Github user dbtsai commented on a diff in the pull request: https://github.com/apache/spark/pull/21850#discussion_r204953202 --- Diff: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/expressions.scala --- @@ -414,6 +414,16 @@ object SimplifyConditionals extends Rule[LogicalPlan] with PredicateHelper { // these branches can be pruned away val (h, t) = branches.span(_._1 != TrueLiteral) CaseWhen( h :+ t.head, None) + + case CaseWhen(branches, elseValue) if branches.length == 1 => + // Using pattern matching like `CaseWhen((cond, branchValue) :: Nil, elseValue)` will not + // work since the implementation of `branches` can be `ArrayBuffer`. A full test is in --- End diff -- `sql("select case when a is null then 1 end col1 from t")` will create `branches` with `ArrayBuffer` implementation.
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