[SPARK-52060][SQL] Make OneRowRelationExec node

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

creates a new OneRowRelationExec node, which is more or less a copy of the RDDScanExec node.

We want a dedicated node because this helps make it more clear when a one row relation, i.e. for patterns like `SELECT version()` is used.

### Why are the changes needed?

this makes it more clear in the code that a one row relation is used and allows us to avoid checking the hard coded "OneRowRelation" string when pattern matching.

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

yes, the plan will now be `OneRowRelationExec` rather than `RDDScanExec`. The plan string should be the same, however.

### How was this patch tested?

added UTs

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

Closes apache#50849 from richardc-db/make_one_row_relation_node.

Lead-authored-by: Richard Chen <r.chen@databricks.com>
Co-authored-by: Wenchen Fan <cloud0fan@gmail.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

Author: richardc-db

sql/core/src/main/scala/org/apache/spark/sql/execution/ExistingRDD.scala

@@ -319,3 +319,46 @@ case class RDDScanExec(
 
   override def getStream: Option[SparkDataStream] = stream
 }
+
+/**
+ * A physical plan node for `OneRowRelation` for scans with no 'FROM' clause.
+ *
+ * We do not extend `RDDScanExec` in order to avoid complexity due to `TreeNode.makeCopy` and
+ * `TreeNode`'s general use of reflection.
+ */
+case class OneRowRelationExec() extends LeafExecNode
+  with InputRDDCodegen {
+
+  override val nodeName: String = s"Scan OneRowRelation"
+
+  override val output: Seq[Attribute] = Nil
+
+  private val rdd: RDD[InternalRow] = {
+    val numOutputRows = longMetric("numOutputRows")
+    session
+      .sparkContext
+      .parallelize(Seq(""), 1)
+      .mapPartitionsInternal { _ =>
+        val proj = UnsafeProjection.create(Seq.empty[Expression])
+        Iterator(proj.apply(InternalRow.empty)).map { r =>
+          numOutputRows += 1
+          r
+        }
+      }
+  }
+
+  override lazy val metrics = Map(
+    "numOutputRows" -> SQLMetrics.createMetric(sparkContext, "number of output rows"))
+
+  protected override def doExecute(): RDD[InternalRow] = rdd
+
+  override def simpleString(maxFields: Int): String = s"$nodeName[]"
+
+  override def inputRDD: RDD[InternalRow] = rdd
+
+  override protected val createUnsafeProjection: Boolean = false
+
+  override protected def doCanonicalize(): SparkPlan = {
+    super.doCanonicalize().asInstanceOf[OneRowRelationExec].copy()
+  }
+}

sql/core/src/main/scala/org/apache/spark/sql/execution/SparkStrategies.scala

@@ -690,8 +690,6 @@ abstract class SparkStrategies extends QueryPlanner[SparkPlan] {
     }
   }
 
-  protected lazy val singleRowRdd = session.sparkContext.parallelize(Seq(InternalRow()), 1)
-
   object InMemoryScans extends Strategy {
     def apply(plan: LogicalPlan): Seq[SparkPlan] = plan match {
       case PhysicalOperation(projectList, filters, mem: InMemoryRelation) =>
@@ -1054,7 +1052,7 @@ abstract class SparkStrategies extends QueryPlanner[SparkPlan] {
           generator, g.requiredChildOutput, outer,
           g.qualifiedGeneratorOutput, planLater(child)) :: Nil
       case _: logical.OneRowRelation =>
-        execution.RDDScanExec(Nil, singleRowRdd, "OneRowRelation") :: Nil
+        execution.OneRowRelationExec() :: Nil
       case r: logical.Range =>
         execution.RangeExec(r) :: Nil
       case r: logical.RepartitionByExpression =>

sql/core/src/main/scala/org/apache/spark/sql/execution/WholeStageCodegenExec.scala

@@ -56,6 +56,7 @@ trait CodegenSupport extends SparkPlan {
     case _: SortMergeJoinExec => "smj"
     case _: BroadcastNestedLoopJoinExec => "bnlj"
     case _: RDDScanExec => "rdd"
+    case _: OneRowRelationExec => "orr"
     case _: DataSourceScanExec => "scan"
     case _: InMemoryTableScanExec => "memoryScan"
     case _: WholeStageCodegenExec => "wholestagecodegen"

sql/core/src/test/scala/org/apache/spark/sql/SQLQuerySuite.scala

@@ -38,7 +38,7 @@ import org.apache.spark.sql.catalyst.optimizer.{ConvertToLocalRelation, NestedCo
 import org.apache.spark.sql.catalyst.parser.ParseException
 import org.apache.spark.sql.catalyst.plans.logical.{LocalLimit, Project, RepartitionByExpression, Sort}
 import org.apache.spark.sql.connector.catalog.CatalogManager.SESSION_CATALOG_NAME
-import org.apache.spark.sql.execution.{CommandResultExec, UnionExec}
+import org.apache.spark.sql.execution.{CommandResultExec, OneRowRelationExec, UnionExec}
 import org.apache.spark.sql.execution.adaptive.AdaptiveSparkPlanHelper
 import org.apache.spark.sql.execution.aggregate._
 import org.apache.spark.sql.execution.columnar.InMemoryTableScanExec
@@ -4962,6 +4962,18 @@ class SQLQuerySuite extends QueryTest with SharedSparkSession with AdaptiveSpark
       parameters = Map("plan" -> "'Aggregate [groupingsets(Vector(0), posexplode(array(col)))]")
     )
   }
+
+  Seq(true, false).foreach { codegenEnabled =>
+    test(s"SPARK-52060: one row relation with codegen enabled - $codegenEnabled") {
+      withSQLConf(SQLConf.WHOLESTAGE_CODEGEN_ENABLED.key -> codegenEnabled.toString) {
+        val df = spark.sql("select 'test' stringCol")
+        checkAnswer(df, Row("test"))
+        val plan = df.queryExecution.executedPlan
+        val oneRowRelationExists = plan.find(_.isInstanceOf[OneRowRelationExec]).isDefined
+        assert(oneRowRelationExists)
+      }
+    }
+  }
 }
 
 case class Foo(bar: Option[String])