From: Axel Polleres <axel.polleres@deri.org>

Date: Thu, 9 Feb 2012 22:10:47 +0100

Message-Id: <DE8DFE24-08E7-487C-B16D-7BAE7E26D3BB@deri.org>

To: SPARQL Working Group <public-rdf-dawg@w3.org>

Date: Thu, 9 Feb 2012 22:10:47 +0100

Message-Id: <DE8DFE24-08E7-487C-B16D-7BAE7E26D3BB@deri.org>

To: SPARQL Working Group <public-rdf-dawg@w3.org>

(in completion of ACTION-587) Dear all, as discussed in the last Telco, we have several options on how to proceed with addressing comment JP-4 [1]. If possible, I would like to get consensus on how to proceed here in the next Telco. In the previous Telco [2], we seemed to have consensus that we do not aim to switch the default behaviour from counting semantics to distinct paths. Now two possibilities to proceed were discussed: Option 1... keep everything as it is in the grammar, and explain which DISTINCT path subqueries can be optimized: As outlined in my email below, it might not be entirely trivial to argue in response to the comment that this would be equivalent to the JP-4 proposed semantics, I am not 100% sure whether/how to define a rewriting to wrap all path expressions into DISTINCT subqueries, such that it would be equivalent to their semantics (e.g. regarding bnode [] shortcuts). Option 2 ... add DISTINCT around paths: It seems that sticking to our intended semantics and allowing - orthogonally to their ALLPATHS keyword proposal the keyword DISTINCT( ) around path expressions switching to existential paths semantics would be equivalent to the JP-4 existential paths semantics as outlined in Section 7.1 of their paper, and thus optimizable. Unlike someone sees a 3rd alternative, I would like to propose to decide between those two options next time and proceed, discussion prior to the call on email would be appreciated. Option 2 might be easier to implement, but also requires us to go for another LC round, as it would change the grammar. I think, in case we skip PR and manage to republish very soon, we would still manage to stay within time limits, but I would also like to know the team contacts' opinion on that. best, Axel 1. http://lists.w3.org/Archives/Public/public-rdf-dawg-comments/2012Jan/0009.html 2. http://www.w3.org/2009/sparql/meeting/2012-02-07#comments On 2 Feb 2012, at 22:01, Axel Polleres wrote: > To get the discussion going, my personal opinion on that one is as follows: > > a) I also think this is new and relevant information. > b) I think it would be important to point to the fact that a naive implementation > of property paths may become very inefficient/blowing up on even relatively harmelsssly looking examples, and that Property PATHs > wrapped into DISTINCT subqueries can be evaluated more efficiently ... I'd be even more than happy > to point in an informal reference to their work, however I feel honestly very uncomfortable with their > title. > c) As for their conclusion, proposing a default semantics that uses distinct paths semantics, > whereas a separate keyword ALL-PATHS would be indicating the current semantics: > it seems that (looking at their results in Section 7.1) that this is orthogonally possible with our current semantics > by just wrapping any TriplesSameSubjectPath containing a property path into a DISTINCT subquery. > It seems their result in section 7.1 indicates something along these lines, but I need some help there: > admittedly don't have a formal proof for this equivalence yet (to be cautious, I am not yet 100% clear how/whether there is > any interference possible with bnodes within TriplesSameSubjectPath and duplicates coming from those bnodes) > > This all said, I unfortunately haven't had the time yet to check all their claims in all detail. > > Axel > > > On 29 Jan 2012, at 20:58, Lee Feigenbaum wrote: > > [...] > > I do see new information in this latest comment, so I'd like to discuss > > it, as it is high priority to know whether it might affect our schedule. > > > > Lee > > > > > End of last call for query is Feb 6th - I don't expect we will be > > > replying until after that point anyway, but, if there's time, we can > > > make a start discussing it. > > > > > > Andy > > > > > > On 29/01/12 19:03, Lee Feigenbaum wrote: > > >> I'd suggest we discuss on our call this week. > > >> > > >> Lee > > >> > > >> On 1/28/2012 12:38 PM, Andy Seaborne wrote: > > >>> A previous conversation of these issues includes: > > >>> > > >>> http://lists.w3.org/Archives/Public/public-rdf-dawg-comments/2011Feb/0005.html > > >>> > > >>> > > >>> > > >>> > > >>> where the