expression: function format incompatible

expression/builtin_string.go

@@ -3019,6 +3019,54 @@ func (b *builtinFormatSig) evalString(row chunk.Row) (string, bool, error) {
 	if isNull || err != nil {
 		return "", isNull, err
 	}
+	if strings.Contains(x, ".") {
+		xArr := strings.Split(x, ".")
+		x1 := xArr[0]
+		x2 := xArr[1]
+		digit, err := strconv.Atoi(d)
+		if err != nil {
+			return "", isNull, err
+		}
+		if len(x2) > digit {
+			t := []byte(x2)
+			carry := false
+			if t[digit] >= '5' {
+				carry = true
+			}
+			if carry == true {
+				for i := digit - 1; i >= 0; i-- {
+					if t[i] == '9' && carry == true {
+						t[i] = '0'
+						carry = true
+					} else if carry == true {
+						t[i] = t[i] + 1
+						carry = false
+					}
+				}
+			}
+			x2 = string(t)
+			t = []byte(x1)
+			if carry == true {
+				for i := len(x1) - 1; i >= 0; i-- {
+					if t[i] == '9' && carry == true {
+						t[i] = '0'
+						carry = true
+					} else if carry == true {
+						t[i] = t[i] + 1
+						carry = false
+					}
+				}
+			}
+			if carry == true {
+				x1 = "1" + string(t)
+			} else {
+				x1 = string(t)
+			}
+		}
+
+		x = x1 + "." + x2
+	}
+
 	formatString, err := mysql.GetLocaleFormatFunction("en_US")(x, d)
 	return formatString, false, err
 }

expression/builtin_string_test.go

@@ -1608,6 +1608,12 @@ func (s *testEvaluatorSuite) TestFormat(c *C) {
 		ret       interface{}
 		warnings  int
 	}{
+		// issue #8796
+		{1.12345, 4, "1.1235", 0},
+		{9.99999, 4, "10.0000", 0},
+		{1.99999, 4, "2.0000", 0},
+		{1.09999, 4, "1.1000", 0},
+
 		{12332.123444, 4, "12,332.1234", 0},
 		{12332.123444, 0, "12,332", 0},
 		{12332.123444, -4, "12,332", 0},