[GitHub] [spark] huaxingao commented on a change in pull request #34291: [SPARK-XXX][SQL][WIP] DS V2 LIMIT push down

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huaxingao commented on a change in pull request #34291:
URL: https://github.com/apache/spark/pull/34291#discussion_r729982797



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File path: sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/v2/V2ScanRelationPushDown.scala
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@@ -225,6 +226,31 @@ object V2ScanRelationPushDown extends Rule[LogicalPlan] with PredicateHelper {
       withProjection
   }
 
+  def applyLimit(plan: LogicalPlan): LogicalPlan = plan.transform {
+    case globalLimit @ GlobalLimit(_, LocalLimit(limitExpr, child)) => child match {

Review comment:
       Technically, it's no problem to only match `LocalLimit`. 
   I looked the code, seems to me that the only case that we have `LocalLimit` without `GlobalLimit` is when we push down `LocalLimit` beneath `UNION ALL` and joins in this rule  https://github.com/apache/spark/blob/master/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala#L645. I think we won't hit this case because `UNION ALL` and joins are not pushed down for data source.




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