[GitHub] [spark] dongjoon-hyun commented on a change in pull request #30018: [SPARK-33122][SQL] Remove redundant aggregates in the Optimzier

[GitBox <gi...@apache.org>]

dongjoon-hyun commented on a change in pull request #30018:
URL: https://github.com/apache/spark/pull/30018#discussion_r598160288



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File path: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
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@@ -488,6 +489,50 @@ 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 = {

Review comment:
       nit. Usually, `isXXX` is better and consistent with Apache Spark convention.




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