On 09/04/14 16:50, Chris Dollin wrote:
On Wednesday, April 09, 2014 12:35:42 PM Walter Travassos wrote:
Recentelly I discoverd the functions on ARQ but I ´m having a little
dificult to use these functions. I´m trying to query the DBpedia using de
function afn:namespaces to find the namespaces of some resources. Here is my
code:
String query =
"PREFIX afn:
<http://jena.hpl.hp.com/ARQ/function#>"+
"select distinct ?s ?p ?namespace " +
"where {?s ?p ?o ." +
"filter (isURI(?s)) " +
"filter (isURI(?o))" +
"filter (afn:namespace(?s) = (afn:namespace(?o)) )}
limit 10";
Handy tip: put newlines (\n) on each line of your query so that
if there's a syntax error the line numbers help.
Note that this query crawls over the entire content of dbpedia.
But I always recieve the error 500.
Server internal error. I expect it ran out of space or time trying to
fetch every triple in the data. Note that the DISTINCT will force it
to scan the data even though there's a LIMIT -- the LIMIT applies
to the distinct results, not the pre-distinct results. If I understand
correctly.
(You should be able tow rite a LIMITed sub-query to get a few results
and then apply the DISTINCT in an outer query.)
Chris
I'm surprised that anything is working. 500 is not good.
The query is not executed by ARQ - it's executed by whatever is at
"http://dbpedia.org/sparql/"; (it's a version of Virtuoso)
afn:namespace is specific to ARQ and not understood by Virtuoso. It
should return 400 if it can't execute it.
A standards compliance way to do it is to use:
replace(str(?x), "(/|#[^/#]*)$", "")
which chops the tail off the string of the URI.
either as
FILTER (replace(str(?s), "(/|#[^/#]*)$", "") =
replace(str(?o), "(/|#[^/#]*)$", "")
or
# BIND to split the line up for readability
BIND(?X AS replace(str(?s), "(/|#[^/#]*)$", ""))
FILTER ( STRSTARTS(str(?o), ?X) )
But as Chris says, your query is very expensive and DBpedia will in all
likelihood timeout during execution giving truncated results.
Andy
