On 2014-04-03 15:00, Kingsley Idehen wrote:
> On 4/3/14 7:47 AM, Bart Vandewoestyne wrote:
>> Hello list,
>>
>> INITIAL REMARK: if this is not the appropriate mailing list for this
>> question, please let me know the best place to ask questions regarding
>> SPARQL queries and their optimization.
>>
>> I'm a beginner when it comes to writing SPARQL queries. I am trying to
>> speed up a certain query that I got from someone, which has the
>> following form:
>>
>> SELECT ?val (COUNT(DISTINCT ?id) as ?vc)
>> WHERE
>> {
>> ?id<http://www.w3.org/1999/02/22-rdf-syntax-ns#type> ?val.
>> ?id ?property ?property_value.
>> ?property_value bif:contains "'foo'".
>> ?id ?property1 ?property_value1.
>> ?property_value1 bif:contains "'bar'".
>> }
>> GROUP BY ?val
>> ORDER BY DESC(?vc)
>>
>>
>> First of all, I noticed that I can write it more elegantly as follows:
>>
>> SELECT ?val (COUNT(DISTINCT ?id) as ?vc)
>> WHERE
>> {
>> ?id<http://www.w3.org/1999/02/22-rdf-syntax-ns#type> ?val ;
>> ?property1 ?property_value1 ;
>> ?property2 ?property_value2 .
>>
>> ?property_value1 bif:contains "'foo'" .
>>
>> ?property_value2 bif:contains "'bar'" .
>> }
>> GROUP BY ?val
>> ORDER BY DESC(?vc)
>>
>> Secondly, looking at
>> http://docs.openlinksw.com/virtuoso/queryingftcols.html my educated
>> guess was that I could replace this query with
>>
>> SELECT ?val (COUNT(DISTINCT ?id) as ?vc)
>> WHERE
>> {
>> ?id<http://www.w3.org/1999/02/22-rdf-syntax-ns#type> ?val ;
>> ?property ?property_value .
>> ?property_value bif:contains 'foo or bar' .
>> }
>> GROUP BY ?val
>> ORDER BY DESC(?vc)
>>
>> and my hope was that this version would run a little faster (don't ask
>> me why... just a wild guess that I would try)
>>
>> Unfortunately, this last version seems to give different query results.
>>
>> My two questions:
>>
>> 1) Why is this last query giving different results? Am I
>> misinterpreting something?
>>
>> 2) Is there a way I can rewrite the original query so that it runs
>> faster?
>>
>> Thanks!
>> Bart
>
> One little issue here is that you don't indicate the role of named
> graphs i.e., do you want this query to be scoped to every named graph or
> to specific named graphs? As you can imagine, this has impact on the
> perceived performance.
>
> You could use the public LOD Cloud instance to demonstrate your quest,
> and share a SPARQL query results URL for accelerated analysis (on our
> side).
>
> [1] http://lod.openlinksw.com/sparql -- LOD Cloud Cache Instance (50
> Billion+ Triples)
Hello Kingsley,
I'm afraid I'm still too new to SPARQL to completely understand your
answer... I'm not really familiar with named graphs yet, but after some
googling around, i think I can conclude that I'm not using any named
graphs for two reasons:
1) My SPARQL queries are as written above, not using a FROM NAMED or
GRAPH keyword.
2) I seem to have no named graphs in my triple store, as demonstrated by
the following query:
SQL> sparql select distinct ?g where { graph ?g {?s ?p ?o} };
g
LONG VARCHAR
_______________________________________________________________________________
0 Rows. -- 1 msec.
So my two questions remain:
1) Why is the rewritten query
SELECT ?val (COUNT(DISTINCT ?id) as ?vc)
WHERE
{
?id<http://www.w3.org/1999/02/22-rdf-syntax-ns#type> ?val ;
?property ?property_value .
?property_value bif:contains 'foo or bar' .
}
GROUP BY ?val
ORDER BY DESC(?vc)
giving me different results than the original one:
SELECT ?val (COUNT(DISTINCT ?id) as ?vc)
WHERE
{
?id<http://www.w3.org/1999/02/22-rdf-syntax-ns#type> ?val.
?id ?property ?property_value.
?property_value bif:contains "'foo'".
?id ?property1 ?property_value1.
?property_value1 bif:contains "'bar'".
}
GROUP BY ?val
ORDER BY DESC(?vc)
2) Is there a way to rewrite the original query so that it runs faster?
Or can I apply other tricks? For as far as I understand from the
Virtuoso docs, I cannot use any indexing tricks to speed this query up
because the default indexing scheme on RDF_QUAD is already active and
should suffice?
If you need further information, feel free to ask.
Kind regards,
Bart
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