code: func upgradeToVer2(s Session) {
  	// Version 2 add two system variable for DistSQL concurrency controlling.
  	// Insert distsql related system variable.
  	distSQLVars := []string{variable.TiDBDistSQLScanConcurrency}
  	values := make([]string, 0, len(distSQLVars))
  	for _, v := range distSQLVars {
  		value := fmt.Sprintf(`("%s", "%s")`, v, variable.SysVars[v].Value)
  		values = append(values, value)
  	}
  	sql := fmt.Sprintf("INSERT HIGH_PRIORITY IGNORE INTO %s.%s VALUES %s;", mysql.SystemDB, mysql.GlobalVariablesTable,
  		strings.Join(values, ", "))
  	mustExecute(s, sql)
  }
docs: // upgradeToVer2 updates to version 2.