Section: New Results

Query Processing for the Web

Participants : Johann Brault-Baron, Arnaud Durand, Nadime Francis, Luc Segoufin, Cristina Sirangelo.

In many applications the output of a query may have a huge size and enumerating all the answers may already consume too many of the allowed resources. In this case it may be appropriate to first output a small subset of the answers and then, on demand, output a subsequent small numbers of answers and so on until all possible answers have been exhausted. To make this even more attractive it is preferable to be able to minimize the time necessary to output the first answers and, from a given set of answers, also minimize the time necessary to output the next set of answers - this second time interval is known as the delay. We have shown that this was doable with a almost linear preprocessing time and constant enumeration delay for first-order queries over structures of low degree [18] . We also presented a survey about this work at the Intl. Symp. on Theoretical Aspects of Computer Science (STACS) [22] .

We have also been interested in querying data structured as graphs, which is nowadays spreading on the Web. Examples are social networks, linked data and the semantic web, via the RDF format. We have tackled the problem of answering queries over graph databases which are available only trough a given set of views. This is a common situation in many applications where access to data needs to be either controlled or optimized. In [19] we have studied when it is possible to rewrite over the views queries issued on the original data, and which query languages are needed for this purpose. We have considered views and queries expressed as Regular path queries, a very common graph query language.