2009/9/8 Richard Cyganiak <[email protected]>:
> On 7 Sep 2009, at 15:36, Bernhard Schandl wrote:
>> is there any way to limit the number of triples returned by a DESCRIBE
>> query? The LIMIT clause of sparql obviously applies to the number of
>> result bindings, but each result binding may lead to an arbitrary
>> number of triples.
>
> Yes. There is no way to limit the number of triples in DESCRIBE,
> because the LIMIT clause in DESCRIBE applies to the number of
> described *resources*, not to the number of triples in those
> descriptions.
>
> I would perhaps execute the SELECT query that you give below, and then
> for each of the results of that query, run a "SELECT * WHERE {<uri> ?
> p ?o}" with a LIMIT clause, followed by "SELECT * WHERE {?s ?p <uri>}".
You won't however pick up the contents of any nested blank nodes using
this method that would have been pulled in by the DESCRIBE
implementations on most SPARQL providers, both because they won't have
URI's to perform the SELECT on and because the final two sets of
queries only pull in one level of nodes. Also, any blank node
references in the results of the final SELECT queries can't be joined
because they are independent. This area of SPARQL is very messy due to
blank nodes, particularly as DESCRIBE hasn't been fully specified and
doesn't have configurable behaviour with respect to the GRAPH patterns
it uses when resolving the triples related to the resource so you
either use DESCRIBE and put up with the lack of a triple LIMIT or use
it and get stuck with everything at once even if you didn't want the
?s ?p <uri> triples (where ?s is a URI) but you did want blank node
recursive closure.
> Also note that LIMIT is not truly meaningful without ORDER BY.
ORDER BY is quite slow on all current SPARQL implementations with
large datasources in the background. It is no wonder people don't use
it even though they technically should. And sometimes you even get
errors using ORDER BY [1] (when you didn't previously ;) ) as I did
when I attempted to add ORDER BY to this query:
SELECT ?concept ?p ?o
WHERE { ?concept rdfs:label "Berlin"@en . ?concept ?p ?o .}
ORDER BY ?concept
LIMIT 5000
I am not sure if that error is a Virtuoso bug or a configuration bug.
Cheers,
Peter
[1] http://tinyurl.com/nbbd94
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