[MARMOTTA-575] SELECT queries over a free subject generated to many CROSS JOINs

• Type: Improvement
• Status: Closed
• Priority: Major
• Resolution: Not A Problem
• Affects Version/s: 3.3.0
• Fix Version/s: 3.3.0
• Component/s: KiWi Triple Store
• Labels: sparql
• Assignee: Sergio Fernández
• Reporter: Sergio Fernández
• Created: 03/Dec/14 08:09
• Updated: 16/Dec/14 09:59
• Resolved: 03/Dec/14 10:42

Description

In the typical usage of SPARQL retrieving all properties for a concrete type:

SELECT ?uri ?var1 ?var2 ?varN
WHERE {
  ?uri a ex:type ;
    ex:var1 ?var1 ;
    ex:var2 ?var2 ;
    ex:varN ?varN .
}

KiWI performs quite bad, because it creates a CROSS JOIN for each property:

SELECT PN.object AS VN, P2.object AS V2, P1.subject AS V1
 FROM triples P1
 CROSS JOIN 
  triples P2
 CROSS JOIN 
  triples PN
 WHERE P1.deleted = false
      AND P1.predicate = 520554470573084672
      AND P1.object = 520588903665881088
      AND (P1.context = 539803406462763008)
       AND P2.deleted = false
      AND P1.subject = P2.subject
      AND P2.predicate = 520554473039335424
      AND (P2.context = 539803406462763008)
       AND PN.deleted = false
      AND P1.subject = PN.subject
      AND PN.predicate = 527860595421151232
      AND (PN.context = 539803406462763008)

Making the subject fixed is actually a quite fast query.