Links' Seminars and Public Events | ![]() ![]() |
2021 | |
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Fri 28th May 10:00 am 11:00 am | Seminar Anastasia Dimou Title: Knowledge graph generation and validation |
Fri 21st May 10:00 am 12:00 pm | Seminar Dimitrios Myrisiotis Title : One-Tape Turing Machine and Branching Program Lower Bounds for MCSP Abstract: eccc.weizmann.ac.il/report/2020/103/ Speaker' webpage : dimyrisiotis.github.io/ zoom |
Fri 7th May 10:00 am 12:00 pm | Seminar Nicole Schweikardt Title: Spanner Evaluation over SLP-Compressed Documents Abstract: We consider the problem of evaluating regular spanners over compressed documents, i.e., we wish to solve evaluation tasks directly on the compressed data, without decompression. As compressed forms of the documents we use straight-line programs (SLPs) -- a lossless compression scheme for textual data widely used in different areas of theoretical computer science and particularly well-suited for algorithmics on compressed data. In terms of data complexity, our results are as follows. For a regular spanner M and an SLP S that represents a document D, we can solve the tasks of model checking and of checking non-emptiness in time O(size(S)). Computing the set M(D) of all span-tuples extracted from D can be done in time O(size(S) size(M(D))), and enumeration of M(D) can be done with linear preprocessing O(size(S)) and a delay of O(depth(S)), where depth(S) is the depth of S's derivation tree. Note that size(S) can be exponentially smaller than the document's size |D|; and, due to known balancing results for SLPs, we can always assume that depth(S) = O(log(|D|)) independent of D's compressibility. Hence, our enumeration algorithm has a delay logarithmic in the size of the non- compressed data and a preprocessing time that is at best (i.e., in the case of highly compressible documents) also logarithmic, but at worst still linear. Therefore, in a big-data perspective, our enumeration algorithm for SLP-compressed documents may nevertheless beat the known linear preprocessing and constant delay algorithms for non-compressed documents. [This is joint work with Markus Schmid, to be presented at PODS'21.] Link to the paper: arxiv.org/pdf/2101.10890.pdf for the paper at least Link to the ACM video: TBA |
Fri 30th Apr 10:00 am 12:00 pm | Présentation de NetworkDisk Je présenterais mon projet avec Bruno: NetworkDisk. Abstract and Title: TBA link to the project: TBA |
Fri 9th Apr 10:00 am 12:00 pm | Seminaire Pascal Weil titre: Problèmes algorithmiques en théorie des groupes infinis resumé: Malgré le titre très général, il s'agira uniquement de problèmes concernant les sous-groupes de groupes infinis, et même juste les sous-groupes de groupes libres. Les résultats et méthodes que je présenterai sont issus de près de 40 ans de littérature et sont dûs à un grand nombre d'auteurs. Je commencerai par poser le paysage, y compris pour ceux qui ne savent plus ce qu'est le groupe libre -- où l'on verra qu'on est, du point de vue algorithmique, dans une variante de la combinatoire des mots. Je présenterai ensuite l'outil central de la plupart des algorithmes efficaces sur les sous-groupes du groupe libre : la représentation de chaque sous-groupe finiment engendré par un graphe étiqueté et enraciné (disons : d'un automate :-)…) unique et facilement calculable à partir d'un ensemble de générateurs du sous-groupe considéré, qu'on appelle le graphe de Stallings. Le jeu consiste ensuite à traduire les problèmes algorithmiques sur les sous-groupes en problèmes algorithmiques sur les graphes de Stallings, et à résoudre ces problèmes de la façon la plus efficace possible. On considèrera notamment les problèmes suivants -- bon, juste le début de cette longue liste. - Le problème du mot généralisé : étant donnés k+1 éléments du groupe libre (ce sont des mots), le dernier appartient-il au sous-groupe engendré par les k premiers ? - Le problème de l'indice : étant donné un tuple d'éléments du groupe libre, le sous-groupe qu'ils engendrent est-il d'indice fini ? - Le problème de la base : étant donné un tuple d'éléments du groupe libre, trouver le rang, et une base du sous-groupe qu'ils engendrent. - Le problème de l'intersection : étant donnés deux tuples d'éléments du groupe libre, calculer l'intersection des sous-groupes qu'ils engendrent (ou calculer une base de cette intersection). - Le problème de la conjugaison : étant donnés deux tuples d'éléments du groupe libre, engendrent-ils le même sous-groupe ? deux sous-groupes conjugués ? - Et de nombreux autres problèmes (mots clés : minimalité de Whitehead, facteur libre, malnormalité, clôture par radical, clôture au sens de la topologie pro-p, etc…) title: Algorithmic problems