Materializing the contents of views has important applications including providing fast access to derived database repositories, optimizing query processing based on cached results, and increasing availability. Maintaining the consistency between materialized views and their base data in the presence of source updates is important to ensure that the materialized views are up-to-date. The straightforward solution for this problem is to recompute the view from scratch over the updated sources.

Many applications, like materialized view maintenance and stream query processing, construct views incrementally over data sources. This results in computed pieces of objects that need to be merged by fusing corresponding objects together. This problem is challenging when dealing with XML data for many reasons including the hierarchical and semi-structured nature of XML data. Also XML query languages (e.g., XQuery) are capable of performing complex operations and transformations such as arbitrary nesting and result reconstruction. Moreover, since XML is an ordered data model, XML order has to be taken into consideration when constructing XML results incrementally. In this paper we study the problem of how to fuse XML pieces (fragments) generated by incrementally processing XML data into XML results. We consider an expressive subset of XQuery language transformations and propose an id-based solution for this problem that supports XML order. We prove the correctness of our approach, in particular that using our mechanism we can correctly yet incrementally merge XML result fragments. We have implemented our proposed semantic identifiers solution. Our experimental results show that it comes with a very small overhead to the query execution time.

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