Completions for property paths with * get error response

[jbuerklin]

On http://qlever.informatik.uni-freiburg.de/Wikidata_Full, when retreiving completions for

PREFIX wdt: <http://www.wikidata.org/prop/direct/>
SELECT WHERE {
  ?x wdt:P31/wdt:P279* [cursor]
}

QLeverUI sends following query:
(The currently available version of QLeverUI on http://qlever.informatik.uni-freiburg.de/ does not build the same query as I'm describing below, but I already pushed an update that will result in queries that look exactly like the following one.)

SELECT ?qleverui_entity (SAMPLE(?qleverui_name) as ?qleverui_name) (SAMPLE(?qleverui_count) as ?qleverui_count) WHERE {
    {
      SELECT ?qleverui_entity (COUNT(?qleverui_entity) AS ?qleverui_count) WHERE {
        ?x <http://www.wikidata.org/prop/direct/P31>/<http://www.wikidata.org/prop/direct/P279>* ?qleverui_entity .
      }
      GROUP BY ?qleverui_entity
    }
    OPTIONAL {
      ?qleverui_entity @en@<http://www.w3.org/2000/01/rdf-schema#label> ?qleverui_name .
    }
    OPTIONAL {
      ?qleverui_entity @en@<http://www.w3.org/2004/02/skos/core#altLabel> ?qleverui_altname .
    }
}
GROUP BY ?qleverui_entity
ORDER BY DESC(?qleverui_count)
LIMIT 40
OFFSET 0

and receives following response:

{
"query": "PREFIX wdt: <http://www.wikidata.org/prop/direct/>\nSELECT ?qleverui_entity (SAMPLE(?qleverui_name) as ?qleverui_name) (SAMPLE(?qleverui_count) as ?qleverui_count) WHERE {\n    {\n      SELECT ?qleverui_entity (COUNT(?qleverui_entity) AS ?qleverui_count) WHERE {\n        ?x <http://www.wikidata.org/prop/direct/P31>/<http://www.wikidata.org/prop/direct/P279>* ?qleverui_entity .\n      }\n      GROUP BY ?qleverui_entity\n    }\n    OPTIONAL {\n      ?qleverui_entity @en@<http://www.w3.org/2000/01/rdf-schema#label> ?qleverui_name .\n    }\n    OPTIONAL {\n      ?qleverui_entity @en@<http://www.w3.org/2004/02/skos/core#altLabel> ?qleverui_altname .\n    }\n}\nGROUP BY ?qleverui_entity\nORDER BY DESC(?qleverui_count)\nLIMIT 40\nOFFSET 0",
"status": "ERROR",
"resultsize": "0",
"time": {
"total": "3801ms",
"computeResult": "3801ms"
},
"exception": "std::bad_alloc"
}

[hannahbast]

I can confirm that the error already occurs for the following simpler query (= the query above without the optional joins and without the final ordering) and even with over 100 GB of RAM still free:

PREFIX wdt: <http://www.wikidata.org/prop/direct/>
SELECT ?o (COUNT(?o) AS ?count) WHERE {
  ?s wdt:P31/wdt:P279* ?o
}
GROUP BY ?o

The last lines written to the log before the crash are as follows:

Sun Jan 19 19:21:09.273	- DEBUG: TransitivePath subresult computation done.
Sun Jan 19 19:21:18.748	- DEBUG: TransitivePath result computation done.
Sun Jan 19 19:21:18.892	- DEBUG: Computing Join result...
Sun Jan 19 19:21:18.895	- DEBUG: Performing join between two tables.
Sun Jan 19 19:21:18.895	- DEBUG: A: width = 2, size = 72,175,797
Sun Jan 19 19:21:18.895	- DEBUG: B: width = 2, size = 62,050,972
Sun Jan 19 19:21:34.155	- DEBUG: Join done.
Sun Jan 19 19:21:34.155	- DEBUG: Result: width = 3, size = 1,419,740,509
Sun Jan 19 19:21:34.155	- DEBUG: Join result computation done.
Sun Jan 19 19:21:34.155	- DEBUG: Union subresult computation done.
Sun Jan 19 19:21:51.470	- DEBUG: Union result computation done.
Sun Jan 19 19:21:51.470	- DEBUG: OrderBy result computation...

The query above without the GROUP BY and the COUNT works and has a result size of 1,491,916,306 (similar, but not the same as in the log above, why?) and uses 60 - 70 GB. With 40 GB of RAM left, the sorting then fails.

[hannahbast]

This is a very hard query with a huge result. QLever has a memory limit for quite some time now and the query above runs into it, now producing a proper error message of the form Tried to allocate XXX MB, but only YYY MB were available. Clear the cache or allow more memory for QLever during startup.