Rob Vesse created JENA-441:
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Summary: In some cases it may be useful to apply DISTINCT before
applying ORDER BY
Key: JENA-441
URL: https://issues.apache.org/jira/browse/JENA-441
Project: Apache Jena
Issue Type: Improvement
Components: ARQ, Optimizer
Affects Versions: Jena 2.10.0
Reporter: Rob Vesse
Assignee: Rob Vesse
Priority: Minor
One of our internal users highlighted an interesting query where changing the
plan makes a big difference in performance.
The query is essentially the following:
SELECT DISTINCT ?p
WHERE
{
?s ?p ?o
} ORDER BY ?p
Leaving the fact that it is a fundamentally dumb query to write the user had an
interesting suggestion about the query plan, currently this generates the
following:
(distinct
(project (?predicate)
(order (?predicate)
(quadpattern (quad <urn:x-arq:DefaultGraphNode> ?s ?predicate ?o)))))
For cases like this it may actually be much more performant to do the distinct
first, because of the associated semantics of the various operators you can't
just simply put distinct before the order but if you rewrite the query as
follows:
SELECT ?p
WHERE
{
{ SELECT DISTINCT ?p WHERE { ?s ?p ?o } }
} ORDER BY ?p
You get the likely much more performant plan:
(project (?predicate)
(order (?predicate)
(distinct
(project (?predicate)
(quadpattern (quad <urn:x-arq:DefaultGraphNode> ?/s ?predicate
?/o))))))
Clearly this optimization does not apply in the general case, I think it only
applies in the case where you have a DISTINCT and all ORDER BY conditions are
simple variables and only those variables are projected in which case I think
you could produce a plan like the following:
(order (?predicate)
(project (?predicate)
(distinct
(quadpattern (quad <urn:x-arq:DefaultGraphNode> ?s ?predicate ?o)))))
I am pretty certain this applies in only a few cases and I haven't reproduced
this with TDB to see if the performance difference is noticeable yet.
Andy - I will try and gather some more information and experiment with this so
don't feel you have to look at this one for the time being.
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