Github user andrewor14 commented on a diff in the pull request:
https://github.com/apache/spark/pull/2839#discussion_r19051180
--- Diff: core/src/main/scala/org/apache/spark/rdd/PairRDDFunctions.scala
---
@@ -238,8 +238,15 @@ class PairRDDFunctions[K, V](self: RDD[(K, V)])
@deprecated("Use reduceByKeyLocally", "1.0.0")
def reduceByKeyToDriver(func: (V, V) => V): Map[K, V] =
reduceByKeyLocally(func)
- /** Count the number of elements for each key, and return the result to
the master as a Map. */
- def countByKey(): Map[K, Long] = self.map(_._1).countByValue()
+ /**
+ * Count the number of elements for each key, collecting the results to
a local Map.
+ *
+ * Note that this method should only be used if the resulting map is
expected to be small, as
+ * the whole thing is loaded into the driver's memory.
+ * To handle very large results, consider using rdd.mapValues(_ =>
1).reduceByKey(_ + _), which
+ * returns an RDD[T, Long] instead of a map.
+ */
+ def countByKey(): Map[K, Long] = self.mapValues(_ => 1L).reduceByKey(_ +
_).collect.toMap
--- End diff --
think you need `collect()`
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