Hisoka-X commented on code in PR #40865:
URL: https://github.com/apache/spark/pull/40865#discussion_r1173210161
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sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/subquery.scala:
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@@ -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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