diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/Analyzer.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/Analyzer.scala
index 023ef2ee17473..ed6fd93de926a 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/Analyzer.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/Analyzer.scala
@@ -249,6 +249,7 @@ class Analyzer(
       ResolveTimeZone(conf) ::
       ResolveRandomSeed ::
       ResolveBinaryArithmetic ::
+      ResolveUnion ::
       TypeCoercion.typeCoercionRules(conf) ++
       extendedResolutionRules : _*),
     Batch("Post-Hoc Resolution", Once, postHocResolutionRules: _*),
@@ -1387,10 +1388,11 @@ class Analyzer(
         i.copy(right = dedupRight(left, right))
       case e @ Except(left, right, _) if !e.duplicateResolved =>
         e.copy(right = dedupRight(left, right))
-      case u @ Union(children) if !u.duplicateResolved =>
+      // Only after we finish by-name resolution for Union
+      case u: Union if !u.byName && !u.duplicateResolved =>
         // Use projection-based de-duplication for Union to avoid breaking the checkpoint sharing
         // feature in streaming.
-        val newChildren = children.foldRight(Seq.empty[LogicalPlan]) { (head, tail) =>
+        val newChildren = u.children.foldRight(Seq.empty[LogicalPlan]) { (head, tail) =>
           head +: tail.map {
             case child if head.outputSet.intersect(child.outputSet).isEmpty =>
               child
@@ -3398,7 +3400,7 @@ object EliminateSubqueryAliases extends Rule[LogicalPlan] {
  */
 object EliminateUnions extends Rule[LogicalPlan] {
   def apply(plan: LogicalPlan): LogicalPlan = plan resolveOperators {
-    case Union(children) if children.size == 1 => children.head
+    case u: Union if u.children.size == 1 => u.children.head
   }
 }
 
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/ResolveUnion.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/ResolveUnion.scala
new file mode 100644
index 0000000000000..693a5a4e75443
--- /dev/null
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/ResolveUnion.scala
@@ -0,0 +1,100 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.sql.catalyst.analysis
+
+import org.apache.spark.sql.AnalysisException
+import org.apache.spark.sql.catalyst.expressions.{Alias, Literal}
+import org.apache.spark.sql.catalyst.optimizer.CombineUnions
+import org.apache.spark.sql.catalyst.plans.logical.{LogicalPlan, Project, Union}
+import org.apache.spark.sql.catalyst.rules.Rule
+import org.apache.spark.sql.internal.SQLConf
+import org.apache.spark.sql.util.SchemaUtils
+
+/**
+ * Resolves different children of Union to a common set of columns.
+ */
+object ResolveUnion extends Rule[LogicalPlan] {
+  private def unionTwoSides(
+      left: LogicalPlan,
+      right: LogicalPlan,
+      allowMissingCol: Boolean): LogicalPlan = {
+    val resolver = SQLConf.get.resolver
+    val leftOutputAttrs = left.output
+    val rightOutputAttrs = right.output
+
+    // Builds a project list for `right` based on `left` output names
+    val rightProjectList = leftOutputAttrs.map { lattr =>
+      rightOutputAttrs.find { rattr => resolver(lattr.name, rattr.name) }.getOrElse {
+        if (allowMissingCol) {
+          Alias(Literal(null, lattr.dataType), lattr.name)()
+        } else {
+          throw new AnalysisException(
+            s"""Cannot resolve column name "${lattr.name}" among """ +
+              s"""(${rightOutputAttrs.map(_.name).mkString(", ")})""")
+        }
+      }
+    }
+
+    // Delegates failure checks to `CheckAnalysis`
+    val notFoundAttrs = rightOutputAttrs.diff(rightProjectList)
+    val rightChild = Project(rightProjectList ++ notFoundAttrs, right)
+
+    // Builds a project for `logicalPlan` based on `right` output names, if allowing
+    // missing columns.
