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)
}