Filter on empty ResultTable seems to break in some situations

[niklas88]

Querying for all the mons (wd:Q429088 mountains on a celestial object) with elevation above 8000 m on Wikidata Full crashes. Searching for mountains (wd:Q8502) on the other hand works. I think this is the same bug @joka921 was also looking at and is related to FILTER on an empty result table under some special circumstances.

PREFIX wd: <http://www.wikidata.org/entity/>
PREFIX wdt: <http://www.wikidata.org/prop/direct/>
PREFIX p: <http://www.wikidata.org/prop/>
PREFIX psn: <http://www.wikidata.org/prop/statement/value-normalized/>
PREFIX wikibase: <http://wikiba.se/ontology#>
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
SELECT ?subj ?label ?coord ?elev WHERE
{ 
  ?subj wdt:P31 wd:Q429088 .
  ?subj p:P2044 ?elev_s .
  ?elev_s psn:P2044 ?elev_v .
  ?elev_v wikibase:quantityAmount ?elev .
  ?subj wdt:P625 ?coord .
  ?subj rdfs:label ?label .

  FILTER (?elev > 8000.0) .
  FILTER langMatches(lang(?label), "en") .  
}
ORDER BY DESC(?elev)

[niklas88]

Another query which triggers it is

PREFIX wdt: <http://www.wikidata.org/prop/direct/>
PREFIX wd: <http://www.wikidata.org/entity/>
PREFIX p: <http://www.wikidata.org/prop/>
PREFIX psv: <http://www.wikidata.org/prop/statement/value/>
PREFIX wikibase: <http://wikiba.se/ontology-beta#>
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
SELECT ?occupation ?gender (AVG(?height) AS ?average_height) (COUNT(?height) AS ?count) WHERE {
  ?x wdt:P31 wd:Q5 .
  ?x p:P2048 ?statement .
  ?x wdt:P21 ?gender_id .
  ?gender_id rdfs:label ?gender .
  FILTER langMatches(lang(?gender), "en") .
  ?statement psv:P2048 ?value .
  ?value wikibase:quantityNormalized ?quantity .
  ?quantity wikibase:quantityUnit wd:Q11573 .
  ?quantity wikibase:quantityAmount ?height .
  ?x wdt:P106 ?occupation_id .
  ?occupation_id rdfs:label ?occupation .
  FILTER langMatches(lang(?occupation), "en") .
  FILTER (?height < 3)
}
GROUP BY ?occupation ?gender
HAVING (?count > 100)
ORDER BY DESC(?average_height)

where fixing PREFIX wikibase: <http://wikiba.se/ontology-beta#> to PREFIX wikibase: <http://wikiba.se/ontology#> makes the query work again because the result is no longer empty.

[floriankramer]

When running the first query on the new wikidata-full backend the result without the filters in not empty, but with the filters it still crashes, so the bug must be related to something else.
The following query reproduces something that looks like the same bug on the scientists dataset:

SELECT ?a ?h WHERE {
 {
  SELECT ?a ?h ?h1 ?h2 ?h3 ?h4 ?h5 WHERE {
 ?a <Height> ?h .
 ?a <Height> ?h1 .
 ?a <Height> ?h2 .
 ?a <Height> ?h3 .
 ?a <Height> ?h4 .
 ?a <Height> ?h5 .
}
}
 ?a <Height> ?h6 .
 FILTER(?h > 1.6)
}

[floriankramer]

Filter were never designed to run on ResultTables with more than 5 columns, as they are always applied as early as possible. Due to the introduction of query subgraphs and having clauses filers now need to be able to run on arbitrarily wide ResultTables.

[niklas88]

@floriankramer interesting, can you point me to a code line where the 5 column assumption becomes apparent?

[floriankramer]

Apparently fixing #... does not auto close issues. This was fixed in #163.