sessionctx/variable: set DEFAULT_VALUE for 'tidb_txn_mode' (#48500)

close #48492

pkg/session/bootstrap.go

@@ -1041,11 +1041,15 @@ const (
 	// version 181
 	//   add column `bdr_role` to `mysql.tidb_ddl_job` and `mysql.tidb_ddl_history`.
 	version181 = 181
+
+	// version 182
+	//   set tidb_txn_mode to Optimistic when tidb_txn_mode is not set.
+	version182 = 182
 )
 
 // currentBootstrapVersion is defined as a variable, so we can modify its value for testing.
 // please make sure this is the largest version
-var currentBootstrapVersion int64 = version181
+var currentBootstrapVersion int64 = version182
 
 // DDL owner key's expired time is ManagerSessionTTL seconds, we should wait the time and give more time to have a chance to finish it.
 var internalSQLTimeout = owner.ManagerSessionTTL + 15
@@ -1202,6 +1206,7 @@ var (
 		upgradeToVer179,
 		upgradeToVer180,
 		upgradeToVer181,
+		upgradeToVer182,
 	}
 )
 
@@ -2948,6 +2953,16 @@ func upgradeToVer181(s sessiontypes.Session, ver int64) {
 	doReentrantDDL(s, "ALTER TABLE mysql.tidb_ddl_history ADD COLUMN `bdr_role` varchar(64)", infoschema.ErrColumnExists)
 }
 
+func upgradeToVer182(s sessiontypes.Session, ver int64) {
+	if ver >= version182 {
+		return
+	}
+	sql := fmt.Sprintf("INSERT HIGH_PRIORITY IGNORE INTO %s.%s VALUES('%s', '%s')",
+		mysql.SystemDB, mysql.GlobalVariablesTable,
+		variable.TiDBTxnMode, variable.OptimisticTxnMode)
+	mustExecute(s, sql)
+}
+
 func writeOOMAction(s sessiontypes.Session) {
 	comment := "oom-action is `log` by default in v3.0.x, `cancel` by default in v4.0.11+"
 	mustExecute(s, `INSERT HIGH_PRIORITY INTO %n.%n VALUES (%?, %?, %?) ON DUPLICATE KEY UPDATE VARIABLE_VALUE= %?`,

pkg/sessionctx/variable/sysvar.go

@@ -1908,7 +1908,7 @@ var defaultSysVars = []*SysVar{
 		s.ConstraintCheckInPlace = TiDBOptOn(val)
 		return nil
 	}},
-	{Scope: ScopeGlobal | ScopeSession, Name: TiDBTxnMode, Value: DefTiDBTxnMode, AllowEmptyAll: true, Type: TypeEnum, PossibleValues: []string{"pessimistic", "optimistic"}, SetSession: func(s *SessionVars, val string) error {
+	{Scope: ScopeGlobal | ScopeSession, Name: TiDBTxnMode, Value: DefTiDBTxnMode, AllowEmptyAll: true, Type: TypeEnum, PossibleValues: []string{PessimisticTxnMode, OptimisticTxnMode}, SetSession: func(s *SessionVars, val string) error {
 		s.TxnMode = strings.ToUpper(val)
 		return nil
 	}},
@@ -3001,6 +3001,11 @@ var SetCharsetVariables = []string{
 const (
 	// MaskPwd is the mask of password for LDAP variables.
 	MaskPwd = "******"
+
+	// PessimisticTxnMode is the name for tidb_txn_mode system variable.
+	PessimisticTxnMode = "pessimistic"
+	// OptimisticTxnMode is the name for tidb_txn_mode system variable.
+	OptimisticTxnMode = "optimistic"
 )
 
 const (

pkg/sessionctx/variable/tidb_vars.go

@@ -1223,7 +1223,7 @@ const (
 	DefTiFlashQuerySpillRatio                      = 0.7
 	DefTiDBEnableTiFlashPipelineMode               = true
 	DefTiDBMPPStoreFailTTL                         = "60s"
-	DefTiDBTxnMode                                 = ""
+	DefTiDBTxnMode                                 = PessimisticTxnMode
 	DefTiDBRowFormatV1                             = 1
 	DefTiDBRowFormatV2                             = 2
 	DefTiDBDDLReorgWorkerCount                     = 4

tests/integrationtest/r/infoschema/tables.result

@@ -230,7 +230,7 @@ innodb_compression_level	GLOBAL	6	8	NULL	NULL	NULL	YES
 SET GLOBAL innodb_compression_level = DEFAULT;
 SELECT * FROM variables_info WHERE variable_name = 'tidb_txn_mode';
 VARIABLE_NAME	VARIABLE_SCOPE	DEFAULT_VALUE	CURRENT_VALUE	MIN_VALUE	MAX_VALUE	POSSIBLE_VALUES	IS_NOOP
-tidb_txn_mode	SESSION,GLOBAL		pessimistic	NULL	NULL	pessimistic,optimistic	NO
+tidb_txn_mode	SESSION,GLOBAL	pessimistic	pessimistic	NULL	NULL	pessimistic,optimistic	NO
 SELECT * FROM variables_info WHERE variable_name = 'max_connections' AND is_noop='NO';
 VARIABLE_NAME	VARIABLE_SCOPE	DEFAULT_VALUE	CURRENT_VALUE	MIN_VALUE	MAX_VALUE	POSSIBLE_VALUES	IS_NOOP
 max_connections	INSTANCE	0	0	0	100000	NULL	NO