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Re: summary on options for JP-4 Comment about the semantics of property paths

From: Matthew Perry <matthew.perry@oracle.com>
Date: Fri, 10 Feb 2012 09:30:00 -0500
Message-ID: <4F3529E8.5070903@oracle.com>
To: public-rdf-dawg@w3.org
Hi Axel,

I would be ok with either option but have preference for option 2. To me, this seems less awkward to use and a little easier to implement.

For example

SELECT ?d
WHERE
{ :a :p1{2,3} ?b .
   { SELECT DISTINCT ?b ?c
      WHERE { ?b :p2* ?c } }
   ?c :p3{1,2} ?d }

is a bit awkward compared to

SELECT ?d
WHERE
{ :a :p1{2,3}/DISTINCT(:p2*)/:p3{1,2} ?d }

Thanks,
Matt


On 2/9/2012 4:10 PM, Axel Polleres wrote:
> (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
Received on Friday, 10 February 2012 14:30:25 GMT

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