https://www.w3.org/TR/sparql11-query/#subqueries says
Due to the bottom-up nature of SPARQL query evaluation, the subqueries are
evaluated logically first, and the results are projected up to the outer query.
I think that this is incorrect. For example, in
SELECT ?x WHERE {
?x :a :b .
FILTER EXISTS {
SELECT ?x WHERE { ?x :a :b } HAVING ( COUNT(*) = 1 )
}
}
The inner select is not even known until the bindings for ?x in the outer
query have been determined because EXISTS uses substitution into the inner
query. (Whether that is a good idea or not is a separate issue.)
I ran into this issue when reading https://scirate.com/arxiv/1606.01441
I believe that the sentence I quote above is the one that the authors indicate
that Fuseki and Blazegraph are using to support their implementation of
subqueries inside EXISTS.
I suggest that there be an erratum removing this sentence.
Peter F. Patel-Schneider
Nuance Communications
