diff --git a/br/pkg/lightning/backend/external/engine.go b/br/pkg/lightning/backend/external/engine.go
index b86b45feed694..b9eb9bd2eb958 100644
--- a/br/pkg/lightning/backend/external/engine.go
+++ b/br/pkg/lightning/backend/external/engine.go
@@ -266,13 +266,15 @@ func (e *Engine) loadBatchRegionData(ctx context.Context, startKey, endKey []byt
 	sortDurHist := metrics.GlobalSortReadFromCloudStorageDuration.WithLabelValues("sort")
 
 	readStart := time.Now()
+	readDtStartKey := e.keyAdapter.Encode(nil, startKey, common.MinRowID)
+	readDtEndKey := e.keyAdapter.Encode(nil, endKey, common.MinRowID)
 	err := readAllData(
 		ctx,
 		e.storage,
 		e.dataFiles,
 		e.statsFiles,
-		startKey,
-		endKey,
+		readDtStartKey,
+		readDtEndKey,
 		e.smallBlockBufPool,
 		e.largeBlockBufPool,
 		&e.memKVsAndBuffers,
@@ -402,7 +404,32 @@ func (e *Engine) ID() string {
 
 // GetKeyRange implements common.Engine.
 func (e *Engine) GetKeyRange() (startKey []byte, endKey []byte, err error) {
-	return e.startKey, e.endKey, nil
+	if _, ok := e.keyAdapter.(common.NoopKeyAdapter); ok {
+		return e.startKey, e.endKey, nil
+	}
+
+	// when duplicate detection feature is enabled, the end key comes from
+	// DupDetectKeyAdapter.Encode or Key.Next(). We try to decode it and check the
+	// error.
+
+	start, err := e.keyAdapter.Decode(nil, e.startKey)
+	if err != nil {
+		return nil, nil, err
+	}
+	end, err := e.keyAdapter.Decode(nil, e.endKey)
+	if err == nil {
+		return start, end, nil
+	}
+	// handle the case that end key is from Key.Next()
+	if e.endKey[len(e.endKey)-1] != 0 {
+		return nil, nil, err
+	}
+	endEncoded := e.endKey[:len(e.endKey)-1]
+	end, err = e.keyAdapter.Decode(nil, endEncoded)
+	if err != nil {
+		return nil, nil, err
+	}
+	return start, kv.Key(end).Next(), nil
 }
 
 // SplitRanges split the ranges by split keys provided by external engine.
@@ -412,6 +439,13 @@ func (e *Engine) SplitRanges(
 	_ log.Logger,
 ) ([]common.Range, error) {
 	splitKeys := e.splitKeys
+	for i, k := range e.splitKeys {
+		var err error
+		splitKeys[i], err = e.keyAdapter.Decode(nil, k)
+		if err != nil {
+			return nil, err
+		}
+	}
 	ranges := make([]common.Range, 0, len(splitKeys)+1)
 	ranges = append(ranges, common.Range{Start: startKey})
 	for i := 0; i < len(splitKeys); i++ {
diff --git a/br/pkg/lightning/backend/external/util.go b/br/pkg/lightning/backend/external/util.go
index 8433dc262aeae..2ee8d9942becb 100644
--- a/br/pkg/lightning/backend/external/util.go
+++ b/br/pkg/lightning/backend/external/util.go
@@ -17,7 +17,6 @@ package external
 import (
 	"bytes"
 	"context"
-	"fmt"
 	"io"
 	"slices"
 	"sort"
@@ -41,11 +40,6 @@ import (
 // file reader, read, parse and skip few smaller keys, and then locate the needed
 // data.
 //
-// To avoid potential data loss, it also checks at least one statistic file has a
-// key larger than or equal to the start key. If not, we are afraid that some
-// paths are missing, and the data between [start key, min(first key of
-// statistic files)) are lost.
-//
 // Caller can specify multiple ascending keys and seekPropsOffsets will return
 // the offsets list per file for each key.
 func seekPropsOffsets(
@@ -64,9 +58,7 @@ func seekPropsOffsets(
 	for i := range offsetsPerFile {
 		offsetsPerFile[i] = make([]uint64, len(starts))
 	}
-	// Record first key if it is smaller than first key of "starts" key argument for
-	// each file, and check all files afterward.
-	firstKeyTooSmallCheckers := make([]kv.Key, len(paths))
+
 	eg, egCtx := util.NewErrorGroupWithRecoverWithCtx(ctx)
 	for i := range paths {
 		i := i
@@ -80,7 +72,6 @@ func seekPropsOffsets(
 			}
 			defer r.Close()
 
-			moved := false
 			keyIdx := 0
 			curKey := starts[keyIdx]
 
