Github user hvanhovell commented on a diff in the pull request:
https://github.com/apache/spark/pull/15101#discussion_r78913008
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
---
@@ -1098,9 +1098,16 @@ object ReplaceExceptWithAntiJoin extends
Rule[LogicalPlan] {
*/
object RemoveLiteralFromGroupExpressions extends Rule[LogicalPlan] {
def apply(plan: LogicalPlan): LogicalPlan = plan transform {
- case a @ Aggregate(grouping, _, _) =>
+ case a @ Aggregate(grouping, _, _) if grouping.nonEmpty =>
val newGrouping = grouping.filter(!_.foldable)
- a.copy(groupingExpressions = newGrouping)
+ if (newGrouping.nonEmpty) {
+ a.copy(groupingExpressions = newGrouping)
+ } else {
+ // Do not rewrite the aggregate if we drop all grouping
expressions, because this can
+ // change the return semantics when the input of the Aggregate is
empty. See SPARK-17114
+ // for more information.
+ a
--- End diff --
Then it might be even better to replace it with something that is trivial
to hash.
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