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Re: Union or not union for the default graph...

From: Andy Seaborne <andy.seaborne@epimorphics.com>
Date: Fri, 13 Apr 2012 11:43:51 +0100
Message-ID: <4F880367.1020007@epimorphics.com>
To: public-rdf-wg@w3.org


On 13/04/12 08:02, Ivan Herman wrote:
> My problem with this is that it becomes a closed possibility provided
> by the store, and not the choice of the dataset provider. I mean: the
> SPARQL service description tells me about the default dataset at the
> SPARQL endpoint. Is there a way to tell the SPARQL engine to use or
> not to use the union of the graphs that are in a specific SPARQL
> query>
>
> What am looking for is a way to tell the system: this is what I want.
> Do I want a quoting or a union semantics for my particular dataset?

Aside from the efficiency, put all the triples from the named graphs 
into the default graph.  That tells system :-)  Seriously this feels 
like a system config issue, not an architectural issue.

David write:
 >> Perhaps we should provide a standard way for an RDF system
 >> to advertise how the default graph.

and that is what SPARQL SD does for SPARQL endpoints.


So if the store does not do what the dataset provider wants, they ought 
to go to a different store. Many systems offer one mode or the other, 
not a choice, and it goes deep, right down to the bytes-on-disk.

So the data publisher makes the data user an offer via the endpoint and 
service description.

If the data user wants something different, don't take the offer made by 
the publisher.  The data user is now taking the responsibility for the 
meaning of the default graph.  So it might have to pull the named graph 
data into a local system that does what it wants.

Some systems allow you to "name" the union-of-named-graphs, as a virtual 
graph (without recursion!).  That's not standard; it's a bit of a hack 
of naming.

In practice, there can more choices: pick a few graphs out of a large 
collection and query over the union of those.  pick all but certain 
graphs or intersections or ...  Once you formalise the 
union-default-graph, it seems a small step to allowing other 
combinations as well.


There is an efficiency thing: the data publisher might wish to say "and 
by the way, make default graph the union" in a TriG simply to avoid 
repeating all the triples if written out long form.

	Andy
Received on Friday, 13 April 2012 10:44:23 UTC

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