Predicate path with (^schema:about)? takes forever

[hannahbast]

The following query requires four minutes and a lot of memory. The runtime info shows a TRANSITIVE PATH operation costing 170 seconds, as well as a large UNION costing 60 seconds.

PREFIX wd: <http://www.wikidata.org/entity/>
PREFIX wdt: <http://www.wikidata.org/prop/direct/>
PREFIX wikibase: <http://wikiba.se/ontology#>
PREFIX schema: <http://schema.org/>
SELECT ?x ?sitelinks WHERE {
  ?x wdt:P31 wd:Q5107 .
  ?x (^schema:about)?/wikibase:sitelinks ?sitelinks .
}

The following query, which replaces the ? via a UNION and the / via a temporary object, both in the natural way, finishes in a reasonable 13 seconds with a cold cache:

PREFIX wd: <http://www.wikidata.org/entity/>
PREFIX wdt: <http://www.wikidata.org/prop/direct/>
PREFIX wikibase: <http://wikiba.se/ontology#>
PREFIX schema: <http://schema.org/>
SELECT ?x ?sitelinks WHERE {
  ?x wdt:P31 wd:Q5107 .
  { ?x wikibase:sitelinks ?sitelinks }
  UNION
  { ?x ^schema:about ?tmp . ?tmp wikibase:sitelinks ?sitelinks }
}