ddl: fix the optimization for the empty table(speed up) (#49629) (#49815

)

close #49679, close #49682

ddl/backfilling.go

@@ -399,6 +399,11 @@ func splitTableRanges(t table.PhysicalTable, store kv.Storage, startKey, endKey
 		zap.Int64("physicalTableID", t.GetPhysicalID()),
 		zap.String("start key", hex.EncodeToString(startKey)),
 		zap.String("end key", hex.EncodeToString(endKey)))
+	if len(startKey) == 0 && len(endKey) == 0 {
+		logutil.BgLogger().Info("split table range from PD, get noop table range", zap.String("category", "ddl"), zap.Int64("physicalTableID", t.GetPhysicalID()))
+		return []kv.KeyRange{}, nil
+	}
+
 	kvRange := kv.KeyRange{StartKey: startKey, EndKey: endKey}
 	s, ok := store.(tikv.Storage)
 	if !ok {
@@ -685,9 +690,6 @@ func (dc *ddlCtx) writePhysicalTableRecord(sessPool *sess.Pool, t table.Physical
 	if err := dc.isReorgRunnable(reorgInfo.Job.ID, false); err != nil {
 		return errors.Trace(err)
 	}
-	if startKey == nil && endKey == nil {
-		return nil
-	}
 
 	failpoint.Inject("MockCaseWhenParseFailure", func(val failpoint.Value) {
 		//nolint:forcetypeassert