in the theory of infinite groups abstract: In spite of the very general title, we will talk only about problems on subgroups of infinite groups, and in fact, only on subgroups of free groups . The results and methods I will present have been obtained over the past 40 years and are due to many researchers. I will start by setting the landscape, including for those who forgot what the free group is --- and we will see that we are dealing here, from the algorithmic point of view, with a variant of combinatorics on words. I will then present the tool that is central to most efficient algorithms on subgroups of free groups: the representation of each finitely generated subgroup by a labeled rooted graph (shall we say… an automaton?) which is unique and easily computable when a tuple of generators of the subgroup under consideration is given. This graph is called the Stallings graph. The game consists, then, in translating algorithmic problems on subgroups into algorithmic problems on Stallings graphs, and in solving these problems as efficiently as possible. We will discuss in particular the following problems (clearly: just the beginning of this long list). - The generalized word problem: given k+1 elements of the free group (these are words), does the last one belong to the subgroup generated by the k first ones? - The index problem: given a tuple of elements of the free group, does the subgroup they generate have finite index? - The basis problem: given a tuple of elements of the free group, find the rank and a basis of the subgroup they generate. - The intersection problem: given two tuples of elements of the free group, compute the intersection of the subgroups they generate (compute a basis of this intersection). - The conjugacy problem: given two tuples of elements of the free group, are the subgroups they generate equal? conjugated? - And many other problems (keywords: Whitehead minimality, free factors, malnormality, closure under radicals, closure in the sense of the pro-p topology, etc…) |
Fri 26th Mar 10:00 am 11:00 am | Séminaire Anne Etien Title: Managing structural and behavioral evolution in relational database: Application of Software Engineering techniques. Abstract: Relational databases play a central role in many information systems. Their schemas usually contain structural and behavioral entity descriptions. However, as any piece of software, they must continuously evolve to adapt to new requirements of a world in constant change. From an evolution point of view, problems are twofold: (1) relational database management systems do not allow inconsistencies i.e., no entity can reference a non existing entity; (2) stored procedures bodies are not described by meta-data i.e., DBMS as PostgreSQL consider stored procedure bodies as plain text and references to entities are unknown. As a consequence, evaluating the impact of an evolution of the database schema is a difficult task. In this seminar, we present a semi-automatic approach based on recommendations (sort of nested code transformations). Recommendations are proposed to architects who select the ones fitting their needs. Selected recommendations are then analysed and compiled to generate SQL script respecting the constraints imposed by the RDBMS. To support recommendations, we designed a meta-model for relational databases easing computation of change impact. We performed an experiment to validate the approach by reproducing a real evolution on a database. The results of our experiment show that our approach is able to reproduce exactly a manual modification in 75% less time. Zoom link: univ-lille-fr.zoom.us/j/95419000064 |
Fri 19th Mar 10:00 am 12:00 pm | Seminar Pablo Ferragin Title: Theory and practice of learning-based compressed data structures Presenter: Giorgio Vinciguerra Abstract: We revisit two fundamental and ubiquitous problems in data structure design: predecessor search and rank/select primitives. We show that real data present a peculiar kind of regularity based on geometric considerations. We name it “approximate linearity”. We thus expand the horizon of compressed data structures by presenting two solutions for the problems above that discover, or “learn”, in a principled algorithmic way, these approximate linearities. We provide a walkthrough of these new theoretical achievements, also with a focus on open-source libraries and their experimental improvements. We conclude by discussing the plethora of research opportunities that these new learning-based approaches to data structure design open up. Zoom link: univ-lille-fr.zoom.us/j/95419000064 |
Fri 12th Mar 10:00 am 12:00 pm | Seminar: Antonio AL SERHALI Title: Can Earliest Query Answering on Nested Streams be achieved in Combined Linear Time? |