WG points out: > > >>> 1/ That aggregation may yield confusing results to a natural query > > >>> 2/ That an optimizer may be given further information via a sub-query > > >>> and DISTINCT. > > >>> > > >>> The purchase order example seems to me to be a reasonable expectation of > > >>> any spreadsheet user. The same price is arrived at several times (two > > >>> ways: via two :item1's and two uses of the same literal 2). > > >>> > > >>> There are two sets of use cases: one set where duplicates are essential, > > >>> and one set where they are redundant. > > >>> > > >>> Jorge's reply then: > > >>> > > >>> http://lists.w3.org/Archives/Public/public-rdf-dawg-comments/2011Feb/0012.html > > >>> > > >>> > > >>> > > >>> > > >>> acknowledged the points in the response. > > >>> > > >>> Andy > > >>> > > >>> -------- Original Message -------- > > >>> Subject: Comments about the semantics of property paths > > >>> Resent-Date: Fri, 27 Jan 2012 14:52:22 +0000 > > >>> Resent-From: public-rdf-dawg-comments@w3.org > > >>> Date: Fri, 27 Jan 2012 11:51:45 -0300 > > >>> From: jorge perez <jorge.perez.rojas@gmail.com> > > >>> To: public-rdf-dawg-comments@w3.org > > >>> CC: Marcelo Arenas <marcelo.arenas1@gmail.com>, Sebastián Conca > > >>> <sconca87@gmail.com>, jorge perez <jorge.perez.rojas@gmail.com> > > >>> > > >>> Dear DAWG members, > > >>> > > >>> We have some comments regarding the semantics of property paths. We > > >>> know that this issue has been raised before, but we think that we can > > >>> provide substantial new information to reconsider it. > > >>> > > >>> We have conducted a thorough study of the current semantics of > > >>> property paths including an empirical analysis. All our results are in > > >>> a paper that has been accepted in WWW 2012. You can find a > > >>> copy of the extended version of this paper at > > >>> http://www.dcc.uchile.cl/~jperez/sub-www-ext.pdf. Given the tight > > >>> schedule of the group, we think that it might be useful to make these > > >>> results public for the group before we have a final published version. > > >>> > > >>> As a summary we can provide two main comments, one from a practical > > >>> perspective and another from a theoretical perspective. > > >>> > > >>> ----------- > > >>> > > >>> - Comment 1: Poor performance of current engines. > > >>> ================================================= > > >>> We tested 4 implementations of property paths: Jena, RDF::Query, > > >>> Sesame, and KGram-Corese (details on the experimental setting can be > > >>> found at http://www.dcc.uchile.cl/~jperez/www_repeatability/). A first > > >>> set of experiments was with synthetic data and other with real data. > > >>> > > >>> In both cases the implementations were not capable to handle even > > >>> small data for the most simple property path queries. > > >>> > > >>> Case A) > > >>> We tested RDF data representing complete graphs. No implementation was > > >>> able to handle a graph with 13 nodes for a query with a single > > >>> property path of the form (:P)* > > >>> > > >>> data1: http://www.dcc.uchile.cl/~jperez/www_repeatability/clique13.n3 > > >>> query1: http://www.dcc.uchile.cl/~jperez/www_repeatability/Cliq-1.rq > > >>> > > >>> See Figure 1 in the paper for the performance of all implementations > > >>> below 13 nodes. The figure suggests that the evaluation time for these > > >>> implementations growths doubly-exponentially w.r.t. the size of the > > >>> input. > > >>> > > >>> Case B) > > >>> We tested real RDF data crawled from a small set of foaf documents. We > > >>> started from Axel's foaf document and retrieve friends, friends of > > >>> friends, etc. following foaf:knows links, and constructed several test > > >>> cases. In this case, no implementation was able to handle an RDF graph > > >>> of 14KB for a query with a single property path (foaf:knows)* > > >>> > > >>> data2: http://www.dcc.uchile.cl/~jperez/www_repeatability/E.n3 > > >>> query2: http://www.dcc.uchile.cl/~jperez/www_repeatability/Foaf-1.rq > > >>> > > >>> See Table 1 in the paper for the performance of all implementations. > > >>> > > >>> - Comment 2: High Computational Complexity. > > >>> =========================================== > > >>> We prove in the paper that for the current semantics of property paths > > >>> in SPARQL the complexity of evaluation is double-exponential (Lemma > > >>> 5.4 and Theorem 5.5). Given that property paths require counting > > >>> paths, we measure the complexity by making use of counting complexity > > >>> classes. The