Problem: A number of Wikidata's properties are essentially inverses—"part of administrative territory" and "contains administrative territory", "has part" and "part of", and so on. This redundancy, however, has led in many instances, and has the potential in many others, to cause an excess amount of bloat—thousands of "owner of" values caused items for Japanese prefectures and India's rail operator to be briefly unmanageable, thousands of "part of" values caused items for chemical elements to be briefly unmanageable, thousands of values for "child astronomical body" are currently making it difficult to manage the item for the Sun, and lots of values for "head of government" on an item X are frequently redundant and go out of sync with the list of items with "position held" (head of government of X). A number of proposals to delete some inverse properties havebeenstalled precisely because they are required for other wikis to display information that is otherwise stored on the target items themselves.
Proposed solution: provide in the Lua interface a function similar to the "haswbstatement" keyword in Wikidata's text search: supposing this function were named "haswbstatement", a call in Lua to haswbstatement('P39', 'Q839078') would return a list of items for individuals that are or were Prime Ministers of Canada.
Who would benefit: all other Wikimedia wikis which use Lua to access Wikidata's data
More comments: ** The Wikidata Query Service's Linked Data Fragments endpoint already allows a property (as a predicate) and an item or other value (as an object) to be specified so that items with the resulting statement (as subjects) can be returned and possibly paged through. (Here 'subject', 'predicate', and 'object' refer to any such entity within Wikidata's RDF triple store.) In addition to the functionality desired above, if it is possible, being able to ask for predicates matching a subject-object pair might also be useful.
There is a gadget which displays "inverse properties", but this does not inherently solve the inverses problem that client wikis have since it does not run on the server side.
