包含了数据库字段和和索引的描述。
which contains descriptions of the database fields and indices.
例如，一个新的业务应用程序要求增加一个数据库字段的长度。
For example, a new business application requires that the length of a database field should be enlarged.
然后依次对每个字段命名，每个字段表示一个数据库字段。
Each field is then named in turn and each represents a database field.
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