[GitHub] [spark] jchen5 commented on a diff in pull request #40865: [SPARK-43156][SQL] Fix `COUNT(*) is null` bug in correlated scalar subquery

["jchen5 (via GitHub)" <gi...@apache.org>]

jchen5 commented on code in PR #40865:
URL: https://github.com/apache/spark/pull/40865#discussion_r1173128023


##########
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/subquery.scala:
##########
@@ -599,10 +600,32 @@ object RewriteCorrelatedScalarSubquery extends Rule[LogicalPlan] with AliasHelpe
         if (Utils.isTesting) {
           assert(mayHaveCountBug.isDefined)
         }
+
+        def queryOutputFoldable(list: Seq[NamedExpression]): Boolean = {
+          trimAliases(list.filter(p => p.exprId.equals(query.output.head.exprId)).head).foldable
+        }
+
+        lazy val resultFoldable = {
+          query match {
+            case Project(expressions, _) =>
+              queryOutputFoldable(expressions)
+            case Aggregate(_, expressions, _) =>
+              queryOutputFoldable(expressions)
+            case _ =>
+              false
+          }
+        }
+
         if (resultWithZeroTups.isEmpty) {
           // CASE 1: Subquery guaranteed not to have the COUNT bug because it evaluates to NULL
           // with zero tuples.
           planWithoutCountBug
+        } else if (resultFoldable) {

Review Comment:
   I just changed this code in https://github.com/apache/spark/pull/40811 so you'll need to merge with that - your check should go after my new one, because if the subquery is something like `select false from ... group by c` then it will still actually return null on empty inputs. I have a test case for it added in my PR.



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