XML Publishing: Bridging Theory and Practice

Abstract. Transforming relational data into XML, as known as XML publishing, is often necessary when one wants to exchange data residing in databases or to create an XML interface of a traditional database. This paper aims to provide an overview of recent advances in XML publish-ing. We present a notion of publishing transducers recently developed for studying the expressive power and complexity of XML publishing languages. In terms of publishing transducers we then characterize XML publishing languages being used in practice. In addition, we address dy-namic aspects of XML publishing, namely, incremental maintenance and update management of XML views published from relational data.

Expressiveness and complexity of xml publishing transducers

A number of languages have been developed for specifying XML publishing, i.e., transformations of relational data into XML trees. These languages generally describe the behaviors of a middleware controller that builds an output tree iteratively, issuing queries to a relational source and expanding the tree with the query results at each step. To study the complexity and expressive power of XML publishing languages, this paper proposes a notion of pub-lishing transducers. Unlike automata for querying XML data, a publishing transducer generates a new XML tree rather than per-forming a query on an existing tree. We study a variety of pub-lishing transducers based on what relational queries a transducer can issue, what temporary stores a transducer can use during tree generation, and whether or not some tree nodes are allowed to be virtual, i.e., excluded from the output tree. We first show how exist-ing XML publishing languages can be characterized by such trans-ducers. We then study the membership, emptiness and equivalence problems for various classes of transducers and existing publish-ing languages. We establish lower and upper bounds, all matching except one, ranging from PTIME to undecidable. Finally, we inves-tigate the expressive power of these transducers and existing lan-guages. We show that when treated as relational query languages, different classes of transducers capture either complexity classes (e.g., PSPACE) or fragments of datalog (e.g., linear datalog). For tree generation, we establish connections between publishing trans-ducers and logical transductions

On the Power of Walking for Querying Tree-Structured Data

XSLT is the prime example of an XML query language based on tree-walking. Indeed, stripped down, XSLT is just a tree-walking tree-transducer equipped with registers and look-ahead. Motivated by this connection, we want to pinpoint the computational power of devices based on tree-walking. We show that in the absence of unique identifiers even very powerful extensions of the tree-walking paradigm are not relationally complete. That is, these extensions do not capture all of first-order logic. In contrast, when unique identifiers are available, we show that various restrictions allow to capture logspace, ptime, pspace, and exptime. These complexity classes are defined w.r.t. a Turing machine model working directly on (attributed) trees. When no attributes are present, relational storage does not add power; whether look-ahead adds power is related to the open question whether tree-walking captures the regular tree languages