Github user gatorsmile commented on a diff in the pull request:
https://github.com/apache/spark/pull/17929#discussion_r115647510
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/PropagateEmptyRelation.scala
---
@@ -68,8 +63,13 @@ object PropagateEmptyRelation extends Rule[LogicalPlan]
with PredicateHelper {
case _: LocalLimit => empty(p)
case _: Repartition => empty(p)
case _: RepartitionByExpression => empty(p)
- // AggregateExpressions like COUNT(*) return their results like 0.
- case Aggregate(_, ae, _) if !ae.exists(containsAggregateExpression)
=> empty(p)
+ // An aggregate with non-empty group expression will return one
output row per group.
--- End diff --
Nit: `An aggregate with non-empty group expression will return one output
row per group.`
->
`An aggregate with non-empty group expression will return one output row
per group when the input to the aggregate is not empty.`
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