The SPARQL LeftJoin abstract operator is not distributive over Union; this limits the algebraic manipulation of graph patterns, which in turn restricts the ability to create query plans for distributed processing or query optimization. In this paper, we present semQA, an algebraic extension for the SPARQL query language for RDF, which overcomes this issue by transforming graph patterns through the use of an idempotent disjunction operator Or as a substitute for Union. This permits the application of a set of equivalences that transform a query into distinct forms. We further present an algorithm to derive the solution set of the original query from the solution set of a query where Union has been substituted by Or. We also analyze the combined complexity of SPARQL, proving it to be NP-complete. It is also shown that the SPARQL query language is not, in the general case, fixed-parameter tractable. Experimental results are presented to validate the query evaluation methodology presented in this paper against the SPARQL standard, to corroborate the complexity analysis, and to illustrate the gains in processing cost reduction that can be obtained through the application of semQA.

中文翻译：

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semQA：具有幂等析取的 SPARQL


SPARQL LeftJoin 抽象运算符在 Union 上不具有分配性；这限制了图形模式的代数操作，进而限制了为分布式处理或查询优化创建查询计划的能力。在本文中，我们提出了 semQA，它是 RDF 的 SPARQL 查询语言的代数扩展，它通过使用幂等析取运算符或替代 Union 来转换图模式，从而克服了这个问题。这允许应用一组等价将查询转换为不同的形式。我们进一步提出了一种算法，用于从查询的解决方案集中导出原始查询的解决方案集，其中 Union 已被 Or 替代。我们还分析了 SPARQL 的组合复杂度，证明它是 NP 完全的。它还表明，在一般情况下，SPARQL 查询语言不是易于处理的固定参数。实验结果旨在根据 SPARQL 标准验证本文提出的查询评估方法，证实复杂性分析，并说明通过应用 semQA 可以降低处理成本。