Assertion !dpTab[k - 1].empty() failed in QueryPlanner.cpp

[dpriskorn]

https://qlever.cs.uni-freiburg.de/wikidata/9IJGud
Assertion !dpTab[k - 1].empty() failed. Please report this to the developers. In file "/local/data-ssd/qlever/qlever-code/src/engine/QueryPlanner.cpp " at line 1228

[hannahbast]

@dpriskorn Thanks for pointing that out to us. The query is impossibly hard (the inner query is a cross-product of the whole index with itself, with a result size of around 1 quintillion, that is 1000 billion billion). But, of course, it should not produce an assertion failure.

May I ask what you intended to do with this query? You gave the variables names which suggest a certain semantics. But without associating these variables with any predicates or entities, they are just free variables that match everything.

[tuukka]

Here's a much easier query that should give the wanted result (based on the variable names), but it runs out of memory when I try to run it: https://qlever.cs.uni-freiburg.de/wikidata/c0KH0F

PREFIX prov: <http://www.w3.org/ns/prov#>
SELECT (COUNT(DISTINCT ?item) AS ?itemsWithReferencedStatements) 
WHERE { 
    ?item ?property ?statement.
    ?statement prov:wasDerivedFrom ?reference.
}

[hannahbast]

@tuukka I agree it's easier than computing a join between two tables of size 30 billion, but it's still a join between a table of size 30 billion (all triples matching ?item ?property ?statement) and a table of size 1 billion (all triples matching ?statement prov:wasDerivedFrom ?reference). With the "lazy group by", "lazy join", etc. that we are currently working on, this will also work with limited RAM, but it's still a very costly query, which will take a long time to process.

[tuukka]

Here's a query that avoids the join and the 30 billion triple table, but it still runs out of memory: https://qlever.cs.uni-freiburg.de/wikidata/MpDkiF

PREFIX prov: <http://www.w3.org/ns/prov#>
PREFIX wds: <http://www.wikidata.org/entity/statement/>

SELECT (COUNT(DISTINCT ?item) AS ?itemsWithReferencedStatements) 
WHERE {
    ?statement prov:wasDerivedFrom ?reference.
    BIND(STRAFTER(STRBEFORE(STR(?statement), "-"), STR(wds:)) AS ?item)
}

[joka921]

@dpriskorn Thanks for reporting this,
As I also recently stumbled upon this issue, I can share some details:
This assertion is triggered as soon as you require a full join of the index with itself (at least two variables with three triples each).
QLever currently refuses to compute such queries (they typically run out of memory anyway), but of course such queries should either work or give a human-readable error, but shouldn't leak an internal Assertion. I will think about what to do
(The trivial fix of "just allowing it" broke some unit tests, which should be easy to fix when I have some time for this.