@@ -100,11 +91,6 @@ func seekPropsOffsets(
 				}
 				propKey := kv.Key(p.firstKey)
 				for propKey.Cmp(curKey) > 0 {
-					if !moved {
-						if firstKeyTooSmallCheckers[i] == nil {
-							firstKeyTooSmallCheckers[i] = propKey
-						}
-					}
 					keyIdx++
 					if keyIdx >= len(starts) {
 						return nil
@@ -112,7 +98,6 @@ func seekPropsOffsets(
 					offsetsPerFile[i][keyIdx] = offsetsPerFile[i][keyIdx-1]
 					curKey = starts[keyIdx]
 				}
-				moved = true
 				offsetsPerFile[i][keyIdx] = p.offset
 				p, err3 = r.nextProp()
 			}
@@ -123,27 +108,6 @@ func seekPropsOffsets(
 		return nil, err
 	}
 
-	hasNil := false
-	for _, k := range firstKeyTooSmallCheckers {
-		if k == nil {
-			hasNil = true
-			break
-		}
-	}
-	if !hasNil {
-		minKey := firstKeyTooSmallCheckers[0]
-		for _, k := range firstKeyTooSmallCheckers[1:] {
-			if k.Cmp(minKey) < 0 {
-				minKey = k
-			}
-		}
-		return nil, fmt.Errorf("start key %s is too small for stat files %v, propKey %s",
-			starts[0].String(),
-			paths,
-			minKey.String(),
-		)
-	}
-
 	// TODO(lance6716): change the caller so we don't need to transpose the result
 	offsetsPerKey := make([][]uint64, len(starts))
 	for i := range starts {
diff --git a/br/pkg/lightning/backend/external/util_test.go b/br/pkg/lightning/backend/external/util_test.go
index 1800a27d23c1a..163d908a2e5c3 100644
--- a/br/pkg/lightning/backend/external/util_test.go
+++ b/br/pkg/lightning/backend/external/util_test.go
@@ -87,15 +87,17 @@ func TestSeekPropsOffsets(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, [][]uint64{{10, 20}, {30, 20}}, got)
 
-	_, err = seekPropsOffsets(ctx, []kv.Key{[]byte("key0")}, []string{file1, file2}, store)
-	require.ErrorContains(t, err, "start key 6b657930 is too small for stat files [/test1 /test2]")
+	got, err = seekPropsOffsets(ctx, []kv.Key{[]byte("key0")}, []string{file1, file2}, store)
+	require.NoError(t, err)
+	require.Equal(t, [][]uint64{{0, 0}}, got)
 
 	got, err = seekPropsOffsets(ctx, []kv.Key{[]byte("key1")}, []string{file1, file2}, store)
 	require.NoError(t, err)
 	require.Equal(t, [][]uint64{{10, 0}}, got)
 
-	_, err = seekPropsOffsets(ctx, []kv.Key{[]byte("key0"), []byte("key1")}, []string{file1, file2}, store)
-	require.ErrorContains(t, err, "start key 6b657930 is too small for stat files [/test1 /test2]")
+	got, err = seekPropsOffsets(ctx, []kv.Key{[]byte("key0"), []byte("key1")}, []string{file1, file2}, store)
+	require.NoError(t, err)
+	require.Equal(t, [][]uint64{{0, 0}, {10, 0}}, got)
 
 	got, err = seekPropsOffsets(ctx, []kv.Key{[]byte("key999")}, []string{file1, file2}, store)
 	require.NoError(t, err)
diff --git a/br/pkg/lightning/backend/local/local.go b/br/pkg/lightning/backend/local/local.go
index 8f41ca6102244..27b422b0481eb 100644
--- a/br/pkg/lightning/backend/local/local.go
+++ b/br/pkg/lightning/backend/local/local.go
@@ -1008,9 +1008,16 @@ func (local *Backend) generateJobForRange(
 		return nil, err
 	}
 	if pairStart == nil {
-		log.FromContext(ctx).Info("There is no pairs in range",
+		logFn := log.FromContext(ctx).Info
+		if _, ok := data.(*external.MemoryIngestData); ok {
+			logFn = log.FromContext(ctx).Warn
+		}
+		logFn("There is no pairs in range",
 			logutil.Key("start", start),
 			logutil.Key("end", end))
+		// trigger cleanup
+		data.IncRef()
+		data.DecRef()
 		return nil, nil
 	}
 
