minor improvements

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

@@ -229,9 +229,8 @@ class PairRDDFunctions[K: ClassTag, V: ClassTag](self: RDD[(K, V)])
     }
     val mergeHLL = (h1: SerializableHyperLogLog, h2: SerializableHyperLogLog) => h1.merge(h2)
 
-    combineByKey(createHLL, mergeValueHLL, mergeHLL, partitioner).map {
-      case (k, v) => (k, v.value.cardinality())
-    }
+    combineByKey(createHLL, mergeValueHLL, mergeHLL, partitioner).mapValues(_.value.cardinality())
+
   }
 
   /**

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

@@ -809,7 +809,9 @@ abstract class RDD[T: ClassTag](
     }
     def mergeCounters(c1: SerializableHyperLogLog, c2: SerializableHyperLogLog) = c1.merge(c2)
 
-    mapPartitions(hllCountPartition).reduce(mergeCounters).value.cardinality()
+    val zeroCounter = new SerializableHyperLogLog(new HyperLogLog(relativeSD))
+    mapPartitions(hllCountPartition).aggregate(zeroCounter)(mergeCounters, mergeCounters)
+      .value.cardinality()
   }
 
   /**