Github user mgaido91 commented on a diff in the pull request:
https://github.com/apache/spark/pull/21291#discussion_r187565401
--- Diff: sql/core/src/main/scala/org/apache/spark/sql/Dataset.scala ---
@@ -2767,7 +2767,12 @@ class Dataset[T] private[sql](
* @since 1.6.0
*/
def count(): Long = withAction("count",
groupBy().count().queryExecution) { plan =>
- plan.executeCollect().head.getLong(0)
+ val collected = plan.executeCollect()
+ if (collected.isEmpty) {
+ 0
+ } else {
+ collected.head.getLong(0)
+ }
--- End diff --
I think it is caused by returning `SinglePartition` when there is no data
(and therefore no partition). So I think we should fix it there and not here.
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