+    val leftChild = if (allowMissingCol) {
+      val missingAttrs = notFoundAttrs.map { attr =>
+        Alias(Literal(null, attr.dataType), attr.name)()
+      }
+      if (missingAttrs.nonEmpty) {
+        Project(leftOutputAttrs ++ missingAttrs, left)
+      } else {
+        left
+      }
+    } else {
+      left
+    }
+    Union(leftChild, rightChild)
+  }
+
+  // Check column name duplication
+  private def checkColumnNames(left: LogicalPlan, right: LogicalPlan): Unit = {
+    val caseSensitiveAnalysis = SQLConf.get.caseSensitiveAnalysis
+    val leftOutputAttrs = left.output
+    val rightOutputAttrs = right.output
+
+    SchemaUtils.checkColumnNameDuplication(
+      leftOutputAttrs.map(_.name),
+      "in the left attributes",
+      caseSensitiveAnalysis)
+    SchemaUtils.checkColumnNameDuplication(
+      rightOutputAttrs.map(_.name),
+      "in the right attributes",
+      caseSensitiveAnalysis)
+  }
+
+  def apply(plan: LogicalPlan): LogicalPlan = plan resolveOperatorsUp {
+    case e if !e.childrenResolved => e
+
+    case Union(children, byName, allowMissingCol) if byName =>
+      val union = children.reduceLeft { (left, right) =>
+        checkColumnNames(left, right)
+        unionTwoSides(left, right, allowMissingCol)
+      }
+      CombineUnions(union)
+  }
+}
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/TypeCoercion.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/TypeCoercion.scala
index 3484108a5503f..604a082be4e55 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/TypeCoercion.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/TypeCoercion.scala
@@ -341,10 +341,10 @@ object TypeCoercion {
         assert(newChildren.length == 2)
         Intersect(newChildren.head, newChildren.last, isAll)
 
-      case s: Union if s.childrenResolved &&
+      case s: Union if s.childrenResolved && !s.byName &&
           s.children.forall(_.output.length == s.children.head.output.length) && !s.resolved =>
         val newChildren: Seq[LogicalPlan] = buildNewChildrenWithWiderTypes(s.children)
-        s.makeCopy(Array(newChildren))
+        s.copy(children = newChildren)
     }
 
     /** Build new children with the widest types for each attribute among all the children */
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
index e481cdbd5fdf4..c4a3f85bbf54b 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
@@ -497,8 +497,8 @@ object LimitPushDown extends Rule[LogicalPlan] {
     // Note: right now Union means UNION ALL, which does not de-duplicate rows, so it is safe to
     // pushdown Limit through it. Once we add UNION DISTINCT, however, we will not be able to
     // pushdown Limit.
-    case LocalLimit(exp, Union(children)) =>
-      LocalLimit(exp, Union(children.map(maybePushLocalLimit(exp, _))))
+    case LocalLimit(exp, u: Union) =>
+      LocalLimit(exp, u.copy(children = u.children.map(maybePushLocalLimit(exp, _))))
     // Add extra limits below OUTER JOIN. For LEFT OUTER and RIGHT OUTER JOIN we push limits to
     // the left and right sides, respectively. It's not safe to push limits below FULL OUTER
     // JOIN in the general case without a more invasive rewrite.
@@ -556,15 +556,15 @@ object PushProjectionThroughUnion extends Rule[LogicalPlan] with PredicateHelper
   def apply(plan: LogicalPlan): LogicalPlan = plan transform {
 
     // Push down deterministic projection through UNION ALL
-    case p @ Project(projectList, Union(children)) =>
-      assert(children.nonEmpty)
+    case p @ Project(projectList, u: Union) =>
+      assert(u.children.nonEmpty)
       if (projectList.forall(_.deterministic)) {
-        val newFirstChild = Project(projectList, children.head)
-        val newOtherChildren = children.tail.map { child =>
-          val rewrites = buildRewrites(children.head, child)
+        val newFirstChild = Project(projectList, u.children.head)
+        val newOtherChildren = u.children.tail.map { child =>
+          val rewrites = buildRewrites(u.children.head, child)
           Project(projectList.map(pushToRight(_, rewrites)), child)
         }
-        Union(newFirstChild +: newOtherChildren)
+        u.copy(children = newFirstChild +: newOtherChildren)
       } else {
         p
       }
@@ -928,19 +928,28 @@ object CombineUnions extends Rule[LogicalPlan] {
   }
 
   private def flattenUnion(union: Union, flattenDistinct: Boolean): Union = {
+    val topByName = union.byName
+    val topAllowMissingCol = union.allowMissingCol
+
     val stack = mutable.Stack[LogicalPlan](union)
     val flattened = mutable.ArrayBuffer.empty[LogicalPlan]
+    // Note that we should only flatten the unions with same byName and allowMissingCol.
