appengine 数据存储区 (java - JDO) 如何存储和返回数据，即是 FIFO 还是 LIFO？如果是先进先出或两者都不是，我怎样才能使它成为后进先出？
我只是想知道如果我按时间顺序存储一些数据，它会按照最近到最近或最近到最近的顺序返回吗？例如，如果我按以下顺序输入数据：A[1], B[2], ...，如果我对表进行简单的select查询，它会将列表返回为 {A, B, ...} 或 {..., B, A} 吗？假设我没有设置任何索引，即索引是由appengine 自动设置的。我的系统设计的一个关键部分取决于此信息，因此我想在继续之前确认此信息。任何帮助将不胜感激。我需要以 LIFO 格式检索数据，如何确保这一点？
- FIFO——先进先出
- LIFO——后进先出
I was just wondering if I store some data chronologically, will it be returned back in the order of most-recent to least-recent OR least-recent to most-recent? For e.g. If I input data in the following order: A[1], B[2], ..., if I do a simple select query of the table, will it return the list as {A, B, ...} or {..., B, A}? Assume that I haven't set any indexes i.e. indexing is automatically set by appengine. A critical part of my system's design depends on this information, so I want to confirm this information before I proceed. Any help will be highly appreciated. I need the data retrieval to be in LIFO format, how can I ensure this?
- FIFO - first in first out
- LIFO - last in first out
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我非常确定应用程序引擎数据存储区没有指定任何保证的顺序，并且您使用的持久性框架（JPA？JDO？）也没有指定任何保证顺序。因此，如果它很重要，则将时间戳与您插入的每个数据一起存储，并向查询添加一个
order by子句，以确保您以适当的顺序检索对象。I'm pretty sure that the app engine datastore doesn't specify any guaranteed order, and that the persistence framework you use (JPA? JDO?) doesn't either. So if it's critical, then store a timestamp along with every data you insert, and add an
order byclause to your queries to make sure you retrieve the objects in the appropriate order.如果不指定排序顺序，数据将按键排序返回。如果您没有指定键名称，则键的排序将按其数字 ID 进行排序，这些 ID 通常是递增的，但由于在运行实例之间分配 ID 的方式，不能保证单调性。如果您需要保证特定的顺序，请在该属性上存储时间戳和顺序。
If you don't specify a sort order, data will be returned sorted by key. If you haven't specified key names, the ordering of the keys will be by their numeric IDs, which are generally increasing but monotonicity is not guaranteed because of the way IDs are allocated among running instances. If you need to guarantee a specific ordering, store a timestamp and order on that property.
App Engine 返回的实体的默认排序顺序（如果您未指定另一个排序顺序）首先按您过滤的任何不等式排序，然后按键排序。如果您想按日期排序，则应包含日期字段并对其进行显式排序。
在keys_only查询中过滤实体的字段是完全有效的。
The default sort order for entities returned by App Engine - if you don't specify another one - is first by any inequality you've filtered on, then by key. If you want to order by date, you should include a date field and sort on it explicitly.
It's perfectly valid to filter on a field of an entity in a keys_only query.