cloud-fan commented on code in PR #49351:
URL: https://github.com/apache/spark/pull/49351#discussion_r1914422660


##########
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/basicLogicalOperators.scala:
##########
@@ -539,48 +588,56 @@ case class Union(
     children.length > 1 && !(byName || allowMissingCol) && childrenResolved && 
allChildrenCompatible
   }
 
-  private lazy val lazyOutput: Seq[Attribute] = computeOutput()
+  override protected def withNewChildrenInternal(newChildren: 
IndexedSeq[LogicalPlan]): Union =
+    copy(children = newChildren)
+}
 
-  private def computeOutput(): Seq[Attribute] = 
Union.mergeChildOutputs(children.map(_.output))
+/**
+ * The logical node for recursion, that contains a initial (anchor) and a 
recursion describing term,
+ * that contains an [[UnionLoopRef]] node.
+ * The node is very similar to [[Union]] because the initial and "generated" 
children are union-ed
+ * and it is also similar to a loop because the recursion continues until the 
last generated child
+ * is not empty.
+ *
+ * @param id The id of the loop, inherited from [[CTERelationDef]]
+ * @param anchor The plan of the initial element of the loop.
+ * @param recursion The plan that describes the recursion with an 
[[UnionLoopRef]] node.
+ * @param limit An optional limit that can be pushed down to the node to stop 
the loop earlier.
+ */
+case class UnionLoop(id: Long,
+    anchor: LogicalPlan,
+    recursion: LogicalPlan,
+    limit: Option[Int] = None) extends UnionBase {
+  override def children: Seq[LogicalPlan] = Seq(anchor, recursion)
+
+  override protected def withNewChildrenInternal(newChildren: 
IndexedSeq[LogicalPlan]): UnionLoop =
+    copy(anchor = newChildren(0), recursion = newChildren(1))
+}
 
-  /**
-   * Maps the constraints containing a given (original) sequence of attributes 
to those with a
-   * given (reference) sequence of attributes. Given the nature of union, we 
expect that the
-   * mapping between the original and reference sequences are symmetric.
-   */
-  private def rewriteConstraints(
-      reference: Seq[Attribute],
-      original: Seq[Attribute],
-      constraints: ExpressionSet): ExpressionSet = {
-    require(reference.size == original.size)
-    val attributeRewrites = AttributeMap(original.zip(reference))
-    constraints.map(_ transform {
-      case a: Attribute => attributeRewrites(a)
-    })
-  }
+/**
+ * The recursive reference in the recursive term of an [[UnionLoop]] node.
+ *
+ * @param loopId The id of the loop, inherited from [[CTERelationRef]]
+ * @param output The output attributes of this recursive reference.
+ * @param accumulated If false the the reference stands for the result of the 
previous iteration.
+ *                    If it is true then then it stands for the union of all 
previous iteration
+ *                    results.
+ */
+case class UnionLoopRef(loopId: Long,
+    override val output: Seq[Attribute],

Review Comment:
   ```suggestion
   case class UnionLoopRef(
       loopId: Long,
       override val output: Seq[Attribute],
   ```



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