If the variable has been set, it USES the value as the automatic response and doesn't bother showing the message.
如果这个变量已经被设置过，它就使用这个值作为自动响应，而不显示这个消息。
2
SP_AUTOEXPAND: The default value is 1, which means enable automatic space expansion; 0 means disable automatic space expansion.
SP_AUTOEXPAND：默认值为1，它意味着支持自动空间扩展；0意味着不支持自动空间扩展。
3
For example, if you set AUTO_MAINT to a value of OFF, all other automatic maintenance parameters are considered to have an effective value of OFF, regardless of whether or not they are set to on.