Seminars

Abstract: We consider the problem of computing a conjunctive query on a large database in a parallel setting with p servers. Unlike traditional query processing, the complexity is no longer dominated by the number of disk accesses. Typically, a query is evaluated by a sufficiently large number of servers such that the entire data can be kept in the main memory of these servers. The dominant cost becomes that of communicating data and synchronizing among the servers.
I will present some interesting results in [1, 2, 3, 4] dealing with the communication complexity of massively parallel computation of a query. The computation is performed in "rounds".
First, I will present the Massively Parallel Communication (MPC) model to analyze the tradeoff between the number of rounds and the amount of communication required in a massively parallel computing environment.
Then I will present the HyperCube (HC) algorithm that computes a full conjunctive query q in one round.
I will discuss the communication complexity [2]. The main result is the optimal load O(m/p1/τ ) where τ is the fractional vertex cover of the hypergraph of q and m the input data size.

References
[1] Parallel Evaluation of Conjunctive Queries. Paris Koutris, Dan Suciu PODS2011
[2] Communication Steps for Parallel Query Processing. Paul Beame, Paris Koutris, Dan Suciu PODS2013
[3] Skew in Parallel Query Processing. Paul Beame, Paris Koutris Dan Suciu PODS'2014
[4] Worst-Case Optimal Algorithms for Parallel Query Processing. Paris Koutris, Paul Beame, Dan Suciu ICDT2016

Abstract:
The challenge that we tackle in this thesis is the problem of how to answer
XPath queries on XML streams with low latency, full coverage, high time
efficiency, and low memory costs. We first propose to approximate earli-
est query answering for navigational XPath queries by compilation to early
nested word automata. It turns out that this leads to almost optimal la-
tency and memory consumption. Second, we contribute a formal semantics
of XPath 3.0. It is obtained by mapping XPath to the new query language
λXP that we introduce. We then show how to compile λXP queries to net-
works of early nested word automata, and develop streaming algorithms for
the latter. Thereby we obtain a streaming algorithm that indeed covers all of
XPath 3.0. Third, we develop an algorithm for projecting XML streams with
respect to the query defined by an early nested word automaton. Thereby
we are able to make our streaming algorithms highly time efficient. We have
implemented all our algorithms with the objective to obtain an industrially
applicable streaming tool. It turns out that our algorithms outperform all
previous approaches in time efficiency, coverage, and latency.

Abstract:
In this talk, I will present a connection between the streaming complexity and the circuit complexity of regular languages through a notion of streaming by block . This result provides tight constructions of boolean circuits computing an automaton, thanks to some classical and recent results on the circuit complexity of regular languages. I will apply this framework to the schema validation in streaming of XML-documents.

Abstract: Higher-order constructions make their way into main stream
programming languages like Java, C++, python, rust... These
constructions bring new challenges to the verification of programs as
they make their control flow more complex.

In this talk, I will present how methods coming from denotational
semantics can prove decidable the verification of certain properties of
higher-order programs. These properties are expressed by means of finite
state automata of the possibly infinite execution trees generated by the
programs and can capture safety properties but also liveness and
fairness properties.

Higher-order pushdown systems and ground tree rewriting systems can
be seen as extensions of suffix word rewriting systems. Both classes
generate infinite graphs with interesting logical
properties. Indeed, the satisfaction of any formula written in
monadic second order logic (respectively first order logic with
reachability predicates) can be decided on such a graph.

The purpose of this talk is to propose a common extension to both
higher-order stack operations and ground tree rewriting. We introduce a model
of higher-order ground tree rewriting over trees labelled by
higher-order stacks (henceforth called stack trees), which syntactically
coincides with ordinary ground tree rewriting at order 1 and with the dynamics
of higher-order pushdown automata over unary trees. The infinite
graphs generated by this class have a decidable first order logic with
reachability.

Formally, an order n stack tree is a tree labelled by order n-1 stacks.
Operations of ground stack tree rewriting are represented by a certain class
of connected DAGs labelled by a set of basic operations over stack trees
describing of the relative application positions of the basic
operations appearing on it. Applying a DAG to a stack tree t intuitively
amounts to paste its input vertices to some leaves of t and to simplify
the obtained structure, applying the basic operations labelling the edges of the
DAG to the leaves they are appended to, until either a new stack tree is
obtained or the process fails, in which case the application of the DAG to t
at the chosen position is deemed impossible. This model is a common extension
to those of higher-order stack operations presented by Carayol and of ground tree
transducers presented by Dauchet and Tison.

As further results we can define a notion of recognisable sets of
operations through a generalisation. The proof that the graphs generated
by a ground stack tree rewriting system have a decidable first order theory
with reachability is inspired by the technique of finite set interpretations
presented by Colcombet and Loding.