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Re: Blank Node Ordering

From: Andy Seaborne <andy.seaborne@epimorphics.com>
Date: Fri, 28 Oct 2011 16:10:31 +0100
Message-ID: <4EAAC5E7.1040009@epimorphics.com>
To: public-rdf-dawg-comments@w3.org
(tying possibly related comments together):

see also:

"str() function should also accept blank node argument"
David Booth
5 Oct 2011

http://lists.w3.org/Archives/Public/public-rdf-dawg-comments/2011Oct/0003.html

and
Oct 18:

http://lists.w3.org/Archives/Public/public-rdf-dawg-comments/2011Oct/0019.html


	Andy


On 27/10/11 15:05, James Leigh wrote:
> Hello,
>
> We recently ran into some unexpected behaviour that we want to bring to
> this groups attention regarding the ORDER BY clause.
>
> When ordering RDF literals and URIs, the same literal or the same URI
> will always be arranged together. However, there is no guarantee with
> blank nodes that the same blank nodes will be arranged together.
>
> The following SPARQL query lists all the vcards addresses in the default
> graph along with their properties. A single address is represented in
> multiple result bindings, one for each property in the data store.
>
> SELECT ?card ?adr ?pred ?obj {
>    ?card a vcard:VCard; vcard:adr ?adr .
>    ?adr ?pred ?obj .
> } ORDER BY ?vcard ?adr ?pred
>
> The (author's) expected result is to have all results bindings ordered
> first by the vcard they belong to and if there are multiple addresses on
> the vcard, each address property is ordered together.
>
> For example the follow bindings sets are a valid result set. Notice that
> the entire home address comes before any of the work address properties.
> This order is predictable because of the ORDER BY clause in the query
> above.
>
> vcard=<me>, adr=<me#home>, pred=vcard:country-name, obj="Australia"
> vcard=<me>, adr=<me#home>, pred=vcard:locality, obj="WonderCity"
> vcard=<me>, adr=<me#home>, pred=vcard:postal-code, obj="5555"
> vcard=<me>, adr=<me#home>, pred=vcard:street-address, obj="111 Lake
> Drive"
> vcard=<me>, adr=<me#work>, pred=vcard:country-name, obj="Australia"
> vcard=<me>, adr=<me#work>, pred=vcard:locality, obj="WonderCity"
> vcard=<me>, adr=<me#work>, pred=vcard:postal-code, obj="5555"
> vcard=<me>, adr=<me#work>, pred=vcard:street-address, obj="33 Enterprise
> Drive"
>
> However, it would be incorrect (in SPARQL 1.0 and SPARQL 1.1 draft) for
> the author to assume the addresses will always be ordered together like
> this.
>
> Consider the result set if blank nodes were used for the address node.
> The result might look like the one below.
>
> vcard=<me>, adr=_:b1, pred=vcard:locality, obj="WonderCity"
> vcard=<me>, adr=_:b1, pred=vcard:street-address, obj="111 Lake Drive"
> vcard=<me>, adr=_:b2, pred=vcard:street-address, obj="33 Enterprise
> Drive"
> vcard=<me>, adr=_:b2, pred=vcard:country-name, obj="Australia"
> vcard=<me>, adr=_:b1, pred=vcard:country-name, obj="Australia"
> vcard=<me>, adr=_:b2, pred=vcard:postal-code, obj="5555"
> vcard=<me>, adr=_:b1, pred=vcard:postal-code, obj="5555"
> vcard=<me>, adr=_:b2, pred=vcard:locality, obj="WonderCity"
>
> Although each result of a vcard is ordered together, because it is a
> URI, the ordering of the adr blank nodes looks random and is
> unpredictable. Sesame 2.x is implemented to appear to randomly arrange
> blank node results when ordering by blank nodes as shown above. When the
> data used contains blank node there is no way to control the ordering.
>
> The author would expect that _:b1 is ordered before or after _:b2, but
> the author would not expect that _:b1 is mixed among _:b2. Although,
> there is no order between _:b1 and _:b2, SPARQL should provide guidance
> on how to arrange blank nodes.
>
> Many people still use blank nodes and this issue causes unexpected
> results for SPARQL users.
>
> My colleagues and I propose that the group seriously consider adding a
> restriction to ORDER BY in SPARQL 1.1 that will ensure ordering of any
> RDF term will guarantee that same terms are arranged together.
>
> Although, an order among different blank nodes could not be fixed.
> SPARQL should fix the same RDF terms to be ordered together.
>
> Thanks,
> James
>
>
>
Received on Friday, 28 October 2011 15:11:06 GMT

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