Github user yhuai commented on a diff in the pull request:
https://github.com/apache/spark/pull/6405#discussion_r31371891
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
---
@@ -264,6 +264,10 @@ object NullPropagation extends Rule[LogicalPlan] {
case e @ Substring(_, Literal(null, _), _) => Literal.create(null,
e.dataType)
case e @ Substring(_, _, Literal(null, _)) => Literal.create(null,
e.dataType)
+ // MaxOf and MinOf can't do null propagation
--- End diff --
Seems for `MaxOf` and `MinOf`, `null` does not mean `unknown`. It will be
good to add scala doc for them to explain the semantic.
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