Fri 19th Feb 10:00 am 11:00 am | Seminar: Bernardo Subercaseau Title: Foundations of Languages for Interpretability. Abstract: The area of interpretability in Machine Learning aims for the design of algorithms that we humans can understand and trust. One of the fundamental questions of interpretability is: given a classifier M, and an input vector x, why did M classify x as M(x)? In order to approximate an answer to this "why" question, many concrete queries, metrics and scores have emerged as proxies, and their complexity has been studied over different classes of models. Many of these analyses are ad-hoc, but they tend to agree on the fact that these queries and scores are hard to compute over Neural Networks, but easy to compute over Decision Trees. It is thus natural to think of a more general approach, like a query language in which users could write an arbitrary number of different queries, and that would allow for a generalized study of the complexity of interpreting different ML models. Our work proposes foundations for such a language, tying to First Order Logic, as a way to have a clear understanding of its expressiveness and complexity. We manage to define a minimalistic structure over FO that allows expressing many natural interpretability queries over models, and we show that evaluating such queries can be done efficiently for Decision Trees, in data-complexity. Zoom link: univ-lille-fr.zoom.us/j/95419000064 |
Fri 12th Feb 10:00 am 12:00 pm | Seminar: Florent Capelli Title: Regularizing the delay of enumeration algorithms Zoom link: univ-lille-fr.zoom.us/j/95419000064 Abstract: Enumeration algorithms are algorithms whose goal is to output the set of all solutions to a given problem. There exists different measures for the quality of such algorithm, whose relevance depends on what the user wants to do with the solutions set. If the goal of the user is to explore some solutions or to transform the solutions as they are outputted with a stream-like algorithm, a relevant measure of the complexity of an enumeration algorithm is the delay between the output of two distinct solutions. Following this line of thoughts, significant efforts have been made by the community to design polynomial delay algorithms, that is, algorithms whose delay between the output of two new solutions is polynomial in the size of the input. While this measure is interesting, it is not always completely necessary to have a bound on the delay and it is enough to ask for a guarantee that running the algorithm for O(t poly(n)) will result in the output of at least t solutions. Of course, by storing each solution seen and outputting them regularly, one can simulate a polynomial delay but if the number of solutions is large, it may result in a blow up in the space used by the enumerator. In this talk, we will present a new technique that allow to transform such algorithm into polynomial delay algorithm using polynomial space. This is joint work with Yann Strozecki. |
Fri 15th Jan 10:00 am 12:00 pm | Séminaire de Kim Nguyễn Titile: The BOLDR project Abstract: I n this presentation, I will give an account of the BOLDR project and perspectives in the field of language integrated queries. Several classes of solutions allow programming languages to express queries: specific APIs such as JDBC, Object-Relational Mappings (ORMs) such as Hibernate, and language-integrated query frameworks such as Microsoft's LINQ. However, most of these solutions do not allow for efficient cross-databases queries, and none allow the use of complex application logic from the programming language in queries. We study the design of a new language-integrated query framework called BOLDR that allows the evaluation in databases of queries written in general-purpose programming languages containing application logic, and targeting several databases following different data models. In this framework, application queries are translated to an intermediate representation. Then, they are typed with a type system extensible by databases in order to detect which database language each subexpression should be translated to. This type system also allows us to detect a class of errors before execution. Next, they are rewritten in order to avoid query avalanches and make the most out of database optimizations. Finally, queries are sent for evaluation to the corresponding databases and the results are converted back to the application. Our experiments show that the techniques we implemented are applicable to real-world database applications, successfully handling a variety of language-integrated queries with good performances. This talk will give an overview of what has been achieved so far (mainly in the context of Julien Lopez' PhD Thesis) and will glimpse at preliminary work that is being done in the context of a collaboration with Oracle Labs. |
