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Date: Fri, 12 Aug 2011 13:41:27 +0000 (UTC)
From: "Andy Seaborne (JIRA)" <jira@apache.org>
To: jena-dev@incubator.apache.org
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Subject: [jira] [Updated] (JENA-89) Avoid a total sort for ORDER BY + LIMIT
 queries
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     [ https://issues.apache.org/jira/browse/JENA-89?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Andy Seaborne updated JENA-89:
------------------------------

    Comment: was deleted

(was: OpBuilder was broken in another was as well: "Op sub = build(list, 3) ;" now fixed.
)

> 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
>
>
> 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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