[SPARK-3394] [SQL] Fix crash in TakeOrdered when limit is 0

This resolves https://issues.apache.org/jira/browse/SPARK-3394

Author: Eric Liang <ekl@google.com>

Closes #2264 from ericl/spark-3394 and squashes the following commits:

c87355b [Eric Liang] refactor
bfb6140 [Eric Liang] change RDD takeOrdered instead
7a51528 [Eric Liang] fix takeordered when limit = 0

core/src/main/scala/org/apache/spark/rdd/RDD.scala

@@ -1127,15 +1127,19 @@ abstract class RDD[T: ClassTag](
    * @return an array of top elements
    */
   def takeOrdered(num: Int)(implicit ord: Ordering[T]): Array[T] = {
-    mapPartitions { items =>
-      // Priority keeps the largest elements, so let's reverse the ordering.
-      val queue = new BoundedPriorityQueue[T](num)(ord.reverse)
-      queue ++= util.collection.Utils.takeOrdered(items, num)(ord)
-      Iterator.single(queue)
-    }.reduce { (queue1, queue2) =>
-      queue1 ++= queue2
-      queue1
-    }.toArray.sorted(ord)
+    if (num == 0) {
+      Array.empty
+    } else {
+      mapPartitions { items =>
+        // Priority keeps the largest elements, so let's reverse the ordering.
+        val queue = new BoundedPriorityQueue[T](num)(ord.reverse)
+        queue ++= util.collection.Utils.takeOrdered(items, num)(ord)
+        Iterator.single(queue)
+      }.reduce { (queue1, queue2) =>
+        queue1 ++= queue2
+        queue1
+      }.toArray.sorted(ord)
+    }
   }
 
   /**

core/src/test/scala/org/apache/spark/rdd/RDDSuite.scala

@@ -521,6 +521,13 @@ class RDDSuite extends FunSuite with SharedSparkContext {
     assert(sortedLowerK === Array(1, 2, 3, 4, 5))
   }
 
+  test("takeOrdered with limit 0") {
+    val nums = Array(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+    val rdd = sc.makeRDD(nums, 2)
+    val sortedLowerK = rdd.takeOrdered(0)
+    assert(sortedLowerK.size === 0)
+  }
+
   test("takeOrdered with custom ordering") {
     val nums = Array(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
     implicit val ord = implicitly[Ordering[Int]].reverse