Navigational queries over RDF data are viewed as one of the main applications of graph query languages, and yet the standard model of graph databases—essentially labeled graphs—is different from the triples-based model of RDF. While encodings of RDF databases into graph data exist, we show that even the most natural ones are bound to lose some functionality when used in conjunction with graph query languages. The solution is to work directly with triples, but then many properties taken for granted in the graph database context (e.g., reachability) lose their natural meaning. Our goal is to introduce languages that work directly over triples and are closed, i.e., they produce sets of triples, rather than graphs. Our basic language is called TriAL, or Triple Algebra: it guarantees closure properties by replacing the product with a family of join operations. We extend TriAL with recursion and explain why such an extension is more intricate for triples than for graphs. We present a declarative language, namely a fragment of datalog, capturing the recursive algebra. For both languages, the combined complexity of query evaluation is given by low-degree polynomials. We compare our language with previously studied graph query languages such as adaptations of XPath, regular path queries, and nested regular expressions; many of these languages are subsumed by the recursive triple algebra. We also provide an implementation of recursive TriAL on top of a relational query engine, and we show its usefulness by running a wide array of navigational queries over real-world RDF data, while at the same time testing how our implementation compares to existing RDF systems.

中文翻译：

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审判

RDF 数据上的导航查询被视为图查询语言的主要应用之一，然而图数据库的标准模型（本质上是标记图）与基于三元组的 RDF 模型不同。虽然存在将 RDF 数据库编码为图形数据的情况，但我们表明，即使是最自然的数据库在与图形查询语言结合使用时也必然会失去一些功能。解决方案是直接使用三元组，但是在图数据库上下文中被视为理所当然的许多属性（例如，可达性）失去了它们的自然意义。我们的目标是引入直接在三元组上工作并且是封闭的语言，即它们产生三元组的集合，而不是图。我们的基本语言称为 TriAL，或三重代数：它通过用一系列连接操作替换产品来保证闭合特性。我们用递归扩展了 TriAL，并解释了为什么这种扩展对于三元组比对图更复杂。我们提出了一种声明性语言，即数据记录的片段，用于捕获递归代数。对于这两种语言，查询评估的综合复杂性由低次多项式给出。我们将我们的语言与之前研究过的图形查询语言（例如 XPath 的改编、正则路径查询和嵌套正则表达式）进行比较；许多这些语言都包含在递归三元代数中。我们还在关系查询引擎之上提供了递归 TriAL 的实现，我们通过在真实世界的 RDF 数据上运行大量导航查询来展示它的实用性，