ddl: disallow dropping auto_increment column attribute

ddl/db_integration_test.go

@@ -1542,11 +1542,15 @@ func (s *testIntegrationSuite3) TestAlterColumn(c *C) {
 	createSQL = result.Rows()[0][1]
 	expected = "CREATE TABLE `mc` (\n  `a` bigint(20) NOT NULL AUTO_INCREMENT,\n  `b` int(11) DEFAULT NULL,\n  PRIMARY KEY (`a`)\n) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin"
 	c.Assert(createSQL, Equals, expected)
-	s.tk.MustExec("alter table mc modify column a bigint") // Drops auto_increment
+	_, err = s.tk.Exec("alter table mc modify column a bigint") // Droppping auto_increment is not allow when @@tidb_allow_remove_auto_inc == 'off'
+	c.Assert(err, NotNil)
+	s.tk.MustExec("set @@tidb_allow_remove_auto_inc = on")
+	s.tk.MustExec("alter table mc modify column a bigint") // Dropping auto_increment is ok when @@tidb_allow_remove_auto_inc == 'on'
 	result = s.tk.MustQuery("show create table mc")
 	createSQL = result.Rows()[0][1]
 	expected = "CREATE TABLE `mc` (\n  `a` bigint(20) NOT NULL,\n  `b` int(11) DEFAULT NULL,\n  PRIMARY KEY (`a`)\n) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin"
 	c.Assert(createSQL, Equals, expected)
+
 	_, err = s.tk.Exec("alter table mc modify column a bigint auto_increment") // Adds auto_increment should throw error
 	c.Assert(err, NotNil)
 

ddl/ddl_api.go

@@ -2690,6 +2690,10 @@ func (d *ddl) getModifiableColumnJob(ctx sessionctx.Context, ident ast.Ident, or
 	if !mysql.HasAutoIncrementFlag(col.Flag) && mysql.HasAutoIncrementFlag(newCol.Flag) {
 		return nil, errUnsupportedModifyColumn.GenWithStackByArgs("set auto_increment")
 	}
+	// Disallow modifying column from auto_increment to not auto_increment if the session variable `AllowRemoveAutoInc` is false.
+	if !ctx.GetSessionVars().AllowRemoveAutoInc && mysql.HasAutoIncrementFlag(col.Flag) && !mysql.HasAutoIncrementFlag(newCol.Flag) {
+		return nil, errUnsupportedModifyColumn.GenWithStackByArgs("to remove auto_increment without @@tidb_allow_remove_auto_inc enabled")
+	}
 
 	// We support modifying the type definitions of 'null' to 'not null' now.
 	var modifyColumnTp byte

sessionctx/variable/session.go

@@ -417,6 +417,9 @@ type SessionVars struct {
 
 	// PrevStmt is used to store the previous executed statement in the current session.
 	PrevStmt string
+
+	// AllowRemoveAutoInc indicates whether a user can drop the auto_increment column attribute or not.
+	AllowRemoveAutoInc bool
 }
 
 // ConnectionInfo present connection used by audit.
@@ -473,6 +476,7 @@ func NewSessionVars() *SessionVars {
 		EnableIndexMerge:            false,
 		EnableNoopFuncs:             DefTiDBEnableNoopFuncs,
 		ReplicaRead:                 kv.ReplicaReadLeader,
+		AllowRemoveAutoInc:          DefTiDBAllowRemoveAutoInc,
 	}
 	vars.Concurrency = Concurrency{
 		IndexLookupConcurrency:     DefIndexLookupConcurrency,
@@ -857,6 +861,8 @@ func (s *SessionVars) SetSystemVar(name string, val string) error {
 		} else if strings.EqualFold(val, "leader") || len(val) == 0 {
 			s.ReplicaRead = kv.ReplicaReadLeader
 		}
+	case TiDBAllowRemoveAutoInc:
+		s.AllowRemoveAutoInc = TiDBOptOn(val)
 	}
 	s.systems[name] = val
 	return nil

sessionctx/variable/sysvar.go

@@ -706,6 +706,7 @@ var defaultSysVars = []*SysVar{
 	{ScopeSession, TiDBExpensiveQueryTimeThreshold, strconv.Itoa(DefTiDBExpensiveQueryTimeThreshold)},
 	{ScopeGlobal | ScopeSession, TiDBEnableNoopFuncs, BoolToIntStr(DefTiDBEnableNoopFuncs)},
 	{ScopeSession, TiDBReplicaRead, "leader"},
+	{ScopeSession, TiDBAllowRemoveAutoInc, BoolToIntStr(DefTiDBAllowRemoveAutoInc)},
 }
 
 // SynonymsSysVariables is synonyms of system variables.

sessionctx/variable/tidb_vars.go

@@ -145,6 +145,9 @@ const (
 
 	// TiDBReplicaRead is used for reading data from replicas, followers for example.
 	TiDBReplicaRead = "tidb_replica_read"
+
+	// TiDBAllowRemoveAutoInc indicates whether a user can drop the auto_increment column attribute or not.
+	TiDBAllowRemoveAutoInc = "tidb_allow_remove_auto_inc"
 )
 
 // TiDB system variable names that both in session and global scope.
@@ -358,6 +361,7 @@ const (
 	DefTiDBWaitSplitRegionFinish       = true
 	DefWaitSplitRegionTimeout          = 300 // 300s
 	DefTiDBEnableNoopFuncs             = false
+	DefTiDBAllowRemoveAutoInc          = false
 )
 
 // Process global variables.

sessionctx/variable/varsutil.go

@@ -574,6 +574,14 @@ func ValidateSetSystemVar(vars *SessionVars, name string, value string) (string,
 		default:
 			return value, ErrWrongValueForVar.GenWithStackByArgs(TiDBTxnMode, value)
 		}
+	case TiDBAllowRemoveAutoInc:
+		switch {
+		case strings.EqualFold(value, "ON") || value == "1":
+			return "on", nil
+		case strings.EqualFold(value, "OFF") || value == "0":
+			return "off", nil
+		}
+		return value, ErrWrongValueForVar.GenWithStackByArgs(name, value)
 	}
 	return value, nil
 }