Andy Seaborne created JENA-653:
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Summary: Filter Placement into union pushes down whole filter but
this fails in a sequence.
Key: JENA-653
URL: https://issues.apache.org/jira/browse/JENA-653
Project: Apache Jena
Issue Type: Bug
Components: ARQ
Affects Versions: Jena 2.11.1
Reporter: Andy Seaborne
Assignee: Andy Seaborne
The filter placement method of pushing into both arms only works when the
filter is directly over the union. If the filter is further out, and for filter
expressions that do not involve variables in the union arms, it should be left
outside as it may be applied elsewhere later.
This is shown in sequence where the union is before some pattern that does bind
the variable.
Example - the key feature is that the {{union}} is first and is joined to a BGP
with the union on the LHS and BGP on the RHS.
{noformat}
PREFIX ex: <http://ex.org/>
PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
SELECT *
WHERE
{ { ?item rdf:type ex:type_a }
UNION
{ ?item rdf:type ex:type_b }
?item ex:label ?label
FILTER ( str(?label) = "a" )
}
{noformat}
Algebra, after join strategy, before filter placement. The joion style is a
{{sequence}}:
{noformat}
(prefix ((ex: <http://ex.org/>)
(rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>))
(filter (= (str ?label) "a")
(sequence
(union
(bgp (triple ?item rdf:type ex:type_a))
(bgp (triple ?item rdf:type ex:type_b)))
(bgp (triple ?item ex:label ?label)))))
{noformat}
which is optimzed (wrongly) as:
{noformat}
(prefix ((ex: <http://ex.org/>)
(rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>))
(sequence
(union
(filter (= (str ?label) "a")
(bgp (triple ?item rdf:type ex:type_a)))
(filter (= (str ?label) "a")
(bgp (triple ?item rdf:type ex:type_b))))
(bgp (triple ?item ex:label ?label))))
{noformat}
The {{(filter (= (str ?label) "a")}} is applied on the {{union}}, not the
later {{(bgp (triple ?item ex:label ?label))))}}.
The problem is in the relationship of {{sequence}} and {{union}}. The {{union}}
can't be treated isolation with the current design. Either the {{union}} needs
a better placement calculated, or failing that (less preferrable), a flag to
change the way filters are pushed own in union depending on nesting context.
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