Revert "Reuse CompactBuffer in UniqueKeyHashedRelation."

This reverts commit 97626a1.

Author: rxin

core/src/main/scala/org/apache/spark/util/collection/CompactBuffer.scala

@@ -49,7 +49,7 @@ private[spark] class CompactBuffer[T] extends Seq[T] with Serializable {
     }
   }
 
-  def update(position: Int, value: T): Unit = {
+  private def update(position: Int, value: T): Unit = {
     if (position < 0 || position >= curSize) {
       throw new IndexOutOfBoundsException
     }

sql/core/src/main/scala/org/apache/spark/sql/execution/joins/HashedRelation.scala

@@ -28,12 +28,6 @@ import org.apache.spark.util.collection.CompactBuffer
  * object.
  */
 private[joins] sealed trait HashedRelation {
-  /**
-   * Get the rows matching the key back as a [[CompactBuffer]]. If no rows match this key,
-   * null is returned.
-   *
-   * The concrete implementation may reuse the returned [[CompactBuffer]].
-   */
   def get(key: Row): CompactBuffer[Row]
 }
 
@@ -55,16 +49,9 @@ private[joins] final class GeneralHashedRelation(hashTable: JavaHashMap[Row, Com
 private[joins] final class UniqueKeyHashedRelation(hashTable: JavaHashMap[Row, Row])
   extends HashedRelation with Serializable {
 
-  private[this] val compactBuf = CompactBuffer[Row](null)
-
   override def get(key: Row) = {
     val v = hashTable.get(key)
-    if (v eq null) {
-      null
-    } else {
-      compactBuf(0) = v
-      compactBuf
-    }
+    if (v eq null) null else CompactBuffer(v)
   }
 
   def getValue(key: Row): Row = hashTable.get(key)