Undesirable SPARQL Jena Query Pattern Behavior with Optionals

[Marco Neumann]

I have just noticed that the following query pattern with optionals yields
undesirable SPARQL query results if the variable (?xLabel) isn't bound.

Let the data be: :x :p :y

SELECT ?x
WHERE{
OPTIONAL{ ?x rdfs:label ?xLabel.}
?x :p ?y.
}

while reversing the order in the query pattern yields a result as expected:
:x

SELECT ?x
WHERE{
?x :p ?y.
OPTIONAL{ ?x rdfs:label ?xLabel.}
}

Is this (the importance of order of optionals) a normal behavior during
query execution here?

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Marco Neumann
KONA