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。
2
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。
3
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.