diff --git a/expression/builtin_compare.go b/expression/builtin_compare.go
index e5c2dcb08fcfa..a3aa3e085b0c3 100644
--- a/expression/builtin_compare.go
+++ b/expression/builtin_compare.go
@@ -642,11 +642,7 @@ func (b *builtinGreatestTimeSig) Clone() builtinFunc {
 
 // evalString evals a builtinGreatestTimeSig.
 // See http://dev.mysql.com/doc/refman/5.7/en/comparison-operators.html#function_greatest
-func (b *builtinGreatestTimeSig) evalString(row chunk.Row) (res string, isNull bool, err error) {
-	var (
-		strRes  string
-		timeRes types.Time
-	)
+func (b *builtinGreatestTimeSig) evalString(row chunk.Row) (strRes string, isNull bool, err error) {
 	sc := b.ctx.GetSessionVars().StmtCtx
 	for i := 0; i < len(b.args); i++ {
 		v, isNull, err := b.args[i].EvalString(b.ctx, row)
@@ -665,16 +661,8 @@ func (b *builtinGreatestTimeSig) evalString(row chunk.Row) (res string, isNull b
 		if i == 0 || strings.Compare(v, strRes) > 0 {
 			strRes = v
 		}
-		if i == 0 || t.Compare(timeRes) > 0 {
-			timeRes = t
-		}
-	}
-	if timeRes.IsZero() {
-		res = strRes
-	} else {
-		res = timeRes.String()
 	}
-	return res, false, nil
+	return strRes, false, nil
 }
 
 type leastFunctionClass struct {
@@ -860,12 +848,7 @@ func (b *builtinLeastTimeSig) Clone() builtinFunc {
 
 // evalString evals a builtinLeastTimeSig.
 // See http://dev.mysql.com/doc/refman/5.7/en/comparison-operators.html#functionleast
-func (b *builtinLeastTimeSig) evalString(row chunk.Row) (res string, isNull bool, err error) {
-	var (
-		// timeRes will be converted to a strRes only when the arguments is a valid datetime value.
-		strRes  string     // Record the strRes of each arguments.
-		timeRes types.Time // Record the time representation of a valid arguments.
-	)
+func (b *builtinLeastTimeSig) evalString(row chunk.Row) (strRes string, isNull bool, err error) {
 	sc := b.ctx.GetSessionVars().StmtCtx
 	for i := 0; i < len(b.args); i++ {
 		v, isNull, err := b.args[i].EvalString(b.ctx, row)
@@ -883,17 +866,9 @@ func (b *builtinLeastTimeSig) evalString(row chunk.Row) (res string, isNull bool
 		if i == 0 || strings.Compare(v, strRes) < 0 {
 			strRes = v
 		}
-		if i == 0 || t.Compare(timeRes) < 0 {
-			timeRes = t
-		}
 	}
 
-	if timeRes.IsZero() {
-		res = strRes
-	} else {
-		res = timeRes.String()
-	}
-	return res, false, nil
+	return strRes, false, nil
 }
 
 type intervalFunctionClass struct {
diff --git a/expression/builtin_compare_test.go b/expression/builtin_compare_test.go
index 9aa5a7a984598..f9ebb4c4c548c 100644
--- a/expression/builtin_compare_test.go
+++ b/expression/builtin_compare_test.go
@@ -312,11 +312,11 @@ func TestGreatestLeastFunc(t *testing.T) {
 		},
 		{
 			[]interface{}{tm, "invalid_time_1", "invalid_time_2", tmWithFsp},
-			curTimeWithFspString, curTimeString, false, false,
+			"invalid_time_2", curTimeString, false, false,
 		},
 		{
 			[]interface{}{tm, "invalid_time_2", "invalid_time_1", tmWithFsp},
-			curTimeWithFspString, curTimeString, false, false,
+			"invalid_time_2", curTimeString, false, false,
 		},
 		{
 			[]interface{}{tm, "invalid_time", nil, tmWithFsp},
diff --git a/expression/integration_test.go b/expression/integration_test.go
index 59984a2c97f61..d071bcc4ec7a1 100644
--- a/expression/integration_test.go
+++ b/expression/integration_test.go
@@ -3942,8 +3942,7 @@ func (s *testIntegrationSuite) TestCompareBuiltin(c *C) {
 	result.Check(testkit.Rows("3 c 1.3 2"))
 	tk.MustQuery("show warnings").Check(testkit.Rows())
 	result = tk.MustQuery(`select greatest(cast("2017-01-01" as datetime), "123", "234", cast("2018-01-01" as date)), greatest(cast("2017-01-01" as date), "123", null)`)
-	// todo: MySQL returns "2018-01-01 <nil>"
-	result.Check(testkit.Rows("2018-01-01 00:00:00 <nil>"))
+	result.Check(testkit.Rows("234 <nil>"))
 	tk.MustQuery("show warnings").Check(testutil.RowsWithSep("|", "Warning|1292|Incorrect time value: '123'", "Warning|1292|Incorrect time value: '234'", "Warning|1292|Incorrect time value: '123'"))
 	// for least
 	result = tk.MustQuery(`select least(1, 2, 3), least("a", "b", "c"), least(1.1, 1.2, 1.3), least("123a", 1, 2)`)
@@ -10433,6 +10432,24 @@ func (s *testIntegrationSuite) TestIssue28643(c *C) {
 	tk.MustQuery("select hour(a) from t;").Check(testkit.Rows("838", "838"))
 }
 
+func (s *testIntegrationSuite) TestIssue29434(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("use test")
+
+	tk.MustExec("drop table if exists t1;")
+	tk.MustExec("create table t1(c1 datetime);")
+	tk.MustExec("insert into t1 values('2021-12-12 10:10:10.000');")
+	tk.MustExec("set tidb_enable_vectorized_expression = on;")
+	tk.MustQuery("select greatest(c1, '99999999999999') from t1;").Check(testkit.Rows("99999999999999"))
+	tk.MustExec("set tidb_enable_vectorized_expression = off;")
+	tk.MustQuery("select greatest(c1, '99999999999999') from t1;").Check(testkit.Rows("99999999999999"))
+
+	tk.MustExec("set tidb_enable_vectorized_expression = on;")
+	tk.MustQuery("select least(c1, '99999999999999') from t1;").Check(testkit.Rows("2021-12-12 10:10:10"))
+	tk.MustExec("set tidb_enable_vectorized_expression = off;")
+	tk.MustQuery("select least(c1, '99999999999999') from t1;").Check(testkit.Rows("2021-12-12 10:10:10"))
+}
+
 func (s *testIntegrationSuite) TestIssue29244(c *C) {
 	tk := testkit.NewTestKit(c, s.store)
 	tk.MustExec("use test")