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Diffstat (limited to 'docs/scala-programming-guide.md')
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diff --git a/docs/scala-programming-guide.md b/docs/scala-programming-guide.md index 3ed86e460c..edaa7d0639 100644 --- a/docs/scala-programming-guide.md +++ b/docs/scala-programming-guide.md @@ -196,6 +196,10 @@ The following tables list the transformations and actions currently supported (s <tr> <td> <b>groupByKey</b>([<i>numTasks</i>]) </td> <td> When called on a dataset of (K, V) pairs, returns a dataset of (K, Seq[V]) pairs. <br /> +<b>Note:</b> If you are grouping in order to perform an aggregation (such as a sum or + average) over each key, using `reduceByKey` or `combineByKey` will yield much better + performance. +<br /> <b>Note:</b> By default, if the RDD already has a partitioner, the task number is decided by the partition number of the partitioner, or else relies on the value of <code>spark.default.parallelism</code> if the property is set , otherwise depends on the partition number of the RDD. You can pass an optional <code>numTasks</code> argument to set a different number of tasks. </td> </tr> |