, Seq.empty[String]),
     (2, Seq("a", "x")),
     (3, Seq.empty[String]),
     (3, Seq("a", "x"))).toDF("x", "s")
   df.groupBy("x").pivot("s").count().show()
   ```
   
   Before:
   
   ```
   Unsupported literal type class scala.collection.mutable.WrappedArray$ofRef 
WrappedArray()
   java.lang.RuntimeException: Unsupported literal type class 
scala.collection.mutable.WrappedArray$ofRef WrappedArray()
        at 
org.apache.spark.sql.catalyst.expressions.Literal$.apply(literals.scala:80)
        at 
org.apache.spark.sql.RelationalGroupedDataset.$anonfun$pivot$2(RelationalGroupedDataset.scala:427)
        at 
scala.collection.TraversableLike.$anonfun$map$1(TraversableLike.scala:237)
        at 
scala.collection.IndexedSeqOptimized.foreach(IndexedSeqOptimized.scala:36)
        at 
scala.collection.IndexedSeqOptimized.foreach$(IndexedSeqOptimized.scala:33)
        at scala.collection.mutable.WrappedArray.foreach(WrappedArray.scala:39)
        at scala.collection.TraversableLike.map(TraversableLike.scala:237)
        at scala.collection.TraversableLike.map$(TraversableLike.scala:230)
        at scala.collection.AbstractTraversable.map(Traversable.scala:108)
        at 
org.apache.spark.sql.RelationalGroupedDataset.pivot(RelationalGroupedDataset.scala:425)
        at 
org.apache.spark.sql.RelationalGroupedDataset.pivot(RelationalGroupedDataset.scala:406)
        at 
org.apache.spark.sql.RelationalGroupedDataset.pivot(RelationalGroupedDataset.scala:317)
        at 
org.apache.spark.sql.DataFramePivotSuite.$anonfun$new$1(DataFramePivotSuite.scala:341)
        at 
scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:23)
   ```
   
   After:
   
   ```
   +---+---+------+
   |  x| []|[a, x]|
   +---+---+------+
   |  3|  1|     1|
   |  2|  1|     1|
   +---+---+------+
   ```
   
   ## How was this patch tested?
   
   Manually tested and unittests were added.
   

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