[SPARK-50659][SQL] Refactor Union output computation out to reuse it …

…in the single-pass Analyzer

### What changes were proposed in this pull request?

Refactor `computeOutput` out to the `Union` companion object.

### Why are the changes needed?

To reuse this piece of code in the single-pass Analyzer.

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

Existing tests.

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes #49283 from vladimirg-db/vladimirg-db/refactor-out-union-compute-output-to-use-in-single-pass-analyzer.

Authored-by: Vladimir Golubev <[email protected]>
Signed-off-by: Wenchen Fan <[email protected]>

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/basicLogicalOperators.scala

@@ -445,6 +445,21 @@ object Union {
   def apply(left: LogicalPlan, right: LogicalPlan): Union = {
     Union (left :: right :: Nil)
   }
+
+  // updating nullability to make all the children consistent
+  def mergeChildOutputs(childOutputs: Seq[Seq[Attribute]]): Seq[Attribute] = {
+    childOutputs.transpose.map { attrs =>
+      val firstAttr = attrs.head
+      val nullable = attrs.exists(_.nullable)
+      val newDt = attrs.map(_.dataType).reduce(StructType.unionLikeMerge)
+      if (firstAttr.dataType == newDt) {
+        firstAttr.withNullability(nullable)
+      } else {
+        AttributeReference(firstAttr.name, newDt, nullable, firstAttr.metadata)(
+          firstAttr.exprId, firstAttr.qualifier)
+      }
+    }
+  }
 }
 
 /**
@@ -526,20 +541,7 @@ case class Union(
 
   private lazy val lazyOutput: Seq[Attribute] = computeOutput()
 
-  // updating nullability to make all the children consistent
-  private def computeOutput(): Seq[Attribute] = {
-    children.map(_.output).transpose.map { attrs =>
-      val firstAttr = attrs.head
-      val nullable = attrs.exists(_.nullable)
-      val newDt = attrs.map(_.dataType).reduce(StructType.unionLikeMerge)
-      if (firstAttr.dataType == newDt) {
-        firstAttr.withNullability(nullable)
-      } else {
-        AttributeReference(firstAttr.name, newDt, nullable, firstAttr.metadata)(
-          firstAttr.exprId, firstAttr.qualifier)
-      }
-    }
-  }
+  private def computeOutput(): Seq[Attribute] = Union.mergeChildOutputs(children.map(_.output))
 
   /**
    * Maps the constraints containing a given (original) sequence of attributes to those with a