maropu commented on a change in pull request #30018:
URL: https://github.com/apache/spark/pull/30018#discussion_r546475732
##########
File path:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
##########
@@ -485,6 +486,46 @@ object RemoveRedundantAliases extends Rule[LogicalPlan] {
def apply(plan: LogicalPlan): LogicalPlan = removeRedundantAliases(plan,
AttributeSet.empty)
}
+/**
+ * Remove redundant aggregates from a query plan. A redundant aggregate is an
aggregate whose
+ * only goal is to keep distinct values, while its parent aggregate would
ignore duplicate values.
+ */
+object RemoveRedundantAggregates extends Rule[LogicalPlan] with AliasHelper {
+ def apply(plan: LogicalPlan): LogicalPlan = plan transformUp {
+ case upper @ Aggregate(_, _, lower: Aggregate) if lowerIsRedundant(upper,
lower) =>
+ val aliasMap = getAliasMap(lower)
+
+ val newAggregate = upper.copy(
+ child = lower.child,
+ groupingExpressions = upper.groupingExpressions.map(replaceAlias(_,
aliasMap)),
+ aggregateExpressions = upper.aggregateExpressions.map(
+ replaceAliasButKeepName(_, aliasMap))
+ )
+
+ // We might have introduces non-deterministic grouping expression
+ if (newAggregate.groupingExpressions.exists(!_.deterministic)) {
+ PullOutNondeterministic.applyLocally.applyOrElse(newAggregate,
identity[LogicalPlan])
+ } else {
+ newAggregate
+ }
+ }
+
+ private def lowerIsRedundant(upper: Aggregate, lower: Aggregate): Boolean = {
+ val upperReferencesOnlyGrouping = upper.references.subsetOf(AttributeSet(
+ lower.aggregateExpressions.filter(!isAggregate(_)).map(_.toAttribute)))
+
+ val upperHasNoAggregateExpressions = upper.aggregateExpressions
+ .forall(_.find(isAggregate).isEmpty)
+
+ upperReferencesOnlyGrouping && upperHasNoAggregateExpressions
Review comment:
nit: to avoid unnecessary object creation (e.g,. `AttributeSet`), how
about checking `upperHasNoAggregateExpressions` first like this?
```
private def lowerIsRedundant(upper: Aggregate, lower: Aggregate): Boolean
= {
val upperHasNoAggregateExpressions = upper.aggregateExpressions
.forall(_.find(isAggregate).isEmpty)
lazy val upperRefsHaveGroupingOnly =
upper.references.subsetOf(AttributeSet(
lower.aggregateExpressions.filter(!isAggregate(_)).map(_.toAttribute)))
upperHasNoAggregateExpressions && upperRefsHaveGroupingOnly
}
```
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