diff --git a/expression/builtin_time.go b/expression/builtin_time.go
index 32a35e021fd25..7f953ce1cda91 100644
--- a/expression/builtin_time.go
+++ b/expression/builtin_time.go
@@ -1684,7 +1684,6 @@ func (c *fromUnixTimeFunctionClass) getFunction(ctx sessionctx.Context, args []E
 	}
 
 	if len(args) > 1 {
-		bf.tp.Flen = args[1].GetType().Flen
 		sig = &builtinFromUnixTime2ArgSig{bf}
 		sig.setPbCode(tipb.ScalarFuncSig_FromUnixTime2Arg)
 		return sig, nil
diff --git a/expression/integration_test.go b/expression/integration_test.go
index d24c10d0fcff0..c45990245334e 100755
--- a/expression/integration_test.go
+++ b/expression/integration_test.go
@@ -6684,6 +6684,23 @@ func (s *testIntegrationSuite) TestIssue18515(c *C) {
 	tk.MustExec("select /*+ TIDB_INLJ(t2) */ t1.a, t1.c, t2.a from t t1, t t2 where t1.c=t2.c;")
 }
 
+func (s *testIntegrationSuite) TestIssue20223(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists t")
+	tk.MustExec("CREATE TABLE t (" +
+		"id int(10) unsigned NOT NULL AUTO_INCREMENT," +
+		"type tinyint(4) NOT NULL," +
+		"create_time int(11) NOT NULL," +
+		"PRIMARY KEY (id)" +
+		")")
+	tk.MustExec("insert into t values (4, 2, 1598584933)")
+	tk.MustQuery("select from_unixtime(create_time,'%Y-%m-%d') as t_day,count(*) as cnt from t where `type` = 1 " +
+		"group by t_day union all " +
+		"select from_unixtime(create_time,'%Y-%m-%d') as t_day,count(*) as cnt from t where `type` = 2 " +
+		"group by t_day").Check(testkit.Rows("2020-08-28 1"))
+}
+
 func (s *testIntegrationSuite) TestIssue18525(c *C) {
 	tk := testkit.NewTestKit(c, s.store)
 	tk.MustExec("use test")
diff --git a/expression/typeinfer_test.go b/expression/typeinfer_test.go
index 51d30d059edc0..b1c2be753779b 100644
--- a/expression/typeinfer_test.go
+++ b/expression/typeinfer_test.go
@@ -1864,7 +1864,7 @@ func (s *testInferTypeSuite) createTestCase4TimeFuncs() []typeInferTestCase {
 		{"from_unixtime(20170101.999)", mysql.TypeDatetime, charset.CharsetBin, mysql.BinaryFlag, mysql.MaxDatetimeWidthWithFsp, 3},
 		{"from_unixtime(20170101.1234567)", mysql.TypeDatetime, charset.CharsetBin, mysql.BinaryFlag, mysql.MaxDatetimeWidthWithFsp, int(types.MaxFsp)},
 		{"from_unixtime('20170101.999')", mysql.TypeDatetime, charset.CharsetBin, mysql.BinaryFlag, mysql.MaxDatetimeWidthWithFsp, int(types.MaxFsp)},
-		{"from_unixtime(20170101.123, '%H')", mysql.TypeVarString, charset.CharsetUTF8MB4, 0, 2, types.UnspecifiedLength},
+		{"from_unixtime(20170101.123, '%H')", mysql.TypeVarString, charset.CharsetUTF8MB4, 0, -1, types.UnspecifiedLength},
 
 		{"extract(day from c_char)", mysql.TypeLonglong, charset.CharsetBin, mysql.BinaryFlag, mysql.MaxIntWidth, 0},
 		{"extract(hour from c_char)", mysql.TypeLonglong, charset.CharsetBin, mysql.BinaryFlag, mysql.MaxIntWidth, 0},