Hi Kingsley,
Thanks for your clarification, but I don't understand why 'first encounter
!= distinct'. I was thinking that DISTINCT just causes duplicate solutions
to be excluded from the result set, just like DISTINCT in SQL. The SPARQL
reference states <http://www.w3.org/TR/rdf-sparql-query/#modDistinct> "The
DISTINCT solution modifier eliminates duplicate solutions. Specifically,
each solution that binds the same variables to the same RDF terms as another
solution is eliminated from the solution set." This sounds like a first
encounter would be added to the result set, and any subsequent encounters
would simply not be added to the result set.
In my (probably naiive) understanding, when the engine sees a solution that
has not yet been added to the result set, the engine would add it to the
result set. At that point with our example query, the LIMIT would be reached
and the result could be returned without traversing any more triples. Am I
missing something? Is there some reason why the engine would need to compute
the entire result set with before applying the limit? Granted, the SPARQL
reference says "duplicates are eliminated before either limit or offset is
applied", but this is in terms of the abstract result set specification and
can be optimized around (i.e. check the LIMIT condition after adding each
solution to the result set) without changing the correctness of the results
returned. Thanks again!
Best regards,
Curran
On Tue, May 24, 2011 at 1:16 PM, Kingsley Idehen <kide...@openlinksw.com>wrote:
> On 5/24/11 1:08 PM, Curran Kelleher wrote:
>
> Greetings,
>
> The problem remains, the following query doesn't execute on the public
> DBPedia
> endpoint<http://dbpedia.org/snorql/?query=select+distinct+%3Fproperty+where+%7B%0D%0A+++++%3Fs+%3Fproperty+%3Fo.%0D%0A%7D+limit+1>,
> even with a limit:
>
> select distinct ?property where {
> ?s ?property ?o.
> } limit 1
>
> Without 'distinct' it does work:
>
> select ?property where {
> ?s ?property ?o.
> } limit 1
>
> Why might this be?
>
>
> Because Distinct requires more work.
>
>
> Shouldn't the engine be able to work this one out quickly even with
> 'distinct', as it needs to only traverse a single triple to compute the
> result?
>
>
> Really? First encounter != distinct :-)
>
> It seems the engine is doing some unnecessary computation to do with
> 'distinct' and is timing out because of it.
>
>
> LIMIT doesn't simply the Distinct computation. It simply limits the
> resultset size.
>
> Kingsley
>
>
>
> Best regards,
> Curran
>
> On Tue, May 24, 2011 at 9:17 AM, Kingsley Idehen
> <kide...@openlinksw.com>wrote:
>
>> On 5/24/11 8:30 AM, Mohamed Morsey wrote:
>> > Hi Sarasi,
>> >
>> > I've performed that query with limit 1000, and it worked on one of our
>> > local endpoints, and it ended within 3 minutes.
>> > So I guess that the maximum time allowed for a query on the official
>> > endpoint is relatively low, but the query itself is executable with
>> limit.
>> >
>> > Hope that helps.
>> >
>> All,
>>
>> If we want to have a live DBpedia endpoint that serves the whole world,
>> we have to have it configured in such a way that it forces use of OFFSET
>> and LIMIT.
>>
>> We are very experienced with DBMS oriented data exposed to massive
>> concurrent users, and this stems to periods prior to the pervasive Web
>> of today, thus we've configured the DBpedia SPARQL endpoint with this
>> experience in hand.
>>
>> Once again, the DBpedia endpoint is for everyone, so we deliberately
>> protect against inconsiderate use e.g. attempting to get massive results
>> sets in single query passes at the expense of others.
>>
>> Also remember, you can make application or service specific instances of
>> the DBpedia SPARQL endpoint via a number of offerings on EC2 if you seek
>> hassle free reconstruction of DBpedia.
>>
>> Links:
>>
>> 1. http://blog.dbpedia.org/2011/01/31/dbpedia-36-ami-available/ .
>>
>> --
>>
>> Regards,
>>
>> Kingsley Idehen
>> President& CEO
>> OpenLink Software
>> Web: http://www.openlinksw.com
>> Weblog: http://www.openlinksw.com/blog/~kidehen
>> Twitter/Identi.ca<http://www.openlinksw.com/blog/%7Ekidehen%0ATwitter/Identi.ca>:
>> kidehen
>>
>>
>>
>>
>>
>>
>>
>> ------------------------------------------------------------------------------
>> vRanger cuts backup time in half-while increasing security.
>> With the market-leading solution for virtual backup and recovery,
>> you get blazing-fast, flexible, and affordable data protection.
>> Download your free trial now.
>> http://p.sf.net/sfu/quest-d2dcopy1
>> _______________________________________________
>> Dbpedia-discussion mailing list
>> Dbpedia-discussion@lists.sourceforge.net
>> https://lists.sourceforge.net/lists/listinfo/dbpedia-discussion
>>
>
>
> ------------------------------------------------------------------------------
> vRanger cuts backup time in half-while increasing security.
> With the market-leading solution for virtual backup and recovery,
> you get blazing-fast, flexible, and affordable data protection.
> Download your free trial now. http://p.sf.net/sfu/quest-d2dcopy1
>
>
> _______________________________________________
> Dbpedia-discussion mailing
> listDbpedia-discussion@lists.sourceforge.nethttps://lists.sourceforge.net/lists/listinfo/dbpedia-discussion
>
>
>
> --
>
> Regards,
>
> Kingsley Idehen
> President & CEO
> OpenLink Software
> Web: http://www.openlinksw.com
>
> Weblog: http://www.openlinksw.com/blog/~kidehen
> Twitter/Identi.ca: kidehen
>
>
>
>
>
>
>
> ------------------------------------------------------------------------------
> vRanger cuts backup time in half-while increasing security.
> With the market-leading solution for virtual backup and recovery,
> you get blazing-fast, flexible, and affordable data protection.
> Download your free trial now.
> http://p.sf.net/sfu/quest-d2dcopy1
> _______________________________________________
> Dbpedia-discussion mailing list
> Dbpedia-discussion@lists.sourceforge.net
> https://lists.sourceforge.net/lists/listinfo/dbpedia-discussion
>
>
------------------------------------------------------------------------------
vRanger cuts backup time in half-while increasing security.
With the market-leading solution for virtual backup and recovery,
you get blazing-fast, flexible, and affordable data protection.
Download your free trial now.
http://p.sf.net/sfu/quest-d2dcopy1
_______________________________________________
Dbpedia-discussion mailing list
Dbpedia-discussion@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/dbpedia-discussion
