[GitHub] dongjoon-hyun commented on a change in pull request #23726: [SPARK-26812][SQL] Report correct nullability for complex datatypes in Union

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dongjoon-hyun commented on a change in pull request #23726: [SPARK-26812][SQL] Report correct nullability for complex datatypes in Union
URL: https://github.com/apache/spark/pull/23726#discussion_r255319845
 
 

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 File path: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/basicLogicalOperators.scala
 ##########
 @@ -230,9 +231,19 @@ case class Union(children: Seq[LogicalPlan]) extends LogicalPlan {
   }
 
   // updating nullability to make all the children consistent
-  override def output: Seq[Attribute] =
-    children.map(_.output).transpose.map(attrs =>
-      attrs.head.withNullability(attrs.exists(_.nullable)))
+  override def output: Seq[Attribute] = {
+    children.map(_.output).transpose.map { attrs =>
+      val firstAttr = attrs.head
+      val outAttr = try {
+        firstAttr.withDataType(attrs.map(_.dataType).reduce(StructType.merge))
+      } catch {
+        // This should never happen, since `output` is called only when the plan is `resolved`, so
 
 Review comment:
   I understand that this is a fallback logic. But, if this should never happen, we should use more explicit ways. We need to find bugs eagerly. For example, `assert()`, `logWarning`, or `not using try` at all.

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