This document discusses efficient translation of SPARQL queries to SQL queries to enable querying relational data stored in databases using SPARQL. It presents an evolution of virtual RDF endpoints that translate SPARQL to SQL, from early systems like D2R that issued multiple SQL queries for one SPARQL query, to more optimized systems like Ultrawrap and Ontop that leverage database optimizations to improve performance. Ontop directly translates SPARQL to a single optimized SQL query and performs optimizations like candidate selection, self-join elimination, and leveraging the database query engine. The document demonstrates this translation process on an example query and evaluates the performance of Ontop compared to other virtual endpoints.

1. Efficient SPARQL to SQL
translation with user
defined mapping
Miloš Chaloupka and Martin Nečaský
XML and Web Engineering Research Group
Faculty of Mathematics and Physics
Charles University

2. Motivation
 RDF is great for publishing data
 Relational DBMS is the standard way how to store data
Efficient SPARQL to SQL translation
with user defined mapping
Rank DBMS Model Score
1 Oracle Relational 1425.56
2 MySQL Relational 1354.03
3 Microsoft SQL Server Relational 1211.55
33 MarkLogic Multi (incl. RDF) 10.15
73 Virtuoso Multi (incl. RDF) 2.50
83 Jena RDF store 2.11
http://db-engines.com/en/ranking

3. Motivation
 How to publish data stored in RDBMS in RDF format?
 Duplicate data
 Periodically create dump and import it to RDF store
 Virtual endpoint
 Provides access to data using a SPARQL endpoint
 Translates every query from SPARQL to SQL
Efficient SPARQL to SQL translation
with user defined mapping
SPARQL SQL

4. Evolution of virtual endpoints
 D2R (initial version from 2004)
 http://d2rq.org/
 „state of the art“
 Multiple SQL queries for a single SPARQL query
Efficient SPARQL to SQL translation
with user defined mapping

5. Evolution of virtual endpoints
 Ultrawrap
 http://capsenta.com/
 Completely relying on the
RDBMS optimization engine
 Outperforms D2R
Efficient SPARQL to SQL translation
with user defined mapping
WITH t(s,s_r,p,o,o_r) AS (
SELECT "http://example.org/student/" + id AS s,
id AS s_r, "rdf:type" AS p, "ex:students" AS o,
NULL AS o_r
FROM students
UNION ALL
SELECT "http://example.org/student/" + id AS s,
id AS s_r, "ex:name" AS p, name AS o, NULL AS o_r
FROM students
)
SELECT t1.s AS iri, t2.o AS name
FROM t AS t1, t AS t2
WHERE
t1.p = "rdf:type" AND
t1.s_r = t2.s_r AND
t2.p = "ex:name"

6. Evolution of virtual endpoints
 Ontop
 http://ontop.inf.unibz.it/
 Single query without „view“
 Outperforms Ultrawrap
 Translates SPARQL query into SQL query and then optimizes it
 Candidate selection
 Self-join elimination
Efficient SPARQL to SQL translation
with user defined mapping

7. Processing query
Parse SPARQL query
Efficient SPARQL to SQL translation
with user defined mapping
SPARQL
SPARQL with mapping
SQL with value bindingsSQL results
SPARQL results
RDBMS
R2RML
SPARQL
SELECT ?iri ?name
WHERE {
?iri a ex:students;
ex:name ?name.
}
Algebra
AND
BGP1 BGP2
?iri rdf:type ex:students ?iri ex:name ?name

8. Processing query
Add mapping information
Efficient SPARQL to SQL translation
with user defined mapping
SPARQL
SPARQL with mapping
SQL with value bindingsSQL results
SPARQL results
RDBMS
R2RML
AND
∪ ∪
?iri rdf:type ex:students ?iri ex:name ?name
BGPr1
1 BGPrn
1
... BGPr1
1 BGPrn
2
...

9. Processing query
Candidate selection
Efficient SPARQL to SQL translation
with user defined mapping
SPARQL
SPARQL with mapping
SQL with value bindingsSQL results
SPARQL results
RDBMS
R2RML
AND
BGPr4
1 ∪
BGPr3
2 BGPr7
2
?iri rdf:type ex:students ?iri ex:name ?name

10. Processing query
Join of unions
Efficient SPARQL to SQL translation
with user defined mapping
SPARQL
SPARQL with mapping
SQL with value bindingsSQL results
SPARQL results
RDBMS
R2RML
∪
AND AND
BGPr4
1 BGPr3
2
BGPr4
1 BGPr7
2
?iri rdf:type ex:students ?iri rdf:type ex:students?iri ex:name ?name ?iri ex:name ?name

11. Processing query
Candidate selection (again)
Efficient SPARQL to SQL translation
with user defined mapping
SPARQL
SPARQL with mapping
SQL with value bindingsSQL results
SPARQL results
RDBMS
R2RML
AND
BGPr4
1 BGPr3
2
?iri rdf:type ex:students ?iri ex:name ?name

12. Processing query
Transform to SQL
Efficient SPARQL to SQL translation
with user defined mapping
SPARQL
SPARQL with mapping
SQL with value bindingsSQL results
SPARQL results
RDBMS
R2RML
𝑖𝑑 𝑖𝑑, 𝑛𝑎𝑚𝑒 ∈ 𝑠𝑡𝑢𝑑𝑒𝑛𝑡𝑠,
𝑛𝑜𝑡𝑛𝑢𝑙𝑙 𝑖𝑑 }
𝑖𝑑, 𝑛𝑎𝑚𝑒 𝑖𝑑, 𝑛𝑎𝑚𝑒 ∈ 𝑠𝑡𝑢𝑑𝑒𝑛𝑡𝑠,
𝑛𝑜𝑡𝑛𝑢𝑙𝑙 𝑖𝑑 , 𝑛𝑜𝑡𝑛𝑢𝑙𝑙(𝑛𝑎𝑚𝑒)}
AND

13. Processing query
Transform to SQL (cont.)
Efficient SPARQL to SQL translation
with user defined mapping
SPARQL
SPARQL with mapping
SQL with value bindingsSQL results
SPARQL results
RDBMS
R2RML
{ 𝑖𝑑1, 𝑖𝑑2, 𝑛𝑎𝑚𝑒2| 𝑖𝑑1, 𝑛𝑎𝑚𝑒1 ∈ 𝑠𝑡𝑢𝑑𝑒𝑛𝑡𝑠, 𝑖𝑑2, 𝑛𝑎𝑚𝑒2
∈ 𝑠𝑡𝑢𝑑𝑒𝑛𝑡𝑠, 𝑛𝑜𝑡𝑛𝑢𝑙𝑙 𝑖𝑑1 , 𝑛𝑜𝑡𝑛𝑢𝑙𝑙 𝑖𝑑2 , 𝑛𝑜𝑡𝑛𝑢𝑙𝑙 𝑛𝑎𝑚𝑒2 ,
𝑖𝑠𝑛𝑢𝑙𝑙 𝑖𝑑1 ∨ 𝑖𝑠𝑛𝑢𝑙𝑙 𝑖𝑑2 ∨ (𝑐𝑜𝑛𝑐𝑎𝑡 ′http://example.org/student/′, 𝑖𝑑1
= 𝑐𝑜𝑛𝑐𝑎𝑡 ′http://example.org/student/′, 𝑖𝑑2 )}

14. Processing query
Simplify conditions
Efficient SPARQL to SQL translation
with user defined mapping
SPARQL
SPARQL with mapping
SQL with value bindingsSQL results
SPARQL results
RDBMS
R2RML
{ 𝑖𝑑1, 𝑖𝑑2, 𝑛𝑎𝑚𝑒2| 𝑖𝑑1, 𝑛𝑎𝑚𝑒1 ∈ 𝑠𝑡𝑢𝑑𝑒𝑛𝑡𝑠, 𝑖𝑑2, 𝑛𝑎𝑚𝑒2
∈ 𝑠𝑡𝑢𝑑𝑒𝑛𝑡𝑠, 𝑛𝑜𝑡𝑛𝑢𝑙𝑙 𝑖𝑑1 , 𝑛𝑜𝑡𝑛𝑢𝑙𝑙 𝑖𝑑2 , 𝑛𝑜𝑡𝑛𝑢𝑙𝑙 𝑛𝑎𝑚𝑒2 , 𝑖𝑠𝑛𝑢𝑙𝑙 𝑖𝑑1
∨ 𝑖𝑠𝑛𝑢𝑙𝑙 𝑖𝑑2 ∨ (𝑖𝑑1 = 𝑖𝑑2)}

15. Processing query
Simplify conditions (cont.)
Efficient SPARQL to SQL translation
with user defined mapping
SPARQL
SPARQL with mapping
SQL with value bindingsSQL results
SPARQL results
RDBMS
R2RML
{ 𝑖𝑑1, 𝑖𝑑2, 𝑛𝑎𝑚𝑒2| 𝑖𝑑1, 𝑛𝑎𝑚𝑒1 ∈ 𝑠𝑡𝑢𝑑𝑒𝑛𝑡𝑠, 𝑖𝑑2, 𝑛𝑎𝑚𝑒2 ∈ 𝑠𝑡𝑢𝑑𝑒𝑛𝑡𝑠, 𝑖𝑑1 = 𝑖𝑑2}

16. Processing query
Self-join optimization
Efficient SPARQL to SQL translation
with user defined mapping
SPARQL
SPARQL with mapping
SQL with value bindingsSQL results
SPARQL results
RDBMS
R2RML
{ 𝑖𝑑1, 𝑛𝑎𝑚𝑒1| 𝑖𝑑1, 𝑛𝑎𝑚𝑒1 ∈ 𝑠𝑡𝑢𝑑𝑒𝑛𝑡𝑠}

17. Processing query
SQL query
Efficient SPARQL to SQL translation
with user defined mapping
SPARQL
SPARQL with mapping
SQL with value bindingsSQL results
SPARQL results
RDBMS
R2RML
SELECT id, name FROM students
?iri 𝑐𝑜𝑛𝑐𝑎𝑡 ′http://example.org/student/′, 𝑖𝑑
?name 𝑛𝑎𝑚𝑒

18. Processing query
SQL query
Efficient SPARQL to SQL translation
with user defined mapping
SPARQL
SPARQL with mapping
SQL with value bindingsSQL results
SPARQL results
RDBMS
R2RML
 SQL query is executed and the
query results are being retriever
 The returned columns does not
contain the SPARQL values directly

19. Processing query
SQL query
Efficient SPARQL to SQL translation
with user defined mapping
SPARQL
SPARQL with mapping
SQL with value bindingsSQL results
SPARQL results
RDBMS
R2RML
?iri 𝑐𝑜𝑛𝑐𝑎𝑡 ′http://example.org/student/′, 𝑖𝑑
?name 𝑛𝑎𝑚𝑒

20. EVI - Evaluation
Efficient SPARQL to SQL translation
with user defined mapping
#1 #2 #3 #4 #5
Evi 38 69 53 128 1291
Ontop 135 118 121 185 1658
1
10
100
1000
10000
Executiontime[ms]
Query

21. Thank you for your attention
Miloš Chaloupka Martin Nečaský
chaloupka@ksi.mff.cuni.cz necasky@ksi.mff.cuni.cz
@ChaloupkaMilos
XML and Web Engineering Research Group
Faculty of Mathematics and Physics
Charles University
Efficient SPARQL to SQL translation
with user defined mapping