Thank you for the link Lorenz. Yes it behaves the same with the same data, in my test it had a different dataset where ?xLabel was bound but with no matching ?x variable.
Someone has recently elaborated on this with regards to Jena here. https://newbedev.com/sparql-optional-query On Thu, Aug 5, 2021 at 7:44 AM Lorenz Buehmann < buehm...@informatik.uni-leipzig.de> wrote: > That shouldn't happen for Blazegraph, especially as they even have a > blog entry for this topic, dubbed "order matters": > https://github.com/blazegraph/database/wiki/SPARQL_Order_Matters > > You can check the Blazegraph query plan as well, just put > &explain=details to the request URL > > You can also try to disable the query optimizer via > > SELECT ... WHERE { > hint:Query hint:optimizer "None". > ... > } > > > If this changes the result, then there is a bug in Blazegraph > > On 03.08.21 19:44, Marco Neumann wrote: > > OK yes, thank you for the algebra hint and implicit empty graph pattern > > Andy. Blazegraph has thrown me off here since it seems to interpret the > > query differently. without preserving the table unit on the left hand > side > > I presume. > > > > On Tue, Aug 3, 2021 at 6:20 PM Andy Seaborne <a...@apache.org> wrote: > > > >> > >> On 03/08/2021 17:38, Marco Neumann wrote: > >>> 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. > >>> } > >> (join > >> (leftjoin > >> (table unit) > >> (bgp (triple ?x rdfs:label ?xLabel))) > >> (bgp (triple ?x :p ?y))))) > >> > >> Note the (table unit) in the leftjoin > >> > >> 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.} > >>> } > >> (project (?x) > >> (leftjoin > >> (bgp (triple ?x :p ?y)) > >> (bgp (triple ?x rdfs:label ?xLabel))))) > >> > >>> Is this (the importance of order of optionals) a normal behavior during > >>> query execution here? > >> Yes - it is correct. They are different queries with different results > >> (this is not an optimizer issue). > >> > >> Andy > >> > > > -- --- Marco Neumann KONA
