From df761ba015717f96ad22be28e03b853a68da183a Mon Sep 17 00:00:00 2001
From: ti-srebot <66930949+ti-srebot@users.noreply.github.com>
Date: Thu, 3 Sep 2020 21:38:08 +0800
Subject: [PATCH] ddl: forbidden changing decimal to int (#19645) (#19682)

---
 ddl/column_test.go | 3 +++
 ddl/ddl_api.go     | 3 +++
 2 files changed, 6 insertions(+)

diff --git a/ddl/column_test.go b/ddl/column_test.go
index d16a1dbd24873..af373407c0f82 100644
--- a/ddl/column_test.go
+++ b/ddl/column_test.go
@@ -948,6 +948,9 @@ func (s *testColumnSuite) TestModifyColumn(c *C) {
 		{"decimal(2,1)", "decimal(3,2)", errUnsupportedModifyColumn.GenWithStackByArgs("can't change decimal column precision")},
 		{"decimal(2,1)", "decimal(2,2)", errUnsupportedModifyColumn.GenWithStackByArgs("can't change decimal column precision")},
 		{"decimal(2,1)", "decimal(2,1)", nil},
+		{"decimal(2,1)", "int", errUnsupportedModifyColumn.GenWithStackByArgs("type int(11) not match origin decimal(2,1)")},
+		{"decimal", "int", errUnsupportedModifyColumn.GenWithStackByArgs("type int(11) not match origin decimal(11,0)")},
+		{"decimal(2,1)", "bigint", errUnsupportedModifyColumn.GenWithStackByArgs("type bigint(20) not match origin decimal(2,1)")},
 	}
 	for _, tt := range tests {
 		ftA := s.colDefStrToFieldType(c, tt.origin)
diff --git a/ddl/ddl_api.go b/ddl/ddl_api.go
index 0ce48000220c8..034a8e9ef6b3c 100644
--- a/ddl/ddl_api.go
+++ b/ddl/ddl_api.go
@@ -2750,6 +2750,9 @@ func checkModifyTypes(origin *types.FieldType, to *types.FieldType, needRewriteC
 			}
 		}
 	case mysql.TypeNewDecimal:
+		if origin.Tp != to.Tp {
+			return errUnsupportedModifyColumn.GenWithStackByArgs(unsupportedMsg)
+		}
 		// The root cause is modifying decimal precision needs to rewrite binary representation of that decimal.
 		if to.Flen != origin.Flen || to.Decimal != origin.Decimal {
 			return errUnsupportedModifyColumn.GenWithStackByArgs("can't change decimal column precision")