diff --git a/executor/window_test.go b/executor/window_test.go
index 970d7f7354100..364d4d0682b2a 100644
--- a/executor/window_test.go
+++ b/executor/window_test.go
@@ -470,3 +470,23 @@ func (s *testSuite7) TestIssue24264(c *C) {
 		"8297270320597030697",
 		"<nil>"))
 }
+
+func (s *testSuite7) TestIssue29947(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("use test")
+	tk.MustExec(`drop table if exists t_tir89b, t_vejdy`)
+
+	tk.MustExec("CREATE TABLE `t_tir89b` (`c_3pcik` int(11) DEFAULT NULL,`c_0b6nxb` text DEFAULT NULL,`c_qytrlc` double NOT NULL,`c_sroc_c` int(11) DEFAULT NULL,PRIMARY KEY (`c_qytrlc`) /*T![clustered_index] NONCLUSTERED */	) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin;")
+
+	tk.MustExec(`INSERT INTO t_tir89b VALUES (66,'cjd1o',87.77,NULL),(134217728,'d_unpd',76.66,NULL),(50,'_13gs',1.46,32),(49,'xclvsc',64.7,48),(7,'1an13',70.86,7),(29,NULL,6.26,6),(8,'hc485b',47.44,2),(84,'d_nlmd',99.3,76),(14,'lbny1c',61.1,47),(45,'9r5bid',25.37,95),(49,'jbz5r',72.99,49),(18,'uode3d',7.21,992),(-8945040,'ftrtib',47.47,20),(29,'algrj',6.28,24),(96,NULL,67.83,24),(5,'s1gfz',89.18,78),(74,'ggqbl',83.89,68),(61,'5n1q7',26.92,6),(10,'4gflb',33.84,28),(48,'xoe0cd',84.71,77),(6,'xkh6i',53.83,19),(5,NULL,89.1,46),(49,'4q6nx',31.5,384),(1,'pgs1',66.8,77),(19,'lltflc',33.49,63),(87,'vd4htc',39.92,-5367008),(47,NULL,28.3,10),(29,'15jqfc',100.11,64),(45,'ii6pm',52.41,61),(0,NULL,85.27,19),(104,'ikpxnb',40.66,955),(40,'gzryzd',36.23,42),(18,'7UPNE',84.27,14),(32,NULL,84.8,53),(51,'2c5lfb',18.98,74),(97,NULL,22.89,6),(70,'guyzyc',96.29,89),(34,'dvdoqb',53.82,1),(94,'6eop6b',81.77,90),(42,'p7vsnd',62.54,NULL);`)
+
+	tk.MustExec("CREATE TABLE `t_vejdy` (`c_iovir` int(11) NOT NULL,`c_r_mw3d` double DEFAULT NULL,`c_uxhghb` int(11) DEFAULT NULL,`c_rb7otb` int(11) NOT NULL,`c_dplyac` int(11) DEFAULT NULL,`c_lmcqed` double DEFAULT NULL,`c_ayaoed` text DEFAULT NULL,`c__zbqr` int(11) DEFAULT NULL,PRIMARY KEY (`c_iovir`,`c_rb7otb`) /*T![clustered_index] NONCLUSTERED */,KEY `t_e1ejcd` (`c_uxhghb`),KEY `t_o6ui_b` (`c_iovir`,`c_r_mw3d`,`c_uxhghb`,`c_rb7otb`,`c_dplyac`,`c_lmcqed`)) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin;")
+
+	tk.MustExec(`INSERT INTO t_vejdy VALUES (49,100.11,68,57,44,17.93,NULL,84),(38,56.91,78,30,0,53.28,'cjd1o',2),(6,NULL,NULL,88,81,93.47,'0jftkb',54),(73,91.51,31,82,3,38.12,'buesob',40),(7,26.73,7,78,9,NULL,'fd5kgd',49),(80,70.57,4,47,43,25.59,'glpoq',44),(79,94.16,15,0,0,79.55,'0ok94d',56),(58,NULL,50,69,2,65.46,'sm6rj',29),(41472,6.51,70,1080,100,43.18,'fofk4c',43),(0,6.2,57,97,2,56.17,'zqpzq',56),(72,76.66,97,88,95,75.47,'hikxqb',34),(27,1.11,134217728,57,25,NULL,'4gflb',0),(64,NULL,47,69,6,72.5,'w7jmhd',45),(-134217679,88.74,33,82,85,59.89,NULL,26),(59,97.98,37,28,33,61.1,'xioxdd',45),(6,47.31,0,0,-19,38.77,'uxmdlc',17),(82,28.62,36,70,39,11.79,'zzi8cc',2),(33,37.3,55,86,69,60.56,'mn_xx',0),(7,NULL,80,0,17,59.79,'5n1q7',97),(88,50.81,15,30,63,25.37,'ordwed',29),(48,4.32,90,48,38,84.62,'lclx',32),(10,NULL,95,75,1,21.64,NULL,85),(62,NULL,0,30,10,NULL,'7bacud',5),(50,38.81,6,0,6,64.28,'gpibn',57),(1,46.8,21,32,46,33.38,NULL,6),(29,NULL,38,7,91,31.5,'pdzdl',24),(54,6.26,1,85,22,75.63,'gl4_7',29),(1,90.37,63,63,6,61.2,'wvw23b',86),(47,NULL,82,73,0,95.79,'uipcf',NULL),(46,48.1,37,6,1,52.33,'gthpic',0),(41,75.1,7,44,5,84.16,'fe_e5',58),(43,87.71,81,32,28,91.98,'9e5nvc',66),(20,58.21,88,75,92,43.64,'kagroc',66),(91,52.75,22,14,80,NULL,'\'_YN6MD\'',6),(72,94.83,0,49,5,57.82,NULL,23),(7,100.11,0,92,13,6.28,NULL,0);`)
+
+	tk.MustExec("begin")
+	tk.MustExec("delete from t_tir89b where t_tir89b.c_3pcik >= t_tir89b.c_sroc_c;")
+	result := tk.MustQuery("select * from (select count(*) over (partition by ref_0.c_0b6nxb order by ref_0.c_3pcik) as c0 from t_tir89b as ref_0) as subq_0 where subq_0.c0 <> 1;")
+	result.Check(testkit.Rows("2", "3"))
+	tk.MustExec("commit")
+}
diff --git a/util/chunk/mutrow.go b/util/chunk/mutrow.go
index 46a9a3a1fe850..c6bb9d6614346 100644
--- a/util/chunk/mutrow.go
+++ b/util/chunk/mutrow.go
@@ -210,12 +210,19 @@ func makeMutRowBytesColumn(bin []byte) *Column {
 	return col
 }
 
