That's the slides with which I presented my paper at the Linked Data on the Web workshop (LDOW) at WWW 2011, Hyderabad, India.

1. How Caching Improves
Efficiency and Result Completeness
for Querying Linked Data
Olaf Hartig
http://olafhartig.de/foaf.rdf#olaf
Database and Information Systems Research Group
Humboldt-Universität zu Berlin

2. Can we query the Web of Data
as of it were a single,
giant database?
SELECT DISTINCT ?i ?label
WHERE {
?prof rdf:type <http://res ... data/dbprofs#DBProfessor> ;
foaf:topic_interest ?i .
}
OPTIONAL {
}
?i rdfs:label ?label
FILTER( LANG(?label)="en" || LANG(?label)="")
ORDER BY ?label
?
Our approach: Link Traversal Based Query Execution
[ISWC'09]
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 2

3. Main Idea
● Intertwine query evaluation with traversal of data links
● We alternate between:
● Evaluate parts of the query (triple patterns)
on a continuously augmented set of data
● Look up URIs in intermediate
solutions and add retrieved data
to the query-local dataset
query-local
dataset
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 3

4. Main Idea
● Intertwine query evaluation with traversal of data links
● We alternate between