[SPARK-20416][SQL] Print UDF names in EXPLAIN

## What changes were proposed in this pull request?
This pr added `withName` in `UserDefinedFunction` for printing UDF names in EXPLAIN

## How was this patch tested?
Added tests in `UDFSuite`.

Author: Takeshi Yamamuro <yamamuro@apache.org>

Closes #17712 from maropu/SPARK-20416.

mllib/src/main/scala/org/apache/spark/ml/feature/Bucketizer.scala

@@ -114,7 +114,7 @@ final class Bucketizer @Since("1.4.0") (@Since("1.4.0") override val uid: String
 
     val bucketizer: UserDefinedFunction = udf { (feature: Double) =>
       Bucketizer.binarySearchForBuckets($(splits), feature, keepInvalid)
-    }
+    }.withName("bucketizer")
 
     val newCol = bucketizer(filteredDataset($(inputCol)).cast(DoubleType))
     val newField = prepOutputField(filteredDataset.schema)

sql/core/src/main/scala/org/apache/spark/sql/expressions/UserDefinedFunction.scala

@@ -47,6 +47,7 @@ case class UserDefinedFunction protected[sql] (
     dataType: DataType,
     inputTypes: Option[Seq[DataType]]) {
 
+  private var _nameOption: Option[String] = None
   private var _nullable: Boolean = true
 
   /**
@@ -67,15 +68,27 @@ case class UserDefinedFunction protected[sql] (
       dataType,
       exprs.map(_.expr),
       inputTypes.getOrElse(Nil),
+      udfName = _nameOption,
       nullable = _nullable))
   }
 
   private def copyAll(): UserDefinedFunction = {
     val udf = copy()
+    udf._nameOption = _nameOption
     udf._nullable = _nullable
     udf
   }
 
+  /**
+   * Updates UserDefinedFunction with a given name.
+   *
+   * @since 2.3.0
+   */
+  def withName(name: String): this.type = {
+    this._nameOption = Option(name)
+    this
+  }
+
   /**
    * Updates UserDefinedFunction with a given nullability.
    *

sql/core/src/test/scala/org/apache/spark/sql/UDFSuite.scala

@@ -263,10 +263,12 @@ class UDFSuite extends QueryTest with SharedSQLContext {
       val sparkPlan = spark.sessionState.executePlan(explain).executedPlan
       sparkPlan.executeCollect().map(_.getString(0).trim).headOption.getOrElse("")
     }
-    val udf1 = "myUdf1"
-    val udf2 = "myUdf2"
-    spark.udf.register(udf1, (n: Int) => { n + 1 })
-    spark.udf.register(udf2, (n: Int) => { n * 1 })
-    assert(explainStr(sql("SELECT myUdf1(myUdf2(1))")).contains(s"UDF:$udf1(UDF:$udf2(1))"))
+    val udf1Name = "myUdf1"
+    val udf2Name = "myUdf2"
+    val udf1 = spark.udf.register(udf1Name, (n: Int) => n + 1)
+    val udf2 = spark.udf.register(udf2Name, (n: Int) => n * 1)
+    assert(explainStr(sql("SELECT myUdf1(myUdf2(1))")).contains(s"UDF:$udf1Name(UDF:$udf2Name(1))"))
+    assert(explainStr(spark.range(1).select(udf1(udf2(functions.lit(1)))))
+      .contains(s"UDF:$udf1Name(UDF:$udf2Name(1))"))
   }
 }