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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.

Regards,

Jeen
Received on Monday, 30 April 2012 00:01:06 GMT

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