[GitHub] [spark] cloud-fan commented on a change in pull request #35710: [SPARK-38391][SQL] Datasource v2 supports complete partial sort pushdown

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cloud-fan commented on a change in pull request #35710:
URL: https://github.com/apache/spark/pull/35710#discussion_r823874415



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File path: sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/v2/V2ScanRelationPushDown.scala
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@@ -362,7 +362,11 @@ object V2ScanRelationPushDown extends Rule[LogicalPlan] with PredicateHelper {
         if (topNPushed) {
           sHolder.pushedLimit = Some(limit)
           sHolder.sortOrders = orders
-          operation
+          sHolder.builder match {
+            case s: SupportsPushDownTopN if s.supportCompleteSortPushDown() =>
+              operation
+            case _ => s

Review comment:
       This makes the top N pushdown look like a hint: we notify the source that a top N is needed, but the Spark query plan is not changed.
   
   This is definitely not optimal: Spark still does a sort, and probably a shuffle if the limit is large. Performance-wise, I think it's the same as pushing down a limit. How about a simpler approach: if we can't push down top N, we just push down limit without changing the plan. Then we don't need to change DS v2 APIs.




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