[Virtuoso-users] How do I iterate over a SPARQL query referring to a graph?

[Beppe Mazzola]

Hi to everybody

I am developing a Virtuoso stored procedure. I want to perform a loop over
the result of a SPARQL query. The problem comes when the query contains a
reference to a graph. In all my attempts I get no error but an empty result
set as well. I tried the following

create procedure R2RML.DBA.try() returns integer{
  for (sparql define input:storage ""
       select ?s ?p
       from <http://example.com/resource>
       where {
         ?s <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> ?o .
       }  LIMIT 5 ) do
    {
      use_the_value("s");
    }};

and also the following

create procedure R2RML.DBA.try() returns integer{
  declare srcgraph varchar;
  srcgraph := 'http://ec.example.com/resource';
  for (sparql define input:storage ""
        select ?s ?p
        where {
        GRAPH `iri(?:srcgraph)`
          {
            ?s <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> ?o .
          }
        }  LIMIT 5 ) do
    {
      use_the_value("s");
    }};

In both cases no iteration is performed, despite the same query, when
executed in the SPARQL endpoint, returns a result.

If I remove the reference to the graph the iterations are executed:

create procedure R2RML.DBA.try() returns integer{
  for (sparql define input:storage ""
        select ?s ?p
        where {
          ?s <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> ?o .
        }  LIMIT 5 ) do
    {
      use_the_value("s");
    }};

I am using the following Virtuoso Open Source Version
Virtuoso Open Source Edition (Column Store) (multi threaded)
Version 7.2.4.2.3217-pthreads as of Feb  3 2017
Compiled for Linux (x86_64-unknown-linux-gnu)

Has anybody an idea of what I get wrong?


Thanks!

Cheers

Beppe

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