Re: Federated query optimization for independent queries (with join)

[Sarven Capadisli]

Hi Diogo,






[1] https://issues.apache.org/jira/browse/JENA-449#comment-13651857

-Sarven

On 05/08/2013 03:06 PM, Diogo FC Patrao wrote:

I couldn't grok VALUES for my particular use (two federated queries) even though Andy explained VALUES. Andy's recommendation [1] to turn off the optimizer, worked! I've tested with --engine=ref. Looking for a way to do it via TDB assembler.

Hi Sarven

My experience with federated queries is exactly the same as yours. I
believe Andy confirmed earlier on the list that there is no optimization on
this type of queries.

I don't remember, but I think I saw somewhere that if you specify the
VALUES keyword (1) on the second federated query, it would optimize the
number of calls.

[1] http://www.w3.org/TR/sparql11-query/#inline-data

--
diogo patrão




On Wed, May 8, 2013 at 9:37 AM, Sarven Capadisli <i...@csarven.ca> wrote:

Hi all,

This is originally entered as a JENA issue (marked as minor) [1], but I
wanted to reach out to Jena users to get an insight:


At this point I'm lead to believe that ARQ is not making an optimization
before making a federated query. Or, well, hopefully the issue has to do
with my SPARQL knowledge

When trying to make two federated queries (may or may not be to same
endpoint), the solution of the first graph pattern in SERVICE is projected
onto the second SERVICE. e.g.,

SELECT DISTINCT ?a ?c ?x ?y
WHERE {
   { SERVICE <http://example.org/sparql> {
     ?a :p ?x .
     ?a skos:exactMatch ?b .
   } }

   { SERVICE <http://example.info/sparql> {
     ?c :p ?y .
     ?c skos:exactMatch ?d .
   } }

   FILTER (?b = ?c || ?d = ?a || ?a = ?c)
}

As there are no overlapping variable names in use, shouldn't each SERVICE
do its own thing independently from one another, and then the results get
joined?

What I'm experiencing is that, if the first query comes back with n number
of triples, the second query is called n times. Expectation is that, each
query is called once.

See also:

http://answers.semanticweb.**com/questions/22550/joining-**
and-filtering-federated-**sparql-queries-with-a-single-**
request-per-service<http://answers.semanticweb.com/questions/22550/joining-and-filtering-federated-sparql-queries-with-a-single-request-per-service>

Thanks,


[1] 
https://issues.apache.org/**jira/browse/JENA-449<https://issues.apache.org/jira/browse/JENA-449>

-Sarven
http://csarven.ca/#i

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