+    // Although we do `UnionCoercion` at analysis phase, we manually run `CombineUnions`
+    // in some places like `Dataset.union`. Flattening unions with different resolution
+    // rules (by position and by name) could cause incorrect results.
     while (stack.nonEmpty) {
       stack.pop() match {
-        case Distinct(Union(children)) if flattenDistinct =>
+        case Distinct(Union(children, byName, allowMissingCol))
+            if flattenDistinct && byName == topByName && allowMissingCol == topAllowMissingCol =>
           stack.pushAll(children.reverse)
-        case Union(children) =>
+        case Union(children, byName, allowMissingCol)
+            if byName == topByName && allowMissingCol == topAllowMissingCol =>
           stack.pushAll(children.reverse)
         case child =>
           flattened += child
       }
     }
-    Union(flattened.toSeq)
+    union.copy(children = flattened)
   }
 }
 
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/PropagateEmptyRelation.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/PropagateEmptyRelation.scala
index 0299646150ff3..d3cdd71eafdb1 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/PropagateEmptyRelation.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/PropagateEmptyRelation.scala
@@ -50,8 +50,8 @@ object PropagateEmptyRelation extends Rule[LogicalPlan] with PredicateHelper wit
   override def conf: SQLConf = SQLConf.get
 
   def apply(plan: LogicalPlan): LogicalPlan = plan transformUp {
-    case p @ Union(children) if children.exists(isEmptyLocalRelation) =>
-      val newChildren = children.filterNot(isEmptyLocalRelation)
+    case p: Union if p.children.exists(isEmptyLocalRelation) =>
+      val newChildren = p.children.filterNot(isEmptyLocalRelation)
       if (newChildren.isEmpty) {
         empty(p)
       } else {
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/basicLogicalOperators.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/basicLogicalOperators.scala
index 039fd9382000a..54ec5859823c3 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/basicLogicalOperators.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/basicLogicalOperators.scala
@@ -220,8 +220,18 @@ object Union {
 
 /**
  * Logical plan for unioning two plans, without a distinct. This is UNION ALL in SQL.
+ *
+ * @param byName          Whether resolves columns in the children by column names.
+ * @param allowMissingCol Allows missing columns in children query plans. If it is true,
+ *                        this function allows different set of column names between two Datasets.
+ *                        This can be set to true only if `byName` is true.
  */
-case class Union(children: Seq[LogicalPlan]) extends LogicalPlan {
+case class Union(
+    children: Seq[LogicalPlan],
+    byName: Boolean = false,
+    allowMissingCol: Boolean = false) extends LogicalPlan {
+  assert(!allowMissingCol || byName, "`allowMissingCol` can be true only if `byName` is true.")
