types: Regard TypeNewDecimal as not a hasVariantFieldLength type.

executor/seqtest/prepared_test.go

@@ -729,6 +729,20 @@ func (s *seqTestSuite) TestPreparedIssue8644(c *C) {
 
 		r := tk.MustQuery(`select * from t`)
 		r.Check(testkit.Rows("a", "aaaaaaaaaaaaaaaaaa"))
+
+		tk.MustExec("drop table if exists t")
+		tk.MustExec("create table t(data decimal)")
+
+		tk.MustExec(`prepare s1 from 'insert t (data) values (?)'`)
+
+		tk.MustExec(`set @a = '1'`)
+		tk.MustExec(`execute stmt1 using @a;`)
+
+		tk.MustExec(`set @b = '11111.11111'`) // '.11111' will be truncated.
+		tk.MustExec(`execute stmt1 using @b;`)
+
+		r = tk.MustQuery(`select * from t`)
+		r.Check(testkit.Rows("1", "11111"))
 	}
 }
 

types/field_type.go

@@ -185,7 +185,7 @@ func DefaultParamTypeForValue(value interface{}, tp *FieldType) {
 func hasVariantFieldLength(tp *FieldType) bool {
 	switch tp.Tp {
 	case mysql.TypeLonglong, mysql.TypeVarString, mysql.TypeDouble, mysql.TypeBlob,
-		mysql.TypeBit, mysql.TypeDuration, mysql.TypeNewDecimal, mysql.TypeEnum, mysql.TypeSet:
+		mysql.TypeBit, mysql.TypeDuration, mysql.TypeEnum, mysql.TypeSet:
 		return true
 	}
 	return false