W3C home > Mailing lists > Public > public-sparql-dev@w3.org > April to June 2016

Re: Improving EXISTS

From: Axel Polleres <droxel@gmail.com>
Date: Thu, 30 Jun 2016 15:07:53 +0200
Cc: Andy Seaborne <andy@apache.org>, public-sparql-dev@w3.org
Message-Id: <BBB3FD9C-D073-467B-9CAA-77165EF0905E@gmail.com>
To: "Peter F. Patel-Schneider" <pfpschneider@gmail.com>
Thanks a lot for the summary, Peter!

Axel
--
url: http://www.polleres.net/  twitter: @AxelPolleres

> On 30 Jun 2016, at 15:05, Peter F. Patel-Schneider <pfpschneider@gmail.com> wrote:
> 
> On 06/30/2016 04:50 AM, Axel Polleres wrote:
>> Dear Andy,
>> 
>> sure, happy to join. I have to admit that I am still unsure whether there is really a *bug* in the specification (in the sense that it would be something that could be fixed by an erratum), or whether just what 
>> the spec describes something in an unambiguous way, but it is just not what people would expect (which indeed would need if we wanted to propose an alternative to change/re-spec). 
>> 
>> I have to admit I couldn't follow the whole discussion but is there a mail/link which summarizes the issues (I am aware of the bnode injection issue, which more there are?) 
>> 
>> Thanks & best regards,
>> Axel
> 
> Hi Axel:
> 
> I don't think that there is an email that is exactly what you asked for so I
> have tried to put together a simple list of problems with EXISTS.  The first
> two problems are directly problems with the spec.  The last three are cases
> where the spec produces what can be considered to be counterintuitive results
> and some implementations diverge from the spec.
> 
> 
> Problem 1: Some uses of EXISTS are undefined
> 
> exists is only defined for graph patterns, but in
>  SELECT ?x WHERE {
>    ?x :p :c .
>    FILTER EXISTS { SELECT ?y { ?y :q :c . } } }
> the argument to exists ends up being a ToMultiSet, which is not listed under
> "Graph Pattern" in the table of SPARQL algebra symbols.
> 
> 
> Problem 2: Substitution happens where definitions are only for variables
> 
> In
>  SELECT ?x WHERE {
>    BIND ( :d AS ?x )
>    FILTER EXISTS { SELECT ?x { :b :p :c } } }
> the substitute ends up producing
>  ToMultiSet(Project(ToList(BGP( :b :p :c )),{:d}))
> However Project is only defined for variables in its second argument.
> 
> This also affects Extend, multisets, BOUND, and maybe other constructs.
> 
> 
> Problem 3: Blank nodes substituted into BGPs act as variables
> 
> In
>  SELECT ?x WHERE {
>    ?x :p :d .
>    FILTER EXISTS { ?x :q :b . } }
> against the graph { _:c :p :d , :e :q :b }
> the substitute ends up producing
>  BGP(_:c :q :b)
> when then matches against :e :q :b.
> 
> This is counterintuitive.  Some implementations diverge from the spec here.
> 
> 
> Problem 4: Substitution can flip MINUS to its disjoint case
> 
> In
>  SELECT ?x WHERE {
>    ?x :p :c .
>    FILTER EXISTS { ?x :p :c . MINUS { ?x :p :c . } } }
> against the graph { :d :p :c } the substitute ends up producing
>  Minus( BGP( :d :p :c ), BGP( :d :p :c ) )
> which produces a non-empty result.
> 
> This is counterintuitive.  Some implementations diverge from the spec here.
> 
> 
> Problem 5: Substitution affects disconnected variables
> 
> In
>  SELECT ?x WHERE {
>    BIND ( :d AS ?x )
>    FILTER EXISTS { BIND ( :e AS ?z )
>                    SELECT ?y WHERE { ?x :p :c } } }
> the substitute ends up producing
>  Join ( Extend( Z, ?z :e ),
>         ToMultiSet( Project( ToList( BGP( :d :p :c ) ), {?y} ) ) )
> 
> This may be counterintuitive.  Some, but not all, implementations diverge
> from the spec here.
Received on Thursday, 30 June 2016 13:08:25 UTC

This archive was generated by hypermail 2.3.1 : Thursday, 30 June 2016 13:08:25 UTC