Re: “Default” vs “unnamed” graph


I had a long chat with Eric after the telecon today. Seema and another colleague of mine, Matt Perry, too joined. Following the discussion, now we are okay with the use of the term "default graph" to refer to the unnamed graph in an R2RML-based RDF store.

So, please go ahead and make the minor changes needed in the current draft to replace unnamed graph with default graph.

If interested, here is how I managed to convince myself in an informal way, by considering triples vs. quads:

    * In general, a default graph (DG, for short) can be thought of as a container of *triples* in a dataset whereas the named graphs contain *quads*.
    * R2RML mapping causes triples and quads to (virtually) come into existence. Among these, the *triples* (by birth) make up the DG of an R2RML-based RDF store.
    * A DG in the context of a SPARQL query on the other hand could consist of triples-by-birth (from an unnamed graph) OR triples-generated-via-UNION-of-SPO-projections-from-quads in an RDF store.
    * So, it is quite possible to have (DG of a SPARQL query against an R2RML-based RDF store) != (DG of the target R2RML-based RDF store). But the two DGs always share the characteristic that both of them consist only of triples -- triples-by-birth only in R2RML and triples-by-birth or triples-by-transformation in SPARQL -- but neither has any quads.

- Souri.

On 3/15/2011 5:18 PM, Richard Cyganiak wrote:
> I'd like to re-iterate my position from this call that we should define the output of an R2RML mapping as an RDF Dataset in the SPARQL sense, as it already says in the introduction, and consistently use the SPARQL's terminology.
> This would imply using the terms “named graph” and “default graph”. The term “unnamed graph” would be removed from the spec.
> The objection raised in the call was that the default graph used in a SPARQL query can actually be constructed on the fly, on a query-by-query basis, by using the FROM keyword or SPARQL protocol parameters.
> This is a valid observation. But I argue that this doesn't conflict at all with the use of the RDF Dataset concept and the term “default graph”.
> To quote from the SPARQL spec [1]:
>> A SPARQL query may specify the dataset to be used for matching by using the FROM clause and the FROM NAMED clause to describe the RDF dataset. If a query provides such a dataset description, then it is used in place of any dataset that the query service would use if no dataset description is provided in a query.
> This makes clear that if FROM/FROM NAMED are used, then one queries a *different* dataset from the one that the query service offers *by default* if FROM/FROM NAMED were not used.
> I'm proposing that we think of the R2RML-generated dataset as the dataset which a query service would use by default in absence of a specific dataset description. This doesn't preclude the possibility of overriding the default graph or any other graph with FROM/FROM NAMED and the SPARQL protocol.
> This would be a simple change in terms of spec text (s/unnamed graph/default graph/ and check the early sections for anyplace that should say “RDF dataset” instead of “RDF graph”). So I propose that we do this before the WD release.
> If there are no objections (on- or off-list), I'll go ahead and do this.
> Best,
> Richard
> [1]

Received on Tuesday, 15 March 2011 22:34:19 UTC