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https://openproceedings.org/2014/conf/icdt/FigueiraFA14.pdf
From a database perspective, however, Core-XPath fails to include the single most important construct in a query language: the join. Without the ability to relate nodes based on the actual data values of the attributes, the logic’s expressive power is inappropriate for many applications.
Recent articles investigate several algorithmic problems
of logics evaluated over data trees. For example, satisfiability and evaluation are discussed in [8, 5]. In particular, all the logics studied in this article have a decidable satisfiability problem [10, 9]; but tools to investigate their expressive power are still lacking. There are good reasons for this: in the presence of joins and data values, classical notions such as Ehrenfeucht-Fra¨ıss´e games or structural bisimulations are difficult to handle. In this article we take the first steps towards understanding the expressive power and model theory of XPath= on data trees.