technical result is that SPARQL 1.1 evaluation is not > > >>> even inside #P (Theorem 5.5), where #P is the counting complexity > > >>> class associated to NP (a prototypical #P-complete problem is the > > >>> problem > > >>> of computing the number of truth assignments that satisfies a > > >>> propositional > > >>> formula, which is more complicated than the prototypical NP-complete > > >>> problem > > >>> which is to verify whether there exists at least one truth assignment > > >>> that > > >>> satisfies a propositional formula). Thus, in informal terms, we prove > > >>> that > > >>> SPARQL 1.1 evaluation considering counting is even more complex than > > >>> solving an NP-complete problem. > > >>> > > >>> We also prove that if only the input data is considered to measure the > > >>> complexity of the problem, then the evaluation problem is #P-hard. > > >>> Notice that without property paths, the evaluation problem for SPARQL > > >>> can be solved in polynomial time (if the complexity is measured only in > > >>> terms of the size of the data). > > >>> > > >>> ------------ > > >>> > > >>> Discussion > > >>> ========== > > >>> > > >>> One of the main conclusions that one can draw from our results is that > > >>> the poor performance exhibited in Cases A) and B) above is not a > > >>> problem of the particular implementations but a problem of the > > >>> specification itself, as our theoretical results imply that every > > >>> implementation that follows the current specification of SPARQL 1.1 > > >>> would have the same poor behavior. > > >>> > > >>> Our results also show that the main source of complexity is the > > >>> requirement of counting paths, and in particular the procedure ALP > > >>> which is the one that gives the semantics for counting. Essentially, > > >>> the counting mechanism produces a number of duplicates that in some > > >>> cases are beyond any naturally feasible number. Table 7 in the paper > > >>> shows a worst case analysis. For instance, for the case > > >>> > > >>> data3: http://www.dcc.uchile.cl/~jperez/www_repeatability/clique7.n3 > > >>> query3: http://www.dcc.uchile.cl/~jperez/www_repeatability/Cliq-2.rq > > >>> > > >>> we formally prove that any implementation that follows the current > > >>> specification should produce an output of 79 Yottabytes (79 trillion > > >>> Terabytes), and thus would not fit in any reasonable storage device. > > >>> Please notice that unfeasible counting can also be obtained with real > > >>> data. For example, for the case > > >>> > > >>> data4: http://www.dcc.uchile.cl/~jperez/www_repeatability/D.n3 > > >>> query2: http://www.dcc.uchile.cl/~jperez/www_repeatability/Foaf-1.rq > > >>> > > >>> ARQ (which was the only implementation that was able to handle this > > >>> case in less than one hour) produced an output of 587MB. Notice that > > >>> data3 is of only 13.2KB. Table 6 in the paper shows the running time > > >>> and the output size. Please notice that this experiment is with real > > >>> data and it is highly probable that property paths will be used in > > >>> practice with this type of queries. > > >>> > > >>> It is worth mentioning that our group is not the only one that have > > >>> formally studied property-path semantics according to the current > > >>> specification, and that have shown negative results about the complexity > > >>> of evaluating it. We are aware that Katja Losemann and Wim Martens > > >>> obtained similar results independently from us. Wim Martens gave a > > >>> talk about this called "The complexity of evaluating path expressions > > >>> in SPARQL" in a Dagstuhl seminar. In that work, the authors also > > >>> studied property-path expressions of the form :P{m,n}, and show that > > >>> the complexity of evaluating them is very high. > > >>> > > >>> We think that we have provided substantial new information to > > >>> reconsider the issue of property path semantics. We have several other > > >>> comments, but we think that the two comments above are the most > > >>> important to consider, and we are open to continue the discussion with > > >>> the group and, if necessary, cooperate with the group to make a proposal > > >>> for property path evaluation that can have an efficient evaluation > > >>> method. > > >>> > > >>> Best regards, > > >>> > > >>> Marcelo Arenas > > >>> Sebastián Conca > > >>> Jorge Pérez > > >>> > > >>> > > >>> > > > > > > > > > >Received on Thursday, 9 February 2012 21:11:23 UTC

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