+
   override def maxRows: Option[Long] = {
     if (children.exists(_.maxRows.isEmpty)) {
       None
@@ -271,7 +281,7 @@ case class Union(children: Seq[LogicalPlan]) extends LogicalPlan {
         child.output.zip(children.head.output).forall {
           case (l, r) => l.dataType.sameType(r.dataType)
         })
-    children.length > 1 && childrenResolved && allChildrenCompatible
+    children.length > 1 && !(byName || allowMissingCol) && childrenResolved && allChildrenCompatible
   }
 
   /**
diff --git a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/analysis/DecimalPrecisionSuite.scala b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/analysis/DecimalPrecisionSuite.scala
index c3e18c7f9557f..d5991ff10ce6c 100644
--- a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/analysis/DecimalPrecisionSuite.scala
+++ b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/analysis/DecimalPrecisionSuite.scala
@@ -73,7 +73,7 @@ class DecimalPrecisionSuite extends AnalysisTest with BeforeAndAfter {
       Union(Project(Seq(Alias(left, "l")()), relation),
         Project(Seq(Alias(right, "r")()), relation))
     val (l, r) = analyzer.execute(plan).collect {
-      case Union(Seq(child1, child2)) => (child1.output.head, child2.output.head)
+      case Union(Seq(child1, child2), _, _) => (child1.output.head, child2.output.head)
     }.head
     assert(l.dataType === expectedType)
     assert(r.dataType === expectedType)
diff --git a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/analysis/ResolveUnionSuite.scala b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/analysis/ResolveUnionSuite.scala
new file mode 100644
index 0000000000000..5c7ad0067a456
--- /dev/null
+++ b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/analysis/ResolveUnionSuite.scala
@@ -0,0 +1,75 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.spark.sql.catalyst.analysis
+
+import org.apache.spark.sql.catalyst.expressions._
+import org.apache.spark.sql.catalyst.plans.logical._
+import org.apache.spark.sql.catalyst.rules.RuleExecutor
+import org.apache.spark.sql.types._
+
+class ResolveUnionSuite extends AnalysisTest {
+  test("Resolve Union") {
+    val table1 = LocalRelation(
+      AttributeReference("i", IntegerType)(),
+      AttributeReference("u", DecimalType.SYSTEM_DEFAULT)(),
+      AttributeReference("b", ByteType)(),
+      AttributeReference("d", DoubleType)())
+    val table2 = LocalRelation(
+      AttributeReference("u", DecimalType.SYSTEM_DEFAULT)(),
+      AttributeReference("b", ByteType)(),
+      AttributeReference("d", DoubleType)(),
+      AttributeReference("i", IntegerType)())
+    val table3 = LocalRelation(
+      AttributeReference("u", DecimalType.SYSTEM_DEFAULT)(),
+      AttributeReference("d", DoubleType)(),
+      AttributeReference("i", IntegerType)())
+    val table4 = LocalRelation(
+      AttributeReference("u", DecimalType.SYSTEM_DEFAULT)(),
+      AttributeReference("i", IntegerType)())
+
+    val rules = Seq(ResolveUnion)
+    val analyzer = new RuleExecutor[LogicalPlan] {
+      override val batches = Seq(Batch("Resolution", Once, rules: _*))
+    }
+
+    // By name resolution
+    val union1 = Union(table1 :: table2 :: Nil, true, false)
+    val analyzed1 = analyzer.execute(union1)
+    val projected1 =
+      Project(Seq(table2.output(3), table2.output(0), table2.output(1), table2.output(2)), table2)
+    val expected1 = Union(table1 :: projected1 :: Nil)
+    comparePlans(analyzed1, expected1)
+
+    // Allow missing column
+    val union2 = Union(table1 :: table3 :: Nil, true, true)
+    val analyzed2 = analyzer.execute(union2)
+    val nullAttr1 = Alias(Literal(null, ByteType), "b")()
+    val projected2 =
+      Project(Seq(table2.output(3), table2.output(0), nullAttr1, table2.output(2)), table3)
+    val expected2 = Union(table1 :: projected2 :: Nil)
+    comparePlans(analyzed2, expected2)
+
+    // Allow missing column + Allow missing column
+    val union3 = Union(union2 :: table4 :: Nil, true, true)
+    val analyzed3 = analyzer.execute(union3)
+    val nullAttr2 = Alias(Literal(null, DoubleType), "d")()
+    val projected3 =
+      Project(Seq(table2.output(3), table2.output(0), nullAttr1, nullAttr2), table4)
+    val expected3 = Union(table1 :: projected2 :: projected3 :: Nil)
+    comparePlans(analyzed3, expected3)
+  }
+}
diff --git a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/optimizer/SetOperationSuite.scala b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/optimizer/SetOperationSuite.scala
