Andy Seaborne created JENA-638:
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Summary: 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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