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

[via GitHub]

Hisoka-X commented on code in PR #40865:
URL: https://github.com/apache/spark/pull/40865#discussion_r1173210161


##########
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:
   Already merged before create this PR. 
   
   > if the subquery is something like `select false from ... group by c` then 
it will still actually return null on empty inputs.
   
   That's why we should judge whether the column returned by subquery is 
foldable. If the result depends on input, the `resultFoldable` will be `false`. 
 It will skip the logic I created.
   
   > I think your fix will fix the COUNT(*) is null case, but break the false 
case, e.g. in a query like select *, (select any_value(false) as result from t1 
where t0.a = t1.c) from t0) Can you confirm this?
   
   Is this answer right?
   <img width="952" alt="image" 
src="https://user-images.githubusercontent.com/32387433/233515508-8a7055f6-7366-413c-b137-2bdcbbabb0a1.png";>
   



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