Conclusion and Issues

This chapter has given an overview of how to integrate relational database systems with XQuery. It introduced the XML datatype and presented both the mixed approach of using XQuery in conjunction with SQL to query XML datatype instances and the top-level XQuery approach of querying collections of XML instances and XML views of relational data. It also provided an insight into the impact of the actual physical data model of the XML datatype on the processing and mapping of XQuery in the context of relational systems.

Many relational database systems at the time of this writing have just started to add native XML support to their support for bidirectional mapping between relational and XML data, and so far no system ships XQuery as described in this chapter. Before long, however, I believe all major relational database systems will provide functionality of the form described (most vendors have already announced support for a built-in XML datatype).

Many open issues need further investigation and are currently active research topics. Among them are the following:

  • Which of the physical mappings of the XML data are the best approaches to support XML inside a relational context? Depending on the data, there may be more than one. In that case the question becomes how to determine the best one and provide support for all the approaches.

  • How should the query-processing model be extended? In particular:

    What additions are needed to the logical algebra?

    What additions can be made to the optimizer to improve efficiency?

    What indices help in improving efficiency?

    How is distributed query processing best supported?

  • Another area of concern is the area of concurrency control. How can the concurrency control of relational data be extended to deal with tree-structured data and provide fine-grained control? Especially in the context of combined relational and XML queries on relational tables containing XML, the current row-level locking granularity is inadequate if access similar to online transaction processing (OLTP) is going to be part of the query mix. Thus, one question is how to improve the parallelism in such queries and how to integrate fine-grained concurrency control of XML data (such as tree-based concurrency control) with the relational concurrency control.



XQuery from the Experts(c) A Guide to the W3C XML Query Language
Beginning ASP.NET Databases Using VB.NET
ISBN: N/A
EAN: 2147483647
Year: 2005
Pages: 102

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