Fri 8th Jan 10:45 am 12:30 pm | Séminaire @ Lê Thành Dũng (Tito) Nguyễn Title: The planar geometry of first-order string transductions (joint work with Pierre Pradic) Abstract: hal.archives-ouvertes......ument We propose a new machine model recognizing star-free languages, with a geometric flavor. Our starting point is the characterization of regular languages using two-way automata (2DFA). The idea is to take seriously the visual representations found throughout the literature of the behavior of a 2DFA on a word ; by putting a total order on the set of states, one can formally define what it means for such a behavior to be planar, in a sense analogous to the planarity of combinatorial maps. Star-free languages are then exactly the languages recognized by "planar 2DFA". We also show that the corresponding planar transducer model characterizes the class of first-order transductions (a.k.a. aperiodic regular functions). If time allows, the talk will briefly discuss the connections of this work with the non-commutative lambda-calculus (cf. our recent paper Aperiodicity in a non-commutative logic, ICALP'20). |
2020 | |
Thu 17th Dec 2:00 pm 4:00 pm | Nofar Carmeli Speaker: Nofar Carmeli (nofar.carme.li/) Zoom link: univ-lille-fr.zoom.us/j/95419000064 Title: The Complexity of Answering Unions of Conjunctive Queries. Abstract: We discuss the fine-grained complexity of enumerating the answers to a query over a relational database. With the ideal guarantees, linear time is required before the first answer to read the input and determine its existence, and then we need to print the answers one by one. Consequently, we wish to identify the queries that can be solved with linear preprocessing time and constant or logarithmic delay between answers. A known dichotomy classifies CQs into those that admit such enumeration and those that do not. The computationally expensive component of query answering is joining tables, which can be done efficiently if and only if the join query is acyclic. However, the join query usually does not appear in a vacuum; for example, it may be part of a larger query, or it may be applied to a database with dependencies. We inspect how the complexity changes in these settings and chart the borders of tractability within. In addition, we consider the task of enumerating query answers with a uniformly random order, and we propose to do so using an efficient random-access structure for representing the set of answers. We also prove conditional lower bounds showing that our algorithms capture all tractable queries in some cases. Among our results, we show that a union of tractable conjunctive queries may be intractable w.r.t. random access; on the other hand, a union of intractable conjunctive queries may be tractable w.r.t. enumeration. |
Fri 11th Dec 10:00 am 11:30 am | Alexandre Vigny Title: Elimination Distance to Bounded Degree on Planar Graphs Link to the zoominar: univ-lille-fr.zoom.us/j/95419000064 Abstract: What does it mean for a graph to almost be planar? Or to almost have bounded degree? On such simple graphs classes, some difficult algorithmic problems become tractable. Ideally, one would like to use (or adapt) existing algorithms for graphs that are "almost" in such a simple class. In this talk, I will discuss the notion of elimination distance to a class C, a notion introduced by Bulian and Dawar (2016). The goals of the talk are: 1) Define this notion, and discuss why it is relevant by presenting some existing results. 2) Show that we can compute the elimination distance of a given planar graph to the class of graph of degree at most d. I.e. answer the question: "Is this graph close to a graph of bounded degree?" The second part is the result of a collaboration with Alexandre Lindermayer and Sebastian Siebertz. |
Fri 4th Dec 10:00 am 11:00 am | Seminar: Pierre Pradic Title: Extracting nested relational queries from implicit definitions Abstract: arxiv.org/pdf/2005.06503.pdf In this talk, I will present results obtained jointly with Michael Benedikt establishing a connection between the Nested Relational Calculus (NRC) and sets implicitly definable using Δ₀ formulas. Call a formula φ(I,O) an implicit definition of the relation O(x,...) in terms of I(y,...) if O is functionally determined by I: for every I, O, O', if both φ(I,O) and φ(I,O') hold, then we have O ≡ O'. When φ is first-order and I and O are relations over base sorts, then Beth's definability theorem states that there is a first-order formula ψ(I,x,...) corresponding to O whenever φ(I,O) holds. Further, this explicit definition