| Links' Seminars and Public Events |   |
| 2022 | |
|---|---|
Fri 18th Nov 11:00 am 11:30 am | Seminar by Sarah Winter CANCELLED for COVID: we will attempt to reschedule this seminar to early 2023 Speaker: Sarah Winter — sarahwinter.net/ Title: A Regular and Complete Notion of Delay for Streaming String Transducers Abstract: The notion of delay between finite transducers is a core element of numerous fundamental results of transducer theory. The goal of this work is to provide a similar notion for more complex abstract machines: we introduce a new notion of delay tailored to measure the similarity between streaming string transducers (SST). We show that our notion is regular: we design a finite automaton that can check whether the delay between any two SSTs executions is smaller than some given bound. As a consequence, our notion enjoys good decidability properties: in particular, while equivalence between non-deterministic SSTs is undecidable, we show that equivalence up to fixed delay is decidable. Moreover, we show that our notion has good completeness properties: we prove that two SSTs are equivalent if and only if they are equivalent up to some (computable) bounded delay. Together with the regularity of our delay notion, it provides an alternative proof that SSTs equivalence is decidable. Finally, the definition of our delay notion is machine-independent, as it only depends on the origin semantics of SSTs. As a corollary, the completeness result also holds for equivalent machine models such as deterministic two-way transducers, or MSO transducers. This is joint work with Emmanuel Filiot, Ismaël Jecker, and Christof Löding. |
Fri 21st Oct 11:00 am 12:00 pm | Online seminar by Pierre Pradic Speaker: Pierre Pradic — perso.ens-lyon.fr/pierre.pradic/ Title: Synthesizing Nested Relational Queries from Implicit Specifications Abstract: Derived datasets can be defined implicitly or explicitly. An implicit definition (of dataset O in terms of datasets I⃗ ) is a logical specification involving the source data I⃗ and the interface data O. It is a valid definition of O in terms of I⃗ , if any two models of the specification agreeing on I⃗ agree on O. In contrast, an explicit definition is a query that produces O from I⃗ . Variants of Beth's theorem state that one can convert implicit definitions to explicit ones. Further, this conversion can be done effectively given a proof witnessing implicit definability in a suitable proof system. In this talk, I will discuss an analogous effective implicit-to-explicit result for nested relations: implicit definitions, given in the natural logic for nested relations, can be effectively converted to explicit definitions in the nested relational calculus NRC. I will first spend some time explaining what NRC is and what logic we use to describe implicit definitions of nested queries. Then I will present the results obtained in our papers, attempt to give some intuitions on the proof of the main theorem and say a few words on in particular the proof-theoretic techniques and concerns that come up (namely, cut-elimination and focussing) and how this can impact the complexity of extracting explicit definitions from proofs of implicit definability. Then if time allows I will discuss a more general model-theoretic result that we first used to give a non-constructive proof of our theorem, and some ideas that we have towards making it constructive and bounding the complexity of extracting explicit definitions. This is Joint work with Michael Benedikt and Christoph Wenhard. The main results I will be discussing were written up in arxiv.org/abs/2005.06503 and arxiv.org/abs/2209.08299.  Online |
Fri 30th Sep 10:00 am 11:30 am | Seminar by Liat Peterfreund Speaker: Liat Peterfreund — sites.google.com/view/liatpeterfreund/ Title: Querying Incomplete Numerical Data: Between Certain and Possible Answers Abstract: Queries with aggregation and arithmetic operations, as well as incomplete data, are common in real-world databases, but we lack a good understanding of how they should interact. On the one hand, systems based on SQL provide ad-hoc rules for numerical nulls, on the other, theoretical research largely concentrates on the standard notions of certain and possible answers which in the presence of numerical attributes and aggregates are often meaningless. In this work, we define a principled compositional framework for databases with numerical nulls and answering queries with arithmetic and aggregations over them. We assume that missing values are given by probability distributions associated with marked nulls, which yields a model of probabilistic bag databases. We concentrate on queries that resemble standard SQL with arithmetic and aggregation and show that they are measurable, and that their outputs have a finite representation. Moreover, since the classical forms of answers provide little information in the numerical setting, we look at the probability that numerical values in output tuples belong to specific intervals. Even though their exact computation is intractable, we show efficient approximation algorithms to compute such probabilities. The talk is based on joint work with Marco Console and Leonid Libkin, and will be presented in PODS 2023. |
Fri 16th Sep 11:00 am 12:00 pm | Seminar Luis Galárraga Speaker : Luis Galárraga — luisgalarraga.de/about/ Title: Computing How-Provenance for SPARQL Queries via Query Rewriting Abstract: Over the past few years, we have witnessed the emergence of large knowledge graphs built by extracting and combining information from multiple sources. This has propelled many advances in query processing over knowledge graphs, however the aspect of providing provenance explanations for query results has so far been mostly neglected. In this talk I will present SPARQLprov, a method based on query rewriting, to compute how-provenance polynomials for SPARQL queries over knowledge graphs. Contrary to existing works, SPARQLprov is system-agnostic and can be applied to standard and already deployed SPARQL engines without the need of customized extensions. To do so, we rely on spm-semirings to compute polynomial annotations that respect the property of commutation with homomorphisms on monotonic and non-monotonic SPARQL queries without aggregate functions. An evaluation on real and synthetic data shows that SPARQLprov over standard engines competes with state-of-the-art solutions for how-provenance computation, while covering a larger fragment of the query language. |
