[GitHub] [spark] huaxingao commented on a diff in pull request #36043: [SPARK-38768][SQL] Remove `Limit` from plan if complete push down limit to data source.

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

huaxingao commented on code in PR #36043:
URL: https://github.com/apache/spark/pull/36043#discussion_r841285841


##########
sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/v2/V2ScanRelationPushDown.scala:
##########
@@ -380,27 +380,32 @@ object V2ScanRelationPushDown extends Rule[LogicalPlan] with PredicateHelper wit
           sHolder.pushedLimit = Some(limit)
           sHolder.sortOrders = orders
           if (isPartiallyPushed) {
-            s
+            (s, isPartiallyPushed)
           } else {
-            operation
+            (operation, isPartiallyPushed)
           }
         } else {
-          s
+          (s, true)
         }
       } else {
-        s
+        (s, true)
       }
     case p: Project =>
-      val newChild = pushDownLimit(p.child, limit)
-      p.withNewChildren(Seq(newChild))
-    case other => other
+      val (newChild, isPartiallyPushed) = pushDownLimit(p.child, limit)
+      (p.withNewChildren(Seq(newChild)), isPartiallyPushed)
+    case other => (other, true)
   }
 
   def pushDownLimits(plan: LogicalPlan): LogicalPlan = plan.transform {
     case globalLimit @ Limit(IntegerLiteral(limitValue), child) =>
-      val newChild = pushDownLimit(child, limitValue)
-      val newLocalLimit = globalLimit.child.asInstanceOf[LocalLimit].withNewChildren(Seq(newChild))
-      globalLimit.withNewChildren(Seq(newLocalLimit))
+      val (newChild, isPartiallyPushed) = pushDownLimit(child, limitValue)
+      if (isPartiallyPushed) {
+        val newLocalLimit =
+          globalLimit.child.asInstanceOf[LocalLimit].withNewChildren(Seq(newChild))
+        globalLimit.withNewChildren(Seq(newLocalLimit))
+      } else {
+        newChild

Review Comment:
   I think there is a problem here. If `isPartiallyPushed` is false, it is assumed that `Limit` is completely pushed down so Spark doesn't do `Limit` any more. However, the `isPartiallyPushed`  false could come from the default case in `PushDownUtils`.`pushLimit`
   
   ```
     def pushLimit(scanBuilder: ScanBuilder, limit: Int): (Boolean, Boolean) = {
       scanBuilder match {
         case s: SupportsPushDownLimit if s.pushLimit(limit) =>
           (true, s.isPartiallyPushed)
         case _ => (false, false)
       }
     }
   ```
   In this case, the `Limit` at Spark is removed wrongly.
   



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