ddl: prohibit modification decimal precision to avoid inconsistency p…

…roblem (#10433)

ddl/column_test.go

@@ -934,6 +934,8 @@ func (s *testColumnSuite) TestModifyColumn(c *C) {
  {"varchar(10)", "varchar(8)", errUnsupportedModifyColumn.GenWithStackByArgs("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.GenWithStack("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

@@ -2192,6 +2192,10 @@ func modifiableCharsetAndCollation(toCharset, toCollate, origCharset, origCollat
 // 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.GenWithStack("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.GenWithStackByArgs(msg)

executor/admin_test.go

@@ -505,6 +505,16 @@ func (s *testSuite1) 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 *testSuite1) TestAdminCheckPrimaryIndex(c *C) {