Data integration is the primary use case for knowledge graphs. However, integrated data are not typically graphs but come in different formats, for example, CSV, XML, or a relational database. Façade-X is a recently proposed method for providing direct access to an open-ended set of data formats. The method includes a meta-model that specialises RDF to fit general data structures. This model allows to express SPARQL queries targeting data sources with those structures. Previous work formalised Façade-X and demonstrated how it can theoretically represent any format expressible with a context-free grammar, as well as the relational model. A reference implementation, SPARQL Anything, demonstrates the feasibility of the approach in practice. It is noteworthy that Façade-X utilises a fraction of RDF, and, consequently, not all SPARQL queries yield a solution (i.e. are satisfiable) when evaluated over a Façade-X graph. In this article, we consolidate Façade-X, and we study the satisfiability of basic graph patterns. The theory is accompanied by an algorithm for deciding the satisfiability of basic graph patterns on Façade-X data sources. Furthermore, we provide extensive experiments with a proof-of-concept implementation, demonstrating practical feasibility, including with real-world queries. Our results pave the way for studying query execution strategies for Façade-X data access with SPARQL and supporting developers to build more efficient data integration systems for knowledge graphs.

翻译：数据集成是知识图谱的主要应用场景。然而，集成数据通常并非图结构，而是以不同格式存在，例如CSV、XML或关系数据库。Façade-X是近期提出的一种为开放式数据格式集合提供直接访问的方法。该方法包含一个元模型，该模型将RDF特化以适应通用数据结构，从而能够表达针对具有这些结构的数据源的SPARQL查询。先前的工作对Façade-X进行了形式化定义，并论证了其理论上可以表示任何可通过上下文无关文法表达的数据格式及关系模型。参考实现SPARQL Anything验证了该方法在实际中的可行性。值得注意的是，Façade-X仅使用RDF的部分特性，因此并非所有SPARQL查询在Façade-X图上评估时都能产生解（即具有可满足性）。本文对Façade-X进行整合，并研究基本图模式的可满足性。我们提出了相应的理论以及判定Façade-X数据源上基本图模式可满足性的算法。此外，我们通过概念验证实现进行了大量实验，证明该方法的实际可行性（包括处理真实世界查询）。研究结果为基于SPARQL的Façade-X数据访问查询执行策略研究奠定了基础，并有助于开发者构建更高效的知识图谱数据集成系统。