[GitHub] [spark] cloud-fan commented on a diff in pull request #40811: [SPARK-43098][SQL] Fix correctness COUNT bug when scalar subquery has group by clause

["cloud-fan (via GitHub)" <gi...@apache.org>]

cloud-fan commented on code in PR #40811:
URL: https://github.com/apache/spark/pull/40811#discussion_r1169413430


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sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/subquery.scala:
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@@ -325,9 +327,20 @@ object PullupCorrelatedPredicates extends Rule[LogicalPlan] with PredicateHelper
     }
 
     plan.transformExpressionsWithPruning(_.containsPattern(PLAN_EXPRESSION)) {
-      case ScalarSubquery(sub, children, exprId, conditions, hint) if children.nonEmpty =>
+      case ScalarSubquery(sub, children, exprId, conditions, hint, mayHaveCountBugOld)
+        if children.nonEmpty =>
         val (newPlan, newCond) = decorrelate(sub, plan)
-        ScalarSubquery(newPlan, children, exprId, getJoinCondition(newCond, conditions), hint)
+        val mayHaveCountBug = if (mayHaveCountBugOld.isDefined) {
+          // For idempotency, we must save this variable the first time this rule is run. Afterward,
+          // the information about whether the subquery has a GROUP BY clause is lost because a
+          // GROUP BY is introduced if one wasn't already present.

Review Comment:
   where do we introduce GROUP BY? There are two code paths here:
   ```
         if (SQLConf.get.decorrelateInnerQueryEnabled) {
           DecorrelateInnerQuery(sub, outer, handleCountBug)
         } else {
           pullOutCorrelatedPredicates(sub, outer)
         }
   ```



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