[spark] Eliminate the union stage when merging into without notMatchedBySource

Author: Aitozi

paimon-spark/paimon-spark-common/src/main/scala/org/apache/paimon/spark/commands/MergeIntoPaimonTable.scala

@@ -172,10 +172,15 @@ case class MergeIntoPaimonTable(
 
       // Add FILE_TOUCHED_COL to mark the row as coming from the touched file, if the row has not been
       // modified and was from touched file, it should be kept too.
-      val targetDSWithFileTouchedCol = createDataset(sparkSession, touchedFileRelation)
+      val touchedDsWithFileTouchedCol = createDataset(sparkSession, touchedFileRelation)
         .withColumn(FILE_TOUCHED_COL, lit(true))
-        .union(createDataset(sparkSession, unTouchedFileRelation)
-          .withColumn(FILE_TOUCHED_COL, lit(false)))
+      val targetDSWithFileTouchedCol = if (notMatchedBySourceActions.nonEmpty) {
+        touchedDsWithFileTouchedCol.union(
+          createDataset(sparkSession, unTouchedFileRelation)
+            .withColumn(FILE_TOUCHED_COL, lit(false)))
+      } else {
+        touchedDsWithFileTouchedCol
+      }
 
       val toWriteDS =
         constructChangedRows(sparkSession, targetDSWithFileTouchedCol).drop(ROW_KIND_COL)