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https://issues.apache.org/jira/browse/JENA-638?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13902489#comment-13902489
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Andy Seaborne commented on JENA-638:
------------------------------------
Keeping open for now - one more area to investigate.
When there is a {{distinct}} and {{project}} and the variables of the project
are all the order and more, can be add the unused variables into the order
(it's legal to change the sort on variables that were entirely
non-deterministic before).
> Optimization OrderByDistinctApplication can enable better TopN optimization.
> ----------------------------------------------------------------------------
>
> Key: JENA-638
> URL: https://issues.apache.org/jira/browse/JENA-638
> Project: Apache Jena
> Issue Type: Improvement
> Components: ARQ, Optimizer
> Affects Versions: Jena 2.11.1
> Reporter: Andy Seaborne
> Assignee: Andy Seaborne
>
> There is a new opportunity for transformation of slice/order to a TopN
> execution.
> Currently, {{OrderByDistinctApplication}} is done after TopN.
> If {{OrderByDistinctApplication}} is done before TopN optimization, then the
> algebra may have the form slice/order/distinct/project which is can be
> executed as a TopN query. This form does not appear in algebra expressions
> directly from queries; the modifier order is slice/distinct/project/order
> which, in general, is not safe for TopN execution.
> However, {{OrderByDistinctApplication}} picks out a condition for it's own
> optimization, that leaves the resultant algebra as further transformable to
> TopN.
> Currently:
> {noformat}
> SELECT DISTINCT ?z
> WHERE
> { ?s ?p ?z }
> ORDER BY ?z
> LIMIT 5
> {noformat}
> Compiled to algebra:
> {noformat}
> (slice _ 5
> (distinct
> (project (?z)
> (order (?z)
> (bgp (triple ?s ?p ?z))))))
> {noformat}
> After optimization: TopN did not apply:
> {noformat}
> (slice _ 5
> (order (?z)
> (distinct
> (project (?z)
> (bgp (triple ?s ?p ?z))))))
> {noformat}
> but this is possible:
> {noformat}
> (top (5 ?z)
> (distinct
> (project (?z)
> (bgp (triple ?s ?p ?z)))))
> {noformat}
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