This query checks whether a given nanopublication has been loaded by the answering Nanopub Query instance. It returns the nanopublication's creation date if the nanopublication is present, and an empty result otherwise. It is designed to be as cheap as possible (a single indexed lookup), so it can be used to poll for the arrival of a just-published nanopublication, for example to delay a cache refresh until the new nanopublication has made its round trip through the network.
prefix npa: <http://purl.org/nanopub/admin/> prefix dct: <http://purl.org/dc/terms/> select ?date where { graph npa:graph { ?_np_iri dct:created ?date . } } limit 1