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https://issues.apache.org/jira/browse/JENA-506?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14315950#comment-14315950
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Rob Vesse commented on JENA-506:
--------------------------------
Should we just close this as won't fix, if someone really wants to {{ORDER BY
RAND()}} in a safe way they can do so using the following pattern
{noformat}
SELECT ?var1 ?var2 # etc.
WHERE
{
# Normal query patterns
BIND(RAND() AS ?sortKey)
}
ORDER BY ?sortKey
{noformat}
This allows sorting at random while ensuring that the sort key is stable and
won't violate Java comparison rules
> SPARQL "ORDER BY RAND()" may lead to a "Comparison method violates its
> general contract!" exception
> ----------------------------------------------------------------------------------------------------
>
> Key: JENA-506
> URL: https://issues.apache.org/jira/browse/JENA-506
> Project: Apache Jena
> Issue Type: Bug
> Components: ARQ
> Affects Versions: Jena 2.10.1
> Environment: OpenJDK 1.7.0_21 (64)
> Reporter: Pasquale Minervini
>
> It seems like the SPARQL "SELECT [..] ORDER BY RAND()" query construct can
> cause troubles when retrieving results; here's a complete stacktrace
> (Specifically, I've been querying the DBLP RDF dataset:
> http://sw.deri.org/~aharth/2004/07/dblp/). I think the construct can be quite
> useful for randomly sampling nodes from the RDF graph, that's the reason for
> the 'Critical'.
> ARQ: VERSION: 2.10.1
> ARQ: BUILD_DATE: 2013-05-11T22:05:51+0100
> Exception in thread "main" java.lang.IllegalArgumentException: Comparison
> method violates its general contract!
> at java.util.TimSort.mergeLo(TimSort.java:747)
> at java.util.TimSort.mergeAt(TimSort.java:483)
> at java.util.TimSort.mergeCollapse(TimSort.java:408)
> at java.util.TimSort.sort(TimSort.java:214)
> at java.util.TimSort.sort(TimSort.java:173)
> at java.util.Arrays.sort(Arrays.java:659)
> at
> org.apache.jena.atlas.data.SortedDataBag.iterator(SortedDataBag.java:204)
> at
> org.apache.jena.atlas.data.SortedDataBag.iterator(SortedDataBag.java:189)
> at
> com.hp.hpl.jena.sparql.engine.iterator.QueryIterSort$SortedBindingIterator.initializeIterator(QueryIterSort.java:99)
> at
> org.apache.jena.atlas.iterator.IteratorDelayedInitialization.init(IteratorDelayedInitialization.java:40)
> at
> org.apache.jena.atlas.iterator.IteratorDelayedInitialization.hasNext(IteratorDelayedInitialization.java:50)
> at
> com.hp.hpl.jena.sparql.engine.iterator.QueryIterPlainWrapper.hasNextBinding(QueryIterPlainWrapper.java:54)
> at
> com.hp.hpl.jena.sparql.engine.iterator.QueryIteratorBase.hasNext(QueryIteratorBase.java:112)
> at
> com.hp.hpl.jena.sparql.engine.iterator.QueryIterSlice.hasNextBinding(QueryIterSlice.java:76)
> at
> com.hp.hpl.jena.sparql.engine.iterator.QueryIteratorBase.hasNext(QueryIteratorBase.java:112)
> at
> com.hp.hpl.jena.sparql.engine.iterator.QueryIteratorWrapper.hasNextBinding(QueryIteratorWrapper.java:40)
> at
> com.hp.hpl.jena.sparql.engine.iterator.QueryIteratorBase.hasNext(QueryIteratorBase.java:112)
> at
> com.hp.hpl.jena.sparql.engine.iterator.QueryIteratorWrapper.hasNextBinding(QueryIteratorWrapper.java:40)
> at
> com.hp.hpl.jena.sparql.engine.iterator.QueryIteratorBase.hasNext(QueryIteratorBase.java:112)
> at
> com.hp.hpl.jena.sparql.engine.ResultSetStream.hasNext(ResultSetStream.java:75)
> I can easily provide a snippet if you really need it.
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