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https://issues.apache.org/jira/browse/JENA-89?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Andy Seaborne updated JENA-89:
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Attachment: (was: JENA-89-TopTests.patch)
> Avoid a total sort for ORDER BY + LIMIT queries
> -----------------------------------------------
>
> Key: JENA-89
> URL: https://issues.apache.org/jira/browse/JENA-89
> Project: Jena
> Issue Type: Improvement
> Components: ARQ
> Reporter: Paolo Castagna
> Assignee: Paolo Castagna
> Priority: Minor
> Labels: arq, optimization, scalability, sparql
> Attachments: ARQ_JENA-89_r1156212.patch,
> JENA-89-Top-FixHeapCollation.patch, JENA-89-TopTests.patch
>
>
> In case of SPARQL queries with ORDER BY + LIMIT, ARQ sorts the entire result
> set and then produces the first N, according to the specified LIMIT.
> As an alternative, discussed on jena-dev [1], we can use a PriorityQueue [2]
> (which is based on a priority heap) to avoid a sort operation.
> ARQ's algebra package contains already a OpTopN [3] operator. The OpExecutor
> [4] will need to use a new QueryIterTopN instead of QueryIterSort +
> QueryIterSlice. A new TransformOrderByLimit to be used by Optimize is also
> necessary.
> ORDER BY + LIMIT queries are typically used to construct the first page when
> results are paginated. Then the following query is ORDER BY + OFFSET + LIMIT.
> (Often users stop at the first page). Ideally, we could cache the ORDER BY
> and implement the OFFSET|LIMIT using results from the cache. However, the
> improvement described by this issue is limited to the ORDER BY + LIMIT case
> for which a priority heap is a good enough solution.
> Hopefully, this would improve the scalability of ORDER BY + LIMIT queries in
> case of small values specified on the LIMIT.
> [1] http://markmail.org/thread/5d2gtazkoxsa2ayv
> [2]
> http://download.oracle.com/javase/6/docs/api/java/util/PriorityQueue.html
> [3]
> https://svn.apache.org/repos/asf/incubator/jena/Jena2/ARQ/trunk/src/com/hp/hpl/jena/sparql/algebra/op/OpTopN.java
> [4]
> https://svn.apache.org/repos/asf/incubator/jena/Jena2/ARQ/trunk/src/com/hp/hpl/jena/sparql/engine/main/OpExecutor.java
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