index ccc30b1d2f8ce..2eea840e21a31 100644
--- a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/optimizer/SetOperationSuite.scala
+++ b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/optimizer/SetOperationSuite.scala
@@ -223,4 +223,21 @@ class SetOperationSuite extends PlanTest {
     val unionCorrectAnswer = unionQuery.analyze
     comparePlans(unionOptimized, unionCorrectAnswer)
   }
+
+  test("CombineUnions only flatten the unions with same byName and allowMissingCol") {
+    val union1 = Union(testRelation :: testRelation :: Nil, true, false)
+    val union2 = Union(testRelation :: testRelation :: Nil, true, true)
+    val union3 = Union(testRelation :: testRelation2 :: Nil, false, false)
+
+    val union4 = Union(union1 :: union2 :: union3 :: Nil)
+    val unionOptimized1 = Optimize.execute(union4)
+    val unionCorrectAnswer1 = Union(union1 :: union2 :: testRelation :: testRelation2 :: Nil)
+    comparePlans(unionOptimized1, unionCorrectAnswer1, false)
+
+    val union5 = Union(union1 :: union1 :: Nil, true, false)
+    val unionOptimized2 = Optimize.execute(union5)
+    val unionCorrectAnswer2 =
+      Union(testRelation :: testRelation :: testRelation :: testRelation :: Nil, true, false)
+    comparePlans(unionOptimized2, unionCorrectAnswer2, false)
+  }
 }
diff --git a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/trees/TreeNodeSuite.scala b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/trees/TreeNodeSuite.scala
index f5259706325eb..ff51bc0071c80 100644
--- a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/trees/TreeNodeSuite.scala
+++ b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/trees/TreeNodeSuite.scala
@@ -583,7 +583,9 @@ class TreeNodeSuite extends SparkFunSuite with SQLHelper {
         JObject(
           "class" -> classOf[Union].getName,
           "num-children" -> 2,
-          "children" -> List(0, 1)),
+          "children" -> List(0, 1),
+          "byName" -> JBool(false),
+          "allowMissingCol" -> JBool(false)),
         JObject(
           "class" -> classOf[JsonTestTreeNode].getName,
           "num-children" -> 0,
diff --git a/sql/core/src/main/scala/org/apache/spark/sql/Dataset.scala b/sql/core/src/main/scala/org/apache/spark/sql/Dataset.scala
index 3472b9fdec9d8..bf124db38edf8 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/Dataset.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/Dataset.scala
@@ -227,7 +227,7 @@ class Dataset[T] private[sql](
     val plan = queryExecution.analyzed match {
       case c: Command =>
         LocalRelation(c.output, withAction("command", queryExecution)(_.executeCollect()))
-      case u @ Union(children) if children.forall(_.isInstanceOf[Command]) =>
+      case u @ Union(children, _, _) if children.forall(_.isInstanceOf[Command]) =>
         LocalRelation(u.output, withAction("command", queryExecution)(_.executeCollect()))
       case _ =>
         queryExecution.analyzed
@@ -2071,51 +2071,9 @@ class Dataset[T] private[sql](
    * @since 3.1.0
    */
   def unionByName(other: Dataset[T], allowMissingColumns: Boolean): Dataset[T] = withSetOperator {
-    // Check column name duplication
-    val resolver = sparkSession.sessionState.analyzer.resolver
-    val leftOutputAttrs = logicalPlan.output
-    val rightOutputAttrs = other.logicalPlan.output
-
-    SchemaUtils.checkColumnNameDuplication(
-      leftOutputAttrs.map(_.name),
-      "in the left attributes",
-      sparkSession.sessionState.conf.caseSensitiveAnalysis)
-    SchemaUtils.checkColumnNameDuplication(
-      rightOutputAttrs.map(_.name),
-      "in the right attributes",
-      sparkSession.sessionState.conf.caseSensitiveAnalysis)
-
-    // Builds a project list for `other` based on `logicalPlan` output names
-    val rightProjectList = leftOutputAttrs.map { lattr =>
-      rightOutputAttrs.find { rattr => resolver(lattr.name, rattr.name) }.getOrElse {
-        if (allowMissingColumns) {
-          Alias(Literal(null, lattr.dataType), lattr.name)()
-        } else {
-          throw new AnalysisException(
-            s"""Cannot resolve column name "${lattr.name}" among """ +
-              s"""(${rightOutputAttrs.map(_.name).mkString(", ")})""")
-        }
-      }
-    }
-
-    // Delegates failure checks to `CheckAnalysis`
-    val notFoundAttrs = rightOutputAttrs.diff(rightProjectList)
-    val rightChild = Project(rightProjectList ++ notFoundAttrs, other.logicalPlan)
-
-    // Builds a project for `logicalPlan` based on `other` output names, if allowing
-    // missing columns.
