Github user ueshin commented on a diff in the pull request:
https://github.com/apache/spark/pull/1428#discussion_r14981973
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
---
@@ -171,6 +171,9 @@ object NullPropagation extends Rule[LogicalPlan] {
case Literal(candidate, _) if candidate == v => true
case _ => false
})) => Literal(true, BooleanType)
+ case e @ Substring(Literal(null, _), _, _) => Literal(null,
e.dataType)
--- End diff --
@rxin, thank you for your comment.
The rule `ConstantFolding` can be applied only if the expression is
foldable, i.e. ALL of the child expressions are foldable, and the
`NullPropagation` covers the case there exists at least one `null` literal
regardless of foldabilities of the others.
So I think the cases are needed.
BTW, `Substring` can be foldable, right?
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