可选地，定义索引来优化查询和保证唯一性。
Indexes are defined to optimize queries and, optionally, to guarantee uniqueness.
此列是定义层次结构的唯一性元素的索引。
This column is the index that defines the unique elements of the hierarchy.
因为你已经确定我推断是三列以确定唯一性(主题行，主体和发件人地址)，所以您将创建包含所有三列的索引。
As you've identified what I infer to be three columns to determine uniqueness (subject line, body, and sender address) you'll create the index to include all three columns.
这个类别为以其他物件值为其索引键值基础的类别提供支援，同时传入的物件可确保产生之资源索引键的唯一性。
This class provides support for classes that base key values on other object values, where the objects passed in assure uniqueness of the generated resource keys.
这个类别为以其他物件值为其索引键值基础的类别提供支援，同时传入的物件可确保产生之资源索引键的唯一性。
This class provides support for classes that base key values on other object values, where the objects passed in assure uniqueness of the generated resource keys.
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