peter-toth commented on code in PR #55927:
URL: https://github.com/apache/spark/pull/55927#discussion_r3258010248


##########
sql/core/src/main/scala/org/apache/spark/sql/execution/joins/ShuffledJoin.scala:
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@@ -28,6 +28,21 @@ import 
org.apache.spark.sql.catalyst.plans.physical.{ClusteredDistribution, Dist
 trait ShuffledJoin extends JoinCodegenSupport {
   def isSkewJoin: Boolean
 
+  private def containsNullSafeJoinMarker(keys: Seq[Expression]): Boolean = {
+    keys.exists(_.exists(_.isInstanceOf[IsNull]))
+  }
+
+  private lazy val canSpreadNullJoinKeys: Boolean = {

Review Comment:
   ~Is this robust enough? What if someone crafts a null handling join 
condition by hand?~
   
   ~Actually, this looks good.~
   
   Actually, why this is needed at all and when can't we spread nulls?
   `<=>` is translated to 2 key pairs `Coalesce(a.k, default), Coalesce(b.k, 
default))`  and `(IsNull(a.k), IsNull(b.k))`, so null never shows up in shuffle 
keys. The join type check seems fair enough.



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