GAE的严重问题

在你动手开发GAE应用之前，最好先考虑下你的应用是否真的可以跑在GAE上，如果不仔细看它的开发手册，你很可能会漏掉一个严重的问题：

Inequality Filters Are Allowed On One Property Only

A query may only use inequality filters (<, <=, >=, >) on one property across all of its filters.

For example, this query is allowed:

select from Person where birthYear >= minBirthYearParam
&& birthYear <= maxBirthYearParam

However, this query is not allowed, because it uses inequality filters on two different properties in the same query:

select from Person where birthYear >= minBirthYearParam
&& height >= minHeightParam // ERROR

Filters can combine equal (==) comparisons for different properties in the same query, including queries with one or more inequality conditions on a property. This is allowed:

select from Person where lastName == lastNameParam
&& city == cityParam
&& birthYear >= minBirthYearParam

The query mechanism relies on all results for a query to be adjacent to one another in the index table, to avoid having to scan the entire table for results. A single index table cannot represent multiple inequality filters on multiple properties while maintaining that all results are consecutive in the table.

这个限制很直接，就是非等于的过滤条件只能应用到一个字段（属性）上，原来在关系型数据库上很简单的一些查询，到了GAE上就变得不可行了。

在开始开发GAE应用前，先考虑清楚你的应用是否会不可避免的碰到这个问题，或者是否可以放弃很多功能，看来便宜不是那么好占的。

作者: Cherami
原载: GAE的严重问题
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