query causing QLever to crash

[pfps]

This query appears to cause the QLever Wikidata service to crash:

SELECT DISTINCT ?first ?firstLabel WHERE {
{ SELECT DISTINCT ?c WHERE {
?c wdt:P31/wdt:P279*/wdt:P31/wdt:P279*/wdt:P31 wd:Q24017465 .
} }
?first wdt:P279* ?c .
OPTIONAL { ?first rdfs:label ?firstLabel . FILTER ( lang(?firstLabel) = 'en' ) }
}

On my local QLever server evaluating the query causes unbounded memory usage.

I think that this query used to work a few months ago.

[pfps]

SELECT DISTINCT ?first ?firstLabel WHERE {
?c wdt:P31/wdt:P279*/wdt:P31/wdt:P279*/wdt:P31 wd:Q24017465 .
?first wdt:P279* ?c .
OPTIONAL { ?first rdfs:label ?firstLabel . FILTER ( lang(?firstLabel) = 'en' ) }
}

may cause the same problem.

[pfps]

I'm also seeing queries that do not cause a memory panic but have a resident memory footprint of 180GB on a 196GB machine when the max memory size is set at 96GB and the max cache size is set at 32GB. Is this expected?

[hannahbast]

@pfps Thanks for pointing this out to us and for the reminder. That is indeed a recent regression and should not happen. We will investigate asap.

Notifying @RobinTF and @joka921

[hannahbast]

We have identified #1595 as the cause of the problem. If it's reverted (which I now did for https://qlever.cs.uni-freiburg.de/wikidata), a query likes https://qlever.cs.uni-freiburg.de/wikidata/Gs948g runs through without problems (even with a rather small amount of RAM).

We will investigate further