ψ can be effectively be computed from a sequent calculus proof witnessing that φ is functional. This allows for practical use of implicit definitions in the context of database programming, as there is a well-established link between fragments of explicitly FO definable relations and relational calculi. NRC is a conservative extension of relational calculi from database theory with limited powerset types in addition to tupling and anonymous base types. NRC expressions thus not only encompass flat relations over primitive datatypes like SQL but also nested collections, while remaining useful in practice. We extend the above correspondence between first-order logic and flat relational queries to NRC and implicit definitions using set-theoretical Δ₀ formulas over (typed) nested collection. Our proof of the equivalence goes through a notion of Δ₀-interpretation and a generalization of Beth definability for multi-sorted structures. This proof is non-constructive and thus does not yield any useful algorithm for converting implicit definitions into NRC terms. Using an approach more closely related to proof-theoretic interpolation, we give a constructive proof of the result restricted to intuitionistic provability, i.e, when the input functionality proof π of φ(I,O) is carried out in intuitionistic logic. Further, if π is cut-free, this can be done efficiently. Whether or not there exists a polynomial-time procedure working with classical proofs of functionality is still an open problem. I will focus on the effective result for the talk, and if time allows, discuss the difficulties with extending it to classical logic. I will not assume any background in either database or model theory. |
Fri 27th Nov 10:00 am 11:30 am | Seminar: Charles Paperman Title: Stackless processing of streamed trees Abstract: In this talk, I will first present the state of the art of efficiency implementation of streaming-text algorithms on modern architecture. Then some recent results on the extraction of information on streamed of structured documents without stack overhead. For more info: paperman.name/data/pub.....d.pdf |
Fri 13th Nov 10:00 am 12:00 pm | Seminar: Mikaël Monet Title: The Complexity of Counting Problems over Incomplete Databases Abstract: In this presentation, I will talk about various counting problems that naturally arise in the context of query evaluation over incomplete databases. Incomplete databases are relational databases that can contain unknown values in the form of labeled nulls. We will assume that the domains of these unknown values are finite and, for a Boolean query $q$, we will consider the following two problems: given as input an incomplete database $D$, (a) return the number of completions of $D$ that satisfy $q$; or (b) return or the number of valuations of the nulls of $D$ yielding a completion that satisfies $q$. We will study the computational complexity of these problems when $q$ is a self-join--free conjunctive query, and study the impact on the complexity of the following two restrictions: (1) every null occurs at most once in $D$ (what is called *Codd tables*); and (2) the domain of each null is the same. Roughly speaking, we will see that counting completions is much harder than counting valuations, and that both (1) and (2) can reduce the complexity of our problems. I will also talk about the approximability of these problems and prove that, while counting valuations can efficiently be approximated, in most cases counting completions cannot. On our way, we will encounter the counting complexity classes #P, Span-P and Span-L. The presentation will be based on joint work with Marcelo Arenas and Pablo Barcelo; see arxiv.org/abs/1912.11064 |
Fri 16th Oct 11:00 am 12:00 pm | Seminar: Aurélien Lemay Title: ShEx Learning from Typed Graphs Abstract: In knowledge graphs, schemas are becoming a new asset to describe the organization of data. The new world-leading format Shex is becoming a de-facto standard in the industry that allows defining flexible and powerful schemas. In this context, the inference of schemas can become a solution to provide shex expressions that describe already existing data. Typically, the inference starts from untyped graphs. However, these tasks appears to be more complex than expected in general, and is possible only for subclasses of Shex. The inference of schemas from typed graph gives a baseline for those algorithms. Its comprehension allows to understand better the underlying difficulties of the task. It presents already unexpected difficulties. We present an algorithm that infers Shex-defined schemas from fully typed graphs. We also present some encountered difficulties, as well as the limitations of the approach. |