Github user gatorsmile commented on a diff in the pull request:
https://github.com/apache/spark/pull/11854#discussion_r56773705
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
---
@@ -1194,6 +1195,41 @@ object RemoveDispensableExpressions extends
Rule[LogicalPlan] {
}
/**
+ * Removes useless Distinct that are not necessary.
+ */
+object EliminateDistinct extends Rule[LogicalPlan] {
+ def apply(plan: LogicalPlan): LogicalPlan = plan transform {
+ // Eliminate the useless distinct.
+ // Distinct has been replaced by Aggregate in the rule
ReplaceDistinctWithAggregate
+ case a @ Aggregate(grouping, aggs, child) if isDistinct(a) &&
isDistinct(child) => child
+ }
+
+ // propagate the distinct property from the child
+ @tailrec
--- End diff --
Another solution is to add a property `isDistinct` to `LogicalPlan`.
However, it could be expensive for recursive calls, compared with the
`@tailrec`. In the future, if the physical plan will use the property
`isDistinct`, we can rewrite it. Actually, this is a very critical property in
runtime. Thanks!
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