[GitHub] [spark] cloud-fan commented on a change in pull request #30368: [SPARK-33442][SQL] Change Combine Limit to Eliminate limit using max row

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

cloud-fan commented on a change in pull request #30368:
URL: https://github.com/apache/spark/pull/30368#discussion_r523931544



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File path: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
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@@ -1452,11 +1452,27 @@ object PushPredicateThroughJoin extends Rule[LogicalPlan] with PredicateHelper {
 }
 
 /**
- * Combines two adjacent [[Limit]] operators into one, merging the
- * expressions into one single expression.
+ * 1. Eliminate [[Limit]] operators if it's child max row <= limit.
+ * 2. Combines two adjacent [[Limit]] operators into one, merging the
+ *    expressions into one single expression.
  */
-object CombineLimits extends Rule[LogicalPlan] {
-  def apply(plan: LogicalPlan): LogicalPlan = plan transform {
+object EliminateLimits extends Rule[LogicalPlan] {
+  private def canEliminate(limitExpr: Expression, child: LogicalPlan): Boolean = {
+    // We skip such case that Sort is after Limit since
+    // SparkStrategies will convert them to TakeOrderedAndProjectExec
+    val skipEliminate = child match {
+      case Sort(_, true, _) => true

Review comment:
       does `TakeOrderedAndProjectExec` really help if we need to get and sort all the output?




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