26 research outputs found
On p/q-recognisable sets
Let p/q be a rational number. Numeration in base p/q is defined by a function
that evaluates each finite word over A_p={0,1,...,p-1} to some rational number.
We let N_p/q denote the image of this evaluation function. In particular, N_p/q
contains all nonnegative integers and the literature on base p/q usually
focuses on the set of words that are evaluated to nonnegative integers; it is a
rather chaotic language which is not context-free. On the contrary, we study
here the subsets of (N_p/q)^d that are p/q-recognisable, i.e. realised by
finite automata over (A_p)^d. First, we give a characterisation of these sets
as those definable in a first-order logic, similar to the one given by the
B\"uchi-Bruy\`ere Theorem for integer bases numeration systems. Second, we show
that the natural order relation and the modulo-q operator are not
p/q-recognisable
On subtrees of the representation tree in rational base numeration systems
International audienc
Run-Based Semantics for RPQs
The formalism of RPQs (regular path queries) is an important building block
of most query languages for graph databases. RPQs are generally evaluated under
homomorphism semantics; in particular only the endpoints of the matched walks
are returned. Practical applications often need the full matched walks to
compute aggregate values. In those cases, homomorphism semantics are not
suitable since the number of matched walks can be infinite. Hence,
graph-database engines adapt the semantics of RPQs, often neglecting
theoretical red flags. For instance, the popular query language Cypher uses
trail semantics, which ensures the result to be finite at the cost of making
computational problems intractable.
We propose a new kind of semantics for RPQs, including in particular
simple-run and binding-trail semantics, as a candidate to reconcile theoretical
considerations with practical aspirations. Both ensure the output to be finite
in a way that is compatible with homomorphism semantics: projection on
endpoints coincides with homomorphism semantics. Hence, testing the emptiness
of result is tractable, and known methods readily apply. Moreover, simple-run
and binding-trail semantics support bag semantics, and enumeration of the bag
of results is tractableComment: 35 page
A Researcher’s Digest of GQL
International audienceGQL (Graph Query Language) is being developed as a new ISO standard for graph query languages to play the same role for graph databases as SQL plays for relational. In parallel, an extension of SQL for querying property graphs, SQL/PGQ, is added to the SQL standard; it shares the graph pattern matching functionality with GQL. Both standards (not yet published) are hard-to-understand specifications of hundreds of pages. The goal of this paper is to present a digest of the language that is easy for the research community to understand, and thus to initiate research on these future standards for querying graphs. The paper concentrates on pattern matching features shared by GQL and SQL/PGQ, as well as querying facilities of GQL
Updating Graph Databases with Cypher
International audienc
Graph Pattern Matching in GQL and SQL/PGQ
As graph databases become widespread, JTC1 -- the committee in joint charge
of information technology standards for the International Organization for
Standardization (ISO), and International Electrotechnical Commission (IEC) --
has approved a project to create GQL, a standard property graph query language.
This complements a project to extend SQL with a new part, SQL/PGQ, which
specifies how to define graph views over an SQL tabular schema, and to run
read-only queries against them.
Both projects have been assigned to the ISO/IEC JTC1 SC32 working group for
Database Languages, WG3, which continues to maintain and enhance SQL as a
whole. This common responsibility helps enforce a policy that the identical
core of both PGQ and GQL is a graph pattern matching sub-language, here termed
GPML.
The WG3 design process is also analyzed by an academic working group, part of
the Linked Data Benchmark Council (LDBC), whose task is to produce a formal
semantics of these graph data languages, which complements their standard
specifications.
This paper, written by members of WG3 and LDBC, presents the key elements of
the GPML of SQL/PGQ and GQL in advance of the publication of these new
standards
PG-Schema: Schemas for Property Graphs
Property graphs have reached a high level of maturity, witnessed by multiple
robust graph database systems as well as the ongoing ISO standardization effort
aiming at creating a new standard Graph Query Language (GQL). Yet, despite
documented demand, schema support is limited both in existing systems and in
the first version of the GQL Standard. It is anticipated that the second
version of the GQL Standard will include a rich DDL. Aiming to inspire the
development of GQL and enhance the capabilities of graph database systems, we
propose PG-Schema, a simple yet powerful formalism for specifying property
graph schemas. It features PG-Types with flexible type definitions supporting
multi-inheritance, as well as expressive constraints based on the recently
proposed PG-Keys formalism. We provide the formal syntax and semantics of
PG-Schema, which meet principled design requirements grounded in contemporary
property graph management scenarios, and offer a detailed comparison of its
features with those of existing schema languages and graph database systems.Comment: 25 page