*: fix 3235 (#3358)

executor/write_test.go

@@ -137,6 +137,14 @@ func (s *testSuite) TestInsert(c *C) {
 	tk.MustExec("update t t1 set id = (select count(*) + 1 from t t2 where t1.id = t2.id)")
 	r = tk.MustQuery("select * from t;")
 	r.Check(testkit.Rows("2"))
+
+	// issue 3235
+	tk.MustExec("drop table if exists t")
+	tk.MustExec("create table t(c decimal(5, 5))")
+	_, err = tk.Exec("insert into t value(0)")
+	c.Assert(err, IsNil)
+	_, err = tk.Exec("insert into t value(1)")
+	c.Assert(types.ErrOverflow.Equal(err), IsTrue)
 }
 
 func (s *testSuite) TestInsertAutoInc(c *C) {

parser/parser.y

@@ -3944,6 +3944,8 @@ CastType:
 		if fopt.Flen == types.UnspecifiedLength {
 			x.Flen = mysql.GetDefaultFieldLength(mysql.TypeNewDecimal)
 			x.Decimal = mysql.GetDefaultDecimal(mysql.TypeNewDecimal)
+		} else if fopt.Decimal == types.UnspecifiedLength {
+			x.Decimal = mysql.GetDefaultDecimal(mysql.TypeNewDecimal)
 		}
 		$$ = x
 	}

util/types/datum.go

@@ -1048,20 +1048,29 @@ func (d *Datum) convertToMysqlDecimal(sc *variable.StatementContext, target *Fie
 	default:
 		return invalidConv(d, target.Tp)
 	}
-	if target.Flen != UnspecifiedLength && target.Decimal != UnspecifiedLength {
+	dec, err = produceDecWithSpecifiedTp(dec, target, sc)
+	ret.SetValue(dec)
+	return ret, errors.Trace(err)
+}
+
+// produceDecToSpecifiedTp produces a new decimal according to `Tp`.
+func produceDecWithSpecifiedTp(dec *MyDecimal, tp *FieldType, sc *variable.StatementContext) (_ *MyDecimal, err error) {
+	flen, decimal := tp.Flen, tp.Decimal
+	if flen != UnspecifiedLength && decimal != UnspecifiedLength {
 		prec, frac := dec.PrecisionAndFrac()
-		if prec-frac > target.Flen-target.Decimal {
-			dec = NewMaxOrMinDec(dec.IsNegative(), target.Flen, target.Decimal)
-			err = ErrOverflow.GenByArgs("DECIMAL", fmt.Sprintf("(%d, %d)", target.Flen, target.Decimal))
-		} else if frac != target.Decimal {
-			dec.Round(dec, target.Decimal, ModeHalfEven)
-			if frac > target.Decimal {
-				err = errors.Trace(handleTruncateError(sc))
+		if !dec.IsZero() && prec-frac > flen-decimal {
+			dec = NewMaxOrMinDec(dec.IsNegative(), flen, decimal)
+			// TODO: we may need a OverlowAsWarning.
+			// select (cast 111 as decimal(1)) causes a warning in MySQL.
+			err = ErrOverflow.GenByArgs("DECIMAL", fmt.Sprintf("(%d, %d)", flen, decimal))
+		} else if frac != decimal {
+			dec.Round(dec, decimal, ModeHalfEven)
+			if !dec.IsZero() && frac > decimal {
+				err = sc.HandleTruncate(ErrTruncated)
 			}
 		}
 	}
-	ret.SetValue(dec)
-	return ret, err
+	return dec, errors.Trace(err)
 }
 
 func (d *Datum) convertToMysqlYear(sc *variable.StatementContext, target *FieldType) (Datum, error) {

util/types/mydecimal.go

@@ -1182,6 +1182,18 @@ func (d *MyDecimal) PrecisionAndFrac() (precision, frac int) {
 	return
 }
 
+// IsZero checks whether it's a zero decimal.
+func (d *MyDecimal) IsZero() bool {
+	isZero := true
+	for _, val := range d.wordBuf {
+		if val != 0 {
+			isZero = false
+			break
+		}
+	}
+	return isZero
+}
+
 // FromBin Restores decimal from its binary fixed-length representation.
 func (d *MyDecimal) FromBin(bin []byte, precision, frac int) (binSize int, err error) {
 	if len(bin) == 0 {