diff --git a/br/pkg/lightning/common/engine.go b/br/pkg/lightning/common/engine.go
index 559f0058e37ab..5920d76fa5d86 100644
--- a/br/pkg/lightning/common/engine.go
+++ b/br/pkg/lightning/common/engine.go
@@ -38,9 +38,13 @@ type Engine interface {
 	KVStatistics() (totalKVSize int64, totalKVCount int64)
 	// ImportedStatistics returns the imported kv size and imported kv count.
 	ImportedStatistics() (importedKVSize int64, importedKVCount int64)
-	// GetKeyRange returns the key range [startKey, endKey) of the engine.
+	// GetKeyRange returns the key range [startKey, endKey) of the engine. If the
+	// duplicate detection is enabled, the keys in engine are encoded by duplicate
+	// detection but the returned keys should not be encoded.
 	GetKeyRange() (startKey []byte, endKey []byte, err error)
-	// SplitRanges splits the range [startKey, endKey) into multiple ranges.
+	// SplitRanges splits the range [startKey, endKey) into multiple ranges. If the
+	// duplicate detection is enabled, the keys in engine are encoded by duplicate
+	// detection but the returned keys should not be encoded.
 	SplitRanges(startKey, endKey []byte, sizeLimit, keysLimit int64, logger log.Logger) ([]Range, error)
 	Close() error
 }
diff --git a/br/pkg/storage/ks3.go b/br/pkg/storage/ks3.go
index 1ecbbb73d4266..919da5e3aa760 100644
--- a/br/pkg/storage/ks3.go
+++ b/br/pkg/storage/ks3.go
@@ -530,6 +530,9 @@ type ks3ObjectReader struct {
 func (r *ks3ObjectReader) Read(p []byte) (n int, err error) {
 	retryCnt := 0
 	maxCnt := r.rangeInfo.End + 1 - r.pos
+	if maxCnt == 0 {
+		return 0, io.EOF
+	}
 	if maxCnt > int64(len(p)) {
 		maxCnt = int64(len(p))
 	}
diff --git a/br/pkg/storage/s3.go b/br/pkg/storage/s3.go
index a1e9b8caed401..f5ffdbf463601 100644
--- a/br/pkg/storage/s3.go
+++ b/br/pkg/storage/s3.go
@@ -898,12 +898,12 @@ type s3ObjectReader struct {
 func (r *s3ObjectReader) Read(p []byte) (n int, err error) {
 	retryCnt := 0
 	maxCnt := r.rangeInfo.End + 1 - r.pos
-	if maxCnt > int64(len(p)) {
-		maxCnt = int64(len(p))
-	}
 	if maxCnt == 0 {
 		return 0, io.EOF
 	}
+	if maxCnt > int64(len(p)) {
+		maxCnt = int64(len(p))
+	}
 	n, err = r.reader.Read(p[:maxCnt])
 	// TODO: maybe we should use !errors.Is(err, io.EOF) here to avoid error lint, but currently, pingcap/errors
 	// doesn't implement this method yet.
diff --git a/pkg/ddl/backfilling_dist_scheduler.go b/pkg/ddl/backfilling_dist_scheduler.go
index a027dd12b20a1..4d9b58b530947 100644
--- a/pkg/ddl/backfilling_dist_scheduler.go
+++ b/pkg/ddl/backfilling_dist_scheduler.go
@@ -375,6 +375,7 @@ func generateGlobalSortIngestPlan(
 	if err != nil {
 		return nil, err
 	}
+	iCnt := int64(len(instanceIDs))
 	metaArr := make([][]byte, 0, 16)
 	for i, g := range kvMetaGroups {
 		if g == nil {
@@ -382,7 +383,7 @@ func generateGlobalSortIngestPlan(
 				zap.Int64("taskID", task.ID))
 			return nil, errors.Errorf("subtask kv group %d is empty", i)
 		}
-		newMeta, err := splitSubtaskMetaForOneKVMetaGroup(ctx, store, g, cloudStorageURI, int64(len(instanceIDs)), logger)
+		newMeta, err := splitSubtaskMetaForOneKVMetaGroup(ctx, store, g, cloudStorageURI, iCnt, logger)
 		if err != nil {
 			return nil, errors.Trace(err)
 		}
diff --git a/tests/realtikvtest/addindextest2/global_sort_test.go b/tests/realtikvtest/addindextest2/global_sort_test.go
index f75c03dbee35c..39e324bb5562a 100644
--- a/tests/realtikvtest/addindextest2/global_sort_test.go
+++ b/tests/realtikvtest/addindextest2/global_sort_test.go
@@ -116,18 +116,23 @@ func TestGlobalSortBasic(t *testing.T) {
 	dom.DDL().SetHook(hook)
 
 	tk.MustExec("alter table t add index idx(a);")
-	dom.DDL().SetHook(origin)
 	tk.MustExec("admin check table t;")
 	<-scheduler.WaitCleanUpFinished
 	checkFileCleaned(t, jobID, cloudStorageURI)
 
 	require.NoError(t, failpoint.Enable("github.com/pingcap/tidb/pkg/ddl/forceMergeSort", "return()"))
 	tk.MustExec("alter table t add index idx1(a);")
-	dom.DDL().SetHook(origin)
 	tk.MustExec("admin check table t;")
 	<-scheduler.WaitCleanUpFinished
+	checkFileCleaned(t, jobID, cloudStorageURI)
 
+	tk.MustExec("alter table t add unique index idx2(a);")
+	tk.MustExec("admin check table t;")
+	<-scheduler.WaitCleanUpFinished
 	checkFileCleaned(t, jobID, cloudStorageURI)
+
+	dom.DDL().SetHook(origin)
+
 	require.NoError(t, failpoint.Disable("github.com/pingcap/tidb/pkg/disttask/framework/scheduler/WaitCleanUpFinished"))
 	require.NoError(t, failpoint.Disable("github.com/pingcap/tidb/pkg/ddl/forceMergeSort"))
 }