code: func (e *GrantExec) grantGlobalPriv(priv *ast.PrivElem, user *ast.UserSpec) error {
  	if priv.Priv == 0 {
  		return nil
  	}
  	asgns, err := composeGlobalPrivUpdate(priv.Priv, "Y")
  	if err != nil {
  		return err
  	}
  	sql := fmt.Sprintf(`UPDATE %s.%s SET %s WHERE User='%s' AND Host='%s'`, mysql.SystemDB, mysql.UserTable, asgns, user.User.Username, user.User.Hostname)
  	_, _, err = e.ctx.(sqlexec.RestrictedSQLExecutor).ExecRestrictedSQL(e.ctx, sql)
  	return err
  }
docs: // grantGlobalPriv manipulates mysql.user table.