diff --git a/util/chunk/chunk.go b/util/chunk/chunk.go
index ce915e423fb9c..0b9395ea9e768 100644
--- a/util/chunk/chunk.go
+++ b/util/chunk/chunk.go
@@ -178,7 +178,7 @@ func (c *Chunk) BatchCopyJoinRowToChunk(isRight bool, chkForJoin *Chunk, outer R
 
 // appendPartialRows appends multiple different rows to the chunk.
 func appendPartialRows(colIdx, outerIdx int, chkForJoin, chk *Chunk, selected []bool) int {
-	rowNum := 0
+	oldLen := chk.NumRows()
 	var columns []*column
 	if colIdx == 0 {
 		columns = chkForJoin.columns[:outerIdx]
@@ -186,12 +186,12 @@ func appendPartialRows(colIdx, outerIdx int, chkForJoin, chk *Chunk, selected []
 		columns = chkForJoin.columns[colIdx:]
 	}
 
-	for i := 0; i < len(selected); i++ {
-		if !selected[i] {
-			continue
-		}
-		for j, rowCol := range columns {
-			chkCol := chk.columns[colIdx+j]
+	for j, rowCol := range columns {
+		chkCol := chk.columns[colIdx+j]
+		for i := 0; i < len(selected); i++ {
+			if !selected[i] {
+				continue
+			}
 			chkCol.appendNullBitmap(!rowCol.isNull(i))
 			chkCol.length++
 
@@ -205,9 +205,8 @@ func appendPartialRows(colIdx, outerIdx int, chkForJoin, chk *Chunk, selected []
 				chkCol.offsets = append(chkCol.offsets, int32(len(chkCol.data)))
 			}
 		}
-		rowNum++
 	}
-	return rowNum
+	return chk.NumRows() - oldLen
 }
 
 // appendPartialSameRows appends same row to the chunk with `rowNum` times.
diff --git a/util/chunk/chunk_test.go b/util/chunk/chunk_test.go
index 2b69cd980faca..efdbc6c8b1b25 100644
--- a/util/chunk/chunk_test.go
+++ b/util/chunk/chunk_test.go
@@ -16,6 +16,7 @@ package chunk
 import (
 	"fmt"
 	"math"
+	"reflect"
 	"testing"
 	"time"
 	"unsafe"
@@ -26,7 +27,6 @@ import (
 	"github.com/pingcap/tidb/types"
 	"github.com/pingcap/tidb/types/json"
 	"github.com/pingcap/tidb/util/hack"
-	"reflect"
 )
 
 func TestT(t *testing.T) {