XML data is stored in a native tree format, separate from the relational data.

XML数据按照原生的树格式与关系数据分开存储。

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Manipulations like this (backed by the proper relational theory) are at the heart of database query optimizers. The resulting tree is shown in Figure 2.

此类操作(由恰当的关系理论作为支持)是数据库查询优化器的核心所在。

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However, due to the mismatch between the XML's tree structure and flat relational tables, mapping XML document to relational database is an involved problem. So it becomes a pop research direction.

然而由于复杂的树形XML文档和简单平坦的关系型数据库表结构之间固有的不匹配，使得XML到关系型数据库的映射实现十分困难，因此成为当前的一个热门研究课题。

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