- From: Claudio Gutierrez <cgutierr@dcc.uchile.cl>
- Date: Thu, 1 Jun 2006 18:22:36 -0400 (CLT)
- To: public-rdf-dawg-comments@w3.org
- cc: marcelo arenas <marenas@ing.puc.cl>, Jorge Adrián Pérez Rojas <jperez@ing.puc.cl>
Dear All, Motivated by database considerations and the discussion in this list --which we have follow closely-- we developed a thorough theoretical study of the core part of SPARQL, namely its pattern matching facility, and submitted the study to review elsewhere. As the deadline of June 6, 2006 is approaching, we thought it was convenient to make public this study with the hope that it can be of some help in the development process of SPARQL. We present in this message general observations, and suggest the interested persons to get the full document with proofs in http://arxiv.org/abs/cs.DB/0605124 Our core suggestion is the need of a formal syntax and semantics for the language SPARQL. The purpose of the following points is to highlight this need: 1) The evaluation process of a language should be directed by a clear semantics and not by the language grammar (http://lists.w3.org/Archives/Public/public-rdf-dawg-comments/2006Apr/0023.html). 2) Ideally, a query language should have a compositional semantics. As D. Maier pointed out for XML: "The meaning of an expression should be the same wherever it appears. Furthermore, expressions with equal result types should be allowed to appear in the same contexts." In SPARQL, the problems brought by a non-compositional semantics are more evident in the nested optional construct. We show in the paper a natural and simple compositional semantics. 3) A precise fromal syntax would be very welcomed by developers. For ex. current SPARQL specification lacks explicit precedence and association rules (they are implicit in the grammar). (http://lists.w3.org/Archives/Public/public-rdf-dawg-comments/2006May/0003.html). The above lack of formal specification can be exemplified by several features, among which the most visible are: i) Non idempotency of sets of graph patterns in the presence of UNION, for ex. implying that for some graph patterns P it is not always the case that {P . P} gives the same result as { P }. We show that UNION is the center of the problem and present solutions. ii) Unrestricted variables in FILTER conditions lead to undetermined results (http://lists.w3.org/Archives/Public/public-rdf-dawg-comments/2006May/0009.html) (http://lists.w3.org/Archives/Public/public-rdf-dawg-comments/2006Apr/0025.html) There are also other curiosities arising from this lack of restriction that we show in the paper. iii) ARQ, a relevant implementation of SPARQL has the following problems: - non compositional (nor conjunctions, nor optionals have compositional properties in ARQ). - the conjunction is not commutative. - FILTER with strange behavior (because of the unrestricted use of variables in conditions). best regards, Jorge Perez, Marcelo Arenas, Claudio Gutierrez.
Received on Thursday, 1 June 2006 22:22:56 UTC