  Nowadays, more and more RDF data is becoming available on the Semantic Web.
While the Semantic Web is generally incomplete by nature, on certain topics, it
already contains complete information and thus, queries may return all answers
that exist in reality. In this paper we develop a technique to check query
completeness based on RDF data annotated with completeness information, taking
into account data-specific inferences that lead to an inference problem which
is $\Pi^P_2$-complete. We then identify a practically relevant fragment of
completeness information, suitable for crowdsourced, entity-centric RDF data
sources such as Wikidata, for which we develop an indexing technique that
allows to scale completeness reasoning to Wikidata-scale data sources. We
verify the applicability of our framework using Wikidata and develop COOL-WD, a
completeness tool for Wikidata, used to annotate Wikidata with completeness
statements and reason about the completeness of query answers over Wikidata.
The tool is available at http://cool-wd.inf.unibz.it/.
