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Re: Usage of datasets and graphs in triple store

From: Eric Prud'hommeaux <eric@w3.org>
Date: Fri, 16 Mar 2018 09:11:08 -0400
To: Axel Polleres <axel@polleres.net>
Cc: Mark Wallace <mark.wallace@semanticarts.com>, Mikael Pesonen <mikael.pesonen@lingsoft.fi>, "semantic-web@w3.org" <semantic-web@w3.org>
Message-ID: <20180316131107.GN8805@w3.org>
* Axel Polleres <axel@polleres.net> [2018-03-16 01:22-0700]
> FWIW, this behaviour of FROM is definitly NOT standardized across triple stores.
> The spec says only
> "Each FROM clause contains an IRI that indicates a graph to be used to form the default graph. This does not put the graph in as a named graph.
> ...
> If a query provides more than one FROM clause, providing more than one IRI to indicate the default graph, then the default graph is the RDF merge <http://www.w3.org/TR/rdf-mt/#graphdefs> of the graphs obtained from representations of the resources identified by the given IRIs."
> 
> while dereferncing and downloading RDF data from the given IRI is one option, what "obtained from representations of the resource identified by the given IRIs" 
> exactly means for a triple store is not defined by the spec and left open to the implementation.

To Axel's point, there are a few things that SPARQL engines do with FROM:

1 fail -- no FROM's are allowed
2 resolve it against locally-known graphs
3 resolve it against web resources

Some engines also treat the default graph as the union of all of the named graphs, but that's more a FROM NAMED behavior than a FROM behavior.

(Slightly related, Fuseki's management web interface calls the server with queries like "...FROM <null>..."; does Fuseki have a special meaning for <null> or does it simply treat nonexistent graphs as empty?)


> just my two cents,
> Axel
> --
> Dr. Axel Polleres	
> url: http://www.polleres.net/  twitter: @AxelPolleres
> 
> > On 15.03.2018, at 09:42, Mark Wallace <mark.wallace@semanticarts.com> wrote:
> > 
> > The FROM clause brings in an RDF graph from a URL, so if the external triple store provides a URL to a data set, then it's possible. 
> > 
> > For example, the RDF4J triple store exposes an HTTP API to access RDF statements in any of its "repositories", so you could do something like:
> > 
> >   select * 
> >   FROM  <http://localhost:8080/rdf4j-server/repositories/myRepo/statements>   
> >   where . . .
> > 
> > So it's definitely possible.  
> > 
> > SERVICE would seem more efficient than fetching all of the repository statements if the data set is large. 
> > 
> > -Mark
> > 
> > -----Original Message-----
> > From: Mikael Pesonen <mikael.pesonen@lingsoft.fi> 
> > Sent: Wednesday, March 14, 2018 10:39 AM
> > To: semantic-web@w3.org
> > Subject: Usage of datasets and graphs in triple store
> > 
> > 
> > Hi,
> > 
> > what would be good reasons to split data to different graphs and also into different datasets?
> > 
> > About sparql FROM clause, is it possible to fetch data from external triple store, or is SERVICE method for that only?
> > 
> > Thank you
> > 
> > --
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-ericP

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Received on Friday, 16 March 2018 13:11:21 UTC

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