From 44dd0bcf8a0040ef1b003a20b1a0edf565d04b32 Mon Sep 17 00:00:00 2001
From: crazycs520 <crazycs520@gmail.com>
Date: Tue, 14 May 2019 20:26:40 +0800
Subject: [PATCH] ddl: prohibit modification decimal precision to avoid
 inconsistency problem (#10433)

---
 ddl/column_test.go     |  2 ++
 ddl/ddl_api.go         |  4 ++++
 executor/admin_test.go | 10 ++++++++++
 3 files changed, 16 insertions(+)

diff --git a/ddl/column_test.go b/ddl/column_test.go
index 0da79df1c9087..e5e649559d0fa 100644
--- a/ddl/column_test.go
+++ b/ddl/column_test.go
@@ -935,6 +935,8 @@ 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)
diff --git a/ddl/ddl_api.go b/ddl/ddl_api.go
index 8d30544a5c2bd..7d79e415cb349 100644
--- a/ddl/ddl_api.go
+++ b/ddl/ddl_api.go
@@ -1670,6 +1670,10 @@ 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)
diff --git a/executor/admin_test.go b/executor/admin_test.go
index b3da8719c2a6e..ac5210e74a0b7 100644
--- a/executor/admin_test.go
+++ b/executor/admin_test.go
@@ -515,6 +515,16 @@ 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) TestAdminCheckPrimaryIndex(c *C) {