ddl: prohibit modification decimal precision to avoid inconsistency p…

…roblem (pingcap#10433)

ddl/column_test.go

@@ -926,6 +926,8 @@ func (s *testColumnSuite) TestModifyColumn(c *C) {
 		{"varchar(10)", "varchar(8)", errUnsupportedModifyColumn.GenByArgs("length 8 is less than origin 10")},
 		{"varchar(10)", "varchar(11)", nil},
 		{"varchar(10) character set utf8 collate utf8_bin", "varchar(10) character set utf8", nil},
+		{"decimal(2,1)", "decimal(3,2)", errUnsupportedModifyColumn.Gen("unsupported modify decimal column precision")},
+		{"decimal(2,1)", "decimal(2,1)", nil},
 	}
 	for _, tt := range tests {
 		ftA := s.colDefStrToFieldType(c, tt.origin)

ddl/ddl_api.go

@@ -1328,6 +1328,10 @@ func (d *ddl) DropColumn(ctx sessionctx.Context, ti ast.Ident, colName model.CIS
 // It returns true if the two types has the same Charset and Collation, the same sign, both are
 // integer types or string types, and new Flen and Decimal must be greater than or equal to origin.
 func modifiable(origin *types.FieldType, to *types.FieldType) error {
+	// The root cause is modifying decimal precision needs to rewrite binary representation of that decimal.
+	if origin.Tp == mysql.TypeNewDecimal && (to.Flen != origin.Flen || to.Decimal != origin.Decimal) {
+		return errUnsupportedModifyColumn.Gen("unsupported modify decimal column precision")
+	}
 	if to.Flen > 0 && to.Flen < origin.Flen {
 		msg := fmt.Sprintf("length %d is less than origin %d", to.Flen, origin.Flen)
 		return errUnsupportedModifyColumn.GenByArgs(msg)

executor/admin_test.go

@@ -466,6 +466,16 @@ func (s *testSuite) TestAdminCheckTable(c *C) {
 	tk.MustExec(`alter table t1 add column b timestamp not null;`)
 	tk.MustExec(`alter table t1 add index(b);`)
 	tk.MustExec(`admin check table t1;`)
+
+	// Test for index with change decimal precision.
+	tk.MustExec(`drop table if exists t1`)
+	tk.MustExec(`create table t1 (a decimal(2,1), index(a))`)
+	tk.MustExec(`insert into t1 set a='1.9'`)
+	_, err := tk.Exec(`alter table t1 modify column a decimal(3,2);`)
+	c.Assert(err, NotNil)
+	c.Assert(err.Error(), Equals, "[ddl:203]unsupported modify decimal column precision")
+	tk.MustExec(`delete from t1;`)
+	tk.MustExec(`admin check table t1;`)
 }
 
 func (s *testSuite) TestAdminShowNextID(c *C) {