Section: New Results

Query Processing for the Web

Participants : Johann Brault-Baron, Arnaud Durand, Nadime Francis, Wojciech Kazana, 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 linear preprocessing time and constant enumeration delay for first-order queries over structures of bounded expansion [27] and for monadic second-order queries over structures of bounded tree-width [15] . We also presented a survey about this work at the Intl. Conf. on Database Theory (ICDT) [19] .

Web data is often structured in the XML format. In [18] we have surveyed results about static analysis of pattern-based queries over XML documents. These queries are analogs of conjunctive queries, their unions and Boolean combinations, in which tree patterns play the role of atomic formulae. These can be viewed as both queries and incomplete documents, and thus static analysis problems can also be viewed as answering queries over such documents. We looked at satisfiability of patterns under schemas, containment of queries for various features of XML used in queries, query answering, and applications of pattern-based queries in reasoning about schema mappings for data exchange.