+func cleanColOfMutRow(col *Column) {
+	for i := range col.offsets {
+		col.offsets[i] = 0
+	}
+	col.nullBitmap[0] = 0
+}
+
 // SetRow sets the MutRow with Row.
 func (mr MutRow) SetRow(row Row) {
 	for colIdx, rCol := range row.c.columns {
 		mrCol := mr.c.columns[colIdx]
+		cleanColOfMutRow(mrCol)
 		if rCol.IsNull(row.idx) {
-			mrCol.nullBitmap[0] = 0
 			continue
 		}
 		elemLen := len(rCol.elemBuf)
@@ -238,8 +245,8 @@ func (mr MutRow) SetValues(vals ...interface{}) {
 // SetValue sets the MutRow with colIdx and value.
 func (mr MutRow) SetValue(colIdx int, val interface{}) {
 	col := mr.c.columns[colIdx]
+	cleanColOfMutRow(col)
 	if val == nil {
-		col.nullBitmap[0] = 0
 		return
 	}
 	switch x := val.(type) {
@@ -285,8 +292,8 @@ func (mr MutRow) SetDatums(datums ...types.Datum) {
 // SetDatum sets the MutRow with colIdx and datum.
 func (mr MutRow) SetDatum(colIdx int, d types.Datum) {
 	col := mr.c.columns[colIdx]
+	cleanColOfMutRow(col)
 	if d.IsNull() {
-		col.nullBitmap[0] = 0
 		return
 	}
 	switch d.Kind() {
diff --git a/util/chunk/mutrow_test.go b/util/chunk/mutrow_test.go
index d55603ff848c6..15f8081ddffd1 100644
--- a/util/chunk/mutrow_test.go
+++ b/util/chunk/mutrow_test.go
@@ -22,6 +22,7 @@ import (
 	"github.com/pingcap/tidb/sessionctx/stmtctx"
 	"github.com/pingcap/tidb/types"
 	"github.com/pingcap/tidb/types/json"
+	"github.com/pingcap/tidb/util/hack"
 )
 
 func (s *testChunkSuite) TestMutRow(c *check.C) {
@@ -87,6 +88,27 @@ func (s *testChunkSuite) TestMutRow(c *check.C) {
 	c.Assert(chk.columns[0].data, check.BytesEquals, mutRow.c.columns[0].data)
 }
 
+func (s *testChunkSuite) TestIssue29947(c *check.C) {
+	mutRow := MutRowFromTypes(allTypes)
+	nilDatum := types.NewDatum(nil)
+
+	dataBefore := make([][]byte, 0, len(mutRow.c.columns))
+	elemBufBefore := make([][]byte, 0, len(mutRow.c.columns))
+	for _, col := range mutRow.c.columns {
+		dataBefore = append(dataBefore, col.data)
+		elemBufBefore = append(elemBufBefore, col.elemBuf)
+	}
+	for i, col := range mutRow.c.columns {
+		mutRow.SetDatum(i, nilDatum)
+		c.Assert(col.IsNull(0), check.IsTrue)
+		for _, off := range col.offsets {
+			c.Assert(off, check.Equals, int64(0))
+		}
+		c.Assert(hack.String(col.data), check.Equals, hack.String(dataBefore[i]))
+		c.Assert(hack.String(col.elemBuf), check.Equals, hack.String(elemBufBefore[i]))
+	}
+}
+
 func BenchmarkMutRowSetRow(b *testing.B) {
 	b.ReportAllocs()
 	rowChk := newChunk(8, 0)