-    val leftChild = if (allowMissingColumns) {
-      val missingAttrs = notFoundAttrs.map { attr =>
-        Alias(Literal(null, attr.dataType), attr.name)()
-      }
-      Project(leftOutputAttrs ++ missingAttrs, logicalPlan)
-    } else {
-      logicalPlan
-    }
-
     // This breaks caching, but it's usually ok because it addresses a very specific use case:
     // using union to union many files or partitions.
-    CombineUnions(Union(leftChild, rightChild))
+    CombineUnions(Union(logicalPlan :: other.logicalPlan :: Nil, true, allowMissingColumns))
   }
 
   /**
diff --git a/sql/core/src/main/scala/org/apache/spark/sql/execution/SparkStrategies.scala b/sql/core/src/main/scala/org/apache/spark/sql/execution/SparkStrategies.scala
index 7b5d8f15962d0..78aa258387daa 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/execution/SparkStrategies.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/execution/SparkStrategies.scala
@@ -683,8 +683,8 @@ abstract class SparkStrategies extends QueryPlanner[SparkPlan] {
         execution.LocalLimitExec(limit, planLater(child)) :: Nil
       case logical.GlobalLimit(IntegerLiteral(limit), child) =>
         execution.GlobalLimitExec(limit, planLater(child)) :: Nil
-      case logical.Union(unionChildren) =>
-        execution.UnionExec(unionChildren.map(planLater)) :: Nil
+      case union: logical.Union =>
+        execution.UnionExec(union.children.map(planLater)) :: Nil
       case g @ logical.Generate(generator, _, outer, _, _, child) =>
         execution.GenerateExec(
           generator, g.requiredChildOutput, outer,
diff --git a/sql/core/src/test/scala/org/apache/spark/sql/execution/SparkPlannerSuite.scala b/sql/core/src/test/scala/org/apache/spark/sql/execution/SparkPlannerSuite.scala
index 9107f8afa83d7..b4cb7e3fce3cf 100644
--- a/sql/core/src/test/scala/org/apache/spark/sql/execution/SparkPlannerSuite.scala
+++ b/sql/core/src/test/scala/org/apache/spark/sql/execution/SparkPlannerSuite.scala
@@ -37,9 +37,9 @@ class SparkPlannerSuite extends SharedSparkSession {
         case ReturnAnswer(child) =>
           planned += 1
           planLater(child) :: planLater(NeverPlanned) :: Nil
-        case Union(children) =>
+        case u: Union =>
           planned += 1
-          UnionExec(children.map(planLater)) :: planLater(NeverPlanned) :: Nil
+          UnionExec(u.children.map(planLater)) :: planLater(NeverPlanned) :: Nil
         case LocalRelation(output, data, _) =>
           planned += 1
           LocalTableScanExec(output, data) :: planLater(NeverPlanned) :: Nil