两个表必须为 InnoDB 类型， 外键和被引用键(referenced key)必须是索引中的第一(FIRST)列。 InnoDB 不会自动为外键和被引用键建立索引，必须明确创建它们。
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如果一个特定参数太大，将被存储在会话中，并且只有引用键被存储在URL中。
If a specific parameter gets too large, it is stored in the session and only the reference key is stored in the URL.
例如，如果存在依赖表(例如表中有一个外键，该外键引用数据要被装载到的目标表)，那么这些表可能被设为CHECK PENDING状态。
For example, if there are any dependent tables (tables that have a foreign key referencing the table into which data is to be loaded, for example), they might be placed in the CHECK PENDING state.
因为这是弱引用最常见的用法，WeakHashMap也被添加到JD k 1.2的类库中，它对键(而不是对值)使用弱引用。
Because this is such a common use for weak references, WeakHashMap, which USES weak references for keys (but not for values), was also added to the class library in JDK 1.2.
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