ddl: add a switch for multi schema change & make it compatible with old test cases

ddl/ddl_api.go

@@ -2315,6 +2315,9 @@ func (d *ddl) AlterTable(ctx sessionctx.Context, ident ast.Ident, specs []*ast.A
 	}
 
 	if len(validSpecs) > 1 {
+		if !ctx.GetSessionVars().EnableChangeMultiSchema {
+			return errRunMultiSchemaChanges
+		}
 		if isSameTypeMultiSpecs(validSpecs) {
 			switch validSpecs[0].Tp {
 			case ast.AlterTableAddColumns:

session/bootstrap.go

@@ -1335,6 +1335,13 @@ func doDMLWorks(s Session) {
 					vVal = string(variable.DynamicOnly)
 				}
 			}
+			if v.Name == variable.TiDBEnableChangeMultiSchema {
+				vVal = variable.BoolOff
+				if flag.Lookup("test.v") != nil || flag.Lookup("check.v") != nil {
+					// enable change multi schema in test case for compatibility with old cases.
+					vVal = variable.BoolOn
+				}
+			}
 			value := fmt.Sprintf(`("%s", "%s")`, strings.ToLower(k), vVal)
 			values = append(values, value)
 		}

session/session.go

@@ -2283,6 +2283,7 @@ var builtinGlobalVariable = []string{
 	variable.TiDBEnableTelemetry,
 	variable.TiDBShardAllocateStep,
 	variable.TiDBEnableChangeColumnType,
+	variable.TiDBEnableChangeMultiSchema,
 	variable.TiDBEnableAmendPessimisticTxn,
 	variable.TiDBMemoryUsageAlarmRatio,
 	variable.TiDBEnableRateLimitAction,

sessionctx/variable/session.go

@@ -572,6 +572,9 @@ type SessionVars struct {
 	// EnableChangeColumnType is used to control whether to enable the change column type.
 	EnableChangeColumnType bool
 
+	// EnableChangeMultiSchema is used to control whether to enable the multi schema change.
+	EnableChangeMultiSchema bool
+
 	// WaitSplitRegionFinish defines the split region behaviour is sync or async.
 	WaitSplitRegionFinish bool
 
@@ -907,6 +910,7 @@ func NewSessionVars() *SessionVars {
 		EnableParallelApply:          DefTiDBEnableParallelApply,
 		ShardAllocateStep:            DefTiDBShardAllocateStep,
 		EnableChangeColumnType:       DefTiDBChangeColumnType,
+		EnableChangeMultiSchema:      DefTiDBChangeMultiSchema,
 		EnableAmendPessimisticTxn:    DefTiDBEnableAmendPessimisticTxn,
 		PartitionPruneMode:           *atomic2.NewString(DefTiDBPartitionPruneMode),
 		TxnScope:                     config.GetGlobalConfig().TxnScope,
@@ -1617,6 +1621,8 @@ func (s *SessionVars) SetSystemVar(name string, val string) error {
 		s.ShardAllocateStep = tidbOptInt64(val, DefTiDBShardAllocateStep)
 	case TiDBEnableChangeColumnType:
 		s.EnableChangeColumnType = TiDBOptOn(val)
+	case TiDBEnableChangeMultiSchema:
+		s.EnableChangeMultiSchema = TiDBOptOn(val)
 	case TiDBEnableAmendPessimisticTxn:
 		s.EnableAmendPessimisticTxn = TiDBOptOn(val)
 	case TiDBTxnScope:

sessionctx/variable/sysvar.go

@@ -1046,6 +1046,7 @@ var defaultSysVars = []*SysVar{
 	{Scope: ScopeSession, Name: TiDBDDLReorgPriority, Value: "PRIORITY_LOW"},
 	{Scope: ScopeGlobal, Name: TiDBMaxDeltaSchemaCount, Value: strconv.Itoa(DefTiDBMaxDeltaSchemaCount), Type: TypeUnsigned, MinValue: 100, MaxValue: 16384, AutoConvertOutOfRange: true},
 	{Scope: ScopeGlobal, Name: TiDBEnableChangeColumnType, Value: BoolToOnOff(DefTiDBChangeColumnType), Type: TypeBool},
+	{Scope: ScopeGlobal, Name: TiDBEnableChangeMultiSchema, Value: BoolToOnOff(DefTiDBChangeMultiSchema), Type: TypeBool},
 	{Scope: ScopeSession, Name: TiDBForcePriority, Value: mysql.Priority2Str[DefTiDBForcePriority]},
 	{Scope: ScopeSession, Name: TiDBEnableRadixJoin, Value: BoolToOnOff(DefTiDBUseRadixJoin), Type: TypeBool},
 	{Scope: ScopeGlobal | ScopeSession, Name: TiDBOptJoinReorderThreshold, Value: strconv.Itoa(DefTiDBOptJoinReorderThreshold), Type: TypeUnsigned, MinValue: 0, MaxValue: 63},

sessionctx/variable/tidb_vars.go

@@ -351,6 +351,9 @@ const (
 	// TiDBEnableChangeColumnType is used to control whether to enable the change column type.
 	TiDBEnableChangeColumnType = "tidb_enable_change_column_type"
 
+	// TiDBEnableChangeMultiSchema is used to control whether to enable the change multi schema.
+	TiDBEnableChangeMultiSchema = "tidb_enable_change_multi_schema"
+
 	// tidb_max_delta_schema_count defines the max length of deltaSchemaInfos.
 	// deltaSchemaInfos is a queue that maintains the history of schema changes.
 	TiDBMaxDeltaSchemaCount = "tidb_max_delta_schema_count"
@@ -560,6 +563,7 @@ const (
 	DefTiDBDDLErrorCountLimit           = 512
 	DefTiDBMaxDeltaSchemaCount          = 1024
 	DefTiDBChangeColumnType             = false
+	DefTiDBChangeMultiSchema            = false
 	DefTiDBHashAggPartialConcurrency    = ConcurrencyUnset
 	DefTiDBHashAggFinalConcurrency      = ConcurrencyUnset
 	DefTiDBWindowConcurrency            = ConcurrencyUnset