Some answers inline, On Fri, Aug 7, 2020 at 6:07 PM Thad Guidry <thadgui...@gmail.com> wrote:
> Very nice David! > > 1. Does the MINUS actually utilize ElasticSearch indexes or just > Blazegraph? > > No, elasticsearch is being used only during the call to the wikibase:mwapi SERVICE. Everything happening outside this call is handled by blazegraph. > I'd like to help the community by writing up a bit better documentation on > our SPARQL pages that talks about FILTER() versus MINUS() if no one has > this info floating around? > The only footnote I saw was: > " MINUS lets you select results that *don’t* fit some graph pattern. FILTER > NOT EXISTS is mostly equivalent (see the SPARQL spec for an example where > they differ), but – at least on WDQS – usually slower by quite a bit." > at the bottom of the SPARQL tutorial > > <https://www.wikidata.org/wiki/Wikidata:SPARQL_tutorial> > and the wiki page SPARQL query service > <https://www.wikidata.org/wiki/Wikidata:SPARQL_query_service/queries#Excluding_subsets> > has: > > Excluding subsets > > SPARQL has three different idioms for excluding subsets: > > - OPTIONAL { ... ?x ... } FILTER(!bound(?x)), > - FILTER NOT EXISTS { ... } > - MINUS { ... } > > Currently, in almost all circumstances, Blazegraph resolves all of these > to the same query plan. > > 2. Is that still a true statement that those 3 above use the same query > plan currently? > I think they indeed serve the same purpose but might vary in subtle ways, for MINUS vs FILTER NOT EXISTS the sparql specs states that they can produce different solutions <https://www.w3.org/TR/sparql11-query/#neg-notexists-minus>. As to which approach is better I can't answer clearly, I tend to prefer MINUS as I find it easier to read/understand. I also tend to avoid plain FILTER(constraint on ?x) when possible as they tend to be rather slow (here the FILTER(!bound(?x)) should be pretty fast though). David.
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