[GitHub] [spark] maropu commented on a change in pull request #31691: [SPARK-34575][SQL] Push down limit through window when partitionSpec is empty

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maropu commented on a change in pull request #31691:
URL: https://github.com/apache/spark/pull/31691#discussion_r585204557



##########
File path: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
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@@ -613,6 +613,15 @@ object LimitPushDown extends Rule[LogicalPlan] {
         case _ => join
       }
       LocalLimit(exp, newJoin)
+
+    case LocalLimit(limitExpr @ IntegerLiteral(limitVal),
+        window @ Window(Seq(Alias(WindowExpression(_: RankLike | _: RowNumber,
+            WindowSpecDefinition(Nil, orderSpec,
+                SpecifiedWindowFrame(RowFrame, UnboundedPreceding, CurrentRow))), _)), _, _, child))
+        if child.maxRows.forall( _ > limitVal) =>
+      LocalLimit(
+        limitExpr = limitExpr,
+        child = window.copy(child = Limit(limitExpr, Sort(orderSpec, true, child))))

Review comment:
       If no partitionSpec specified, I think the planner inserts an exchange for making a single partition. So, we don't need a global sort here?




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