Formalising openCypher Graph Queries in Relational Algebra

Abstract

Graph database systems are increasingly adapted for storing and processing heterogeneous network-like datasets. However, due to the novelty of such systems, no standard data model or query language has yet emerged. Consequently, migrating datasets or applications even between related technologies often requires a large amount of manual work or ad-hoc solutions, thus subjecting the users to the possibility of vendor lock-in. To avoid this threat, vendors are working on supporting existing standard languages (e.g. SQL) or standardising languages. In this paper, we present a formal specification for openCypher, a highlevel declarative graph query language with an ongoing standardisation effort. We introduce relational graph algebra, which extends relational operators by adapting graph-specific operators and define a mapping from core openCypher constructs to this algebra. We propose an algorithm that allows systematic compilation of openCypher queries.

Towards an Integrated Graph Algebra for Graph Pattern Matching with Gremlin

“…Furthermore, there also exist certain applicationspecific extensions of algebra operators, such as the α and β operators, for graph data aggregation (used in complex graph network analysis) defined by [7]. We present graph-specific operators, some of which have been adapted from [9,13] and propose additional operators based on the algebra defined by [18,17].…”

Model-Driven Engineering of an OpenCypher Engine: Using Graph Queries to Compile Graph Queries

“…We gave a formal specification for the core subset of the openCypher language in [23] using relational graph algebra, which extends relational algebra with graph-specific operators. Here, we give a brief summary of the operators in relational graph algebra, which operates on multisets (bags) [15] of tuples, that form graph relations.…”

“…Rules for RETURN also apply to WITH. Table 1: Mapping from openCypher constructs to relational algebra [23]. Variables, labels, types and literals are typeset as «v».…”

“…3.2. Exact semantics of the other unary operators are given in [23]. DuplicateElimination, Grouping, Sort and Top operators work like their corresponding SQL clauses.…”

“…Details are given in [23], but essentially a query part contains clauses up to the next WITH or RETURN clause and defines a result set of the attributes listed, which is then fed into the next query part as its input. For example, the query in List.…”