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https://issues.apache.org/jira/browse/JENA-108?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13096215#comment-13096215
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Paolo Castagna commented on JENA-108:
-------------------------------------
Andy, I am sorry but I am not sure I understand what you are suggesting.
> Optimise DISTINCT + ORDER BY + LIMIT queries
> --------------------------------------------
>
> Key: JENA-108
> URL: https://issues.apache.org/jira/browse/JENA-108
> Project: Jena
> Issue Type: Improvement
> Components: ARQ
> Reporter: Paolo Castagna
> Assignee: Paolo Castagna
> Priority: Minor
> Labels: optimization, sparql
>
> When users what to find the top 10 most 'something' 'things' in their data
> write queries of this form: SELECT DISTINCT * { ... } ORDER BY ... LIMIT 10
> We can add a specific low level optimization for this type of queries using a
> new QueryIterTopNDistinct iterator.
> This SPARQL query:
> SELECT * { ... } ORDER BY ... LIMIT 10
> has this algebra:
> (slice _ 10
> (order (...)
> (bgp (...))))
> which is now transformed into (see JENA-89):
> (top (10 ...)
> (bgp (...)))
> This query:
> SELECT DISTINCT * { ... } ORDER BY ...
> has this algebra:
> (distinct
> (order (...)
> (bgp (...))))
> which is now transformed into (see JENA-90):
> (reduced
> (order (...)
> (bgp (...))))
> What about:
> SELECT DISTINCT * { ... } ORDER BY ... LIMIT 10 ?
> The algebra expression is:
> (slice _ 10
> (distinct
> (order (?p ?o)
> (bgp (triple ?s ?p ?o)))))
> which is (currently) transformed into:
> (slice _ 10
> (reduced
> (order (?p ?o)
> (bgp (triple ?s ?p ?o)))))
> The optimization in JENA-90 kicks in, but it would be better to use the top
> operator for small limits.
> However, it must be a slightly different top implementation (which accepts
> only distinct bindings).
> As Andy suggested in a comment on JENA-90 (12 August 2011), we can transform
> the algebra into:
> (top (10 ...)
> (distinct
> (bgp (...))))
> and then use a QueryIterTopNDistinct when OpExecutor(OpTop) sees an
> opTop.getSubOp() instanceof OpDistinct.
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