IMPLEMENTAT ION OF WEB QUERY LANGUAGES RECONSIDERED beyond tree and single-language algebras

Visions of the next generation Web such as the “Semantic Web” or the “Web 2.0” have triggered the emergence of a multitude of data formats. ¿ese formats have di erent characteristics as far as the shape of data is concerned (for example treevs. graph-shaped). ¿ey are accompanied by a puzzlingly large number of query languages each limited to one data format.¿us, a key feature of theWeb, namely to make it possible to access anything published by anyone, is compromised. ¿is thesis is devoted to versatile query languages capable of accessing data in a variety of Web formats. ¿e issue is addressed from three angles: language design, common, yet uniform semantics, and common, yet uniform evaluation. First, we consider the query language Xcerpt as an example of the advocated class of versatile Web query languages. Using this concrete exemplar allows us to clarify and discuss the vision of versatility in detail. Second, a number of query languages, XPath, XQuery, SPARQL, and Xcerpt, are translated into a common intermediary language, CIQLog. ¿is language has a purely logical semantics, which makes it easily amenable to optimizations. As a side e ect, this provides the, to the best of our knowledge, rst logical semantics for XQuery and SPARQL. It is a very useful tool for understanding the commonalities and di erences of the considered languages. ¿ird, the intermediate logical language is translated into a query algebra, CIQCAG. ¿e core feature of CIQCAG is that it scales from treeto graphshaped data and queries without e ciency losses when tree-data and queries are considered: it is shown that, in these cases, optimal complexities are achieved. CIQCAG is also shown to evaluate each of the aforementioned query languages with a complexity at least as good as the best known evaluation methods so far. For example, navigational XPath is evaluated with space complexityO(q ⋅ d) and time complexityO(q ⋅ n) where q is the query size, n the data size, and d the depth of the (tree-shaped) data. CIQCAG is further shown to provide linear time and space evaluation of tree-shaped queries for a larger class of graph-shaped data than any method previously proposed.¿is larger class of graph-shaped data, called continuous-image graphs, short cigs, is introduced for the rst time in this thesis. A (directed) graph is a cig if its nodes can be totally ordered in such a manner that, for this order, the children of any node form a continuous