On 19/03/12 10:18, Milorad Tosic wrote:
Hi,
I am trying to understand how NamedGraphs work in Jena+TDB and how to use
SPARQL for the purpose. The API that I found as an eventual candidate that
would fit the task is GraphStore. My initial understanding was that GraphStore
is a kind of facade on top of the TDB store that should be used in a case of
incoming UPDATE query if there is a chance that the query eventually deals with
NamedGraphs. However, the test gives somewhat unexpected results (suggesting
that the GraphStore may actually be deep copy of the store?).
What am I missing here? I am afraid that my understanding of the conceptual
model of NamedGraphs in Jena+TDB is not correct. How should the SPARQL query
processing be organized in the case of TDB store working with named graphs?
Regards,
Milorad
Hi there,
There is a problem in this line:
GraphStore graphStore = GraphStoreFactory.create(m_triplestore) ;
which takes a model, without knowing the dataset it comes from, and asks
for a GraphStore. A temporary dataset is created which in actually
another in-memory dataset unconnected to the original m_dataset.
Datasets are the unit of collections of graphs.
Ideally, try to use datasets everywhere and think of the models as views
into the dataset. If you want to work across the model container, work
on a dataset.
But, beware, there is second trap waiting for you :-)
TDB does not know about empty graphs - if it has no triples, as far as
TDB is concerned, it does not exist. CREATE GRAPH is a no-op for TDB.
There is no separate management of graphs, only the triples and quads
tables. No entries => does not show up in
SELECT ?g WHERE { GRAPH ?g { } }
(you don't need DISTINCT)
GraphStore is actually just a wrapper around the dataset - it's
stateless but it works on datasets.
Andy
