Github user mn-mikke commented on a diff in the pull request: https://github.com/apache/spark/pull/21704#discussion_r200113984 --- Diff: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/collectionOperations.scala --- @@ -2007,7 +2007,14 @@ case class Concat(children: Seq[Expression]) extends Expression { } } - override def dataType: DataType = children.map(_.dataType).headOption.getOrElse(StringType) + override def dataType: DataType = { + val dataTypes = children.map(_.dataType) + dataTypes.headOption.map { + case ArrayType(et, _) => + ArrayType(et, dataTypes.exists(_.asInstanceOf[ArrayType].containsNull)) --- End diff -- Yes, it should work (see a [test](https://github.com/ueshin/apache-spark/blob/30d5aed41085cfb82e3da43099adf972b953ece8/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/CollectionExpressionsSuite.scala#L951) for it). Did we miss anything?
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