Role of foreign keys

Foreign keys play a crucial role for optimizing the saturated mapping assertions by doing some query containment checks.

Let us now consider the case where foreign keys are missing. Please download the following files: university-no-fk.ttl, university-no-fk.obda and university-no-fk.properties files.

Let us consider the case of foaf:Person. If you run the following SPARQL query in the absence of foreign keys:

PREFIX foaf: <http://xmlns.com/foaf/0.1/>
SELECT ?p {
  ?p a foaf:Person .
}

you will obtain a SQL query similar to the following one (after some minor reformatting):

SELECT 1 AS "pQuestType", NULL AS "pLang",
       ('http://example.org/uni2/person/' || QVIEW1."lab_teacher") AS "p"
FROM   "uni2"."course" QVIEW1
WHERE  QVIEW1."lab_teacher" IS NOT NULL
UNION ALL
SELECT 1 AS "pQuestType", NULL AS "pLang",
       ('http://example.org/uni2/person/' || QVIEW1."lecturer") AS "p"
FROM   "uni2"."course" QVIEW1
WHERE  QVIEW1."lecturer" IS NOT NULL
UNION ALL
SELECT 1 AS "pQuestType", NULL AS "pLang",
       ('http://example.org/uni1/academic/' || QVIEW1."a_id") AS "p"
FROM   "uni1"."teaching" QVIEW1
WHERE  QVIEW1."a_id" IS NOT NULL
UNION ALL
SELECT 1 AS "pQuestType", NULL AS "pLang",
       ('http://example.org/uni2/person/' || QVIEW1."pid") AS "p"
FROM   "uni2"."registration" QVIEW1
WHERE  QVIEW1."pid" IS NOT NULL
UNION ALL
SELECT 1 AS "pQuestType", NULL AS "pLang",
       ('http://example.org/uni1/student/' || QVIEW1."s_id") AS "p"
FROM   "uni1"."course-registration" QVIEW1
WHERE  QVIEW1."s_id" IS NOT NULL
UNION ALL
SELECT 1 AS "pQuestType", NULL AS "pLang",
       ('http://example.org/uni1/student/' || QVIEW1."s_id") AS "p"
FROM   "uni1"."student" QVIEW1
WHERE  QVIEW1."s_id" IS NOT NULL
UNION ALL
SELECT 1 AS "pQuestType", NULL AS "pLang",
       ('http://example.org/uni1/academic/' || QVIEW1."a_id") AS "p"
FROM   "uni1"."academic" QVIEW1
WHERE  QVIEW1."a_id" IS NOT NULL
UNION ALL
SELECT 1 AS "pQuestType", NULL AS "pLang",
       ('http://example.org/uni2/person/' || QVIEW1."pid") AS "p"
FROM   "uni2"."person" QVIEW1
WHERE  QVIEW1."pid" IS NOT NULL

This SQL query is the union of eight sub-queries. Basically, it queries the tables uni1.student, uni1.academic, uni2.person as expected, but also uni1.teaching, uni1.course-registration, uni2.course and uni2.registration. Recall that the presence of the four latter tables is due to the fact that, according to the ontology, the respective domains of the properties :givesLecture, :givesLab and :attends are subsumed by foaf:Person.

With the setting of the first session which includes foreign keys, the generated SQL query contains only three unions:

SELECT 1 AS "pQuestType", NULL AS "pLang",
       ('http://example.org/uni1/student/' || QVIEW1."s_id") AS "p"
FROM   "uni1"."student" QVIEW1
WHERE  QVIEW1."s_id" IS NOT NULL
UNION ALL
SELECT 1 AS "pQuestType", NULL AS "pLang",
       ('http://example.org/uni1/academic/' || QVIEW1."a_id") AS "p"
FROM   "uni1"."academic" QVIEW1
WHERE  QVIEW1."a_id" IS NOT NULL
UNION ALL
SELECT 1 AS "pQuestType", NULL AS "pLang",
       ('http://example.org/uni2/person/' || QVIEW1."pid") AS "p"
FROM   "uni2"."person" QVIEW1
WHERE  QVIEW1."pid" IS NOT NULL

Side note

If we now consider also the first and the last names of persons, foreign keys are not needed to optimize the query.

Try:

PREFIX foaf: <http://xmlns.com/foaf/0.1/>
SELECT ?p ?firstName ?lastName {
  ?p a foaf:Person ;
     foaf:firstName ?firstName ;
     foaf:lastName ?lastName .
}

and observe that the query produces only three unions.

Why? Because the domain of foaf:firstName and foaf:lastName is foaf:Person. The SPARQL query can thus be safely rewritten as follow:

PREFIX foaf: <http://xmlns.com/foaf/0.1/>
SELECT ?p ?firstName ?lastName {
  ?p foaf:firstName ?firstName ;
     foaf:lastName ?lastName .
}