[Wikidata-bugs] [Maniphest] [Closed] T116298: SPARQL endpoint should gracefully handle cycles and loops in transitive properties

[Smalyshev]

Smalyshev closed this task as "Resolved". Smalyshev added a comment.

This works now:

SELECT DISTINCT ?city WHERE {
  ?city wdt:P31/wdt:P279* wd:Q515 . # find instances of subclasses of city
  ?city wdt:P131* wd:Q1202 .
  hint:Prior hint:gearing "forward" .
}

See also https://wiki.blazegraph.com/wiki/index.php/QueryHints

Basically what is going on is that query optimizer doesn't always knows which way it should traverse the path queries, especially if there are many clauses. Playing with gearing hint allows to tell it the right way.

If the query doesn't work with any direction, please add it here and I'll take a look, but for now I think this resolves the task. The optimizer may be improved, but I imagine there will always be cases where we need to give some hints.

TASK DETAIL

https://phabricator.wikimedia.org/T116298

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To: Smalyshev
Cc: abian, Nikki, Jheald, Aklapper, daniel, QZanden, EBjune, merbst, Avner, debt, Gehel, D3r1ck01, Jonas, FloNight, Xmlizer, Izno, jkroll, Smalyshev, Wikidata-bugs, Jdouglas, aude, Deskana, Manybubbles, Mbch331

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