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Re: Additional comments on the semantics of property paths

From: Jeen Broekstra <jeen.broekstra@gmail.com>
Date: Mon, 30 Apr 2012 12:00:31 +1200
Message-ID: <4F9DD61F.9010105@gmail.com>
To: Lee Feigenbaum <lee@thefigtrees.net>
CC: public-rdf-dawg-comments@w3.org
On 12/04/12 23:58, Lee Feigenbaum wrote:
> The changes from the current Last Call working draft are as follows:
>     The semantics of *, +, and ? are changed to be non-counting (they no
> longer preserve duplicates)
>     The /, |, and ! remain unchanged as in the current draft (they
> preserve duplicates)
>     The curly brace forms -- {n}, {n,m}, {n,}, {,m} -- have all been
> removed
> There are several motivations for these design changes:
>     Changing the semantics of * and + to non-counting is expected to
> address the evaluation performance concerns raised by your comment.
>     The /, |, and ! operators are often used as shortcuts for writing
> out equivalent graph patterns longhand. By leaving these operators with
> counting semantics (just as the equivalent graph pattern expansions
> have), SPARQL 1.1 property paths will continue to yield intuitive
> results. Two examples that the Working Group has considered in coming to
> this conclusion are:
>    SELECT (sum(?cost) AS ?total)
>    {
>      :order :hasItem/:price ?cost
>    }
> and
>    SELECT ?member { ?list rdf:rest*/rdf:first ?member }
> ...when applied to a list with duplicate items, such as (1 2 1). (This
> type of query was one motivation for the property paths feature that the
> WG documented in the SPARQL 1.1 New Features & Rationale document:
> http://www.w3.org/TR/sparql-features/#Property_paths)
> In both cases, users' intuitive expectations demands that '/' preserve
> duplicates.
>     The group decided to remove the curly brace forms due to a lack of
> experience with appropriate counting/non-counting semantics for these
> forms. We believe that implementations will likely extend SPARQL 1.1
> Property Paths with support for these forms, and we will gather
> implementation experience to use for future standardization work. We've
> included this item on a list of work items for a future working group -
> http://www.w3.org/2009/sparql/wiki/Future_Work_Items
> We would be grateful if you would acknowledge that your comment has been
> answered by sending a reply to this mailing list.

First of all, my thanks to the WG for taking a long hard look at this
complex issue again.

I am satisfied that my comment has been answered by the WG and am
reasonably satisfied with the outcome from an implementation point of view.


Received on Monday, 30 April 2012 00:01:06 UTC

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