code: func (c *Context) GetGlobalSysVar(ctx sessionctx.Context, name string) (string, error) {
  	v := variable.GetSysVar(name)
  	if v == nil {
  		return "", variable.UnknownSystemVar.GenWithStackByArgs(name)
  	}
  	return v.Value, nil
  }
docs: // GetGlobalSysVar implements GlobalVarAccessor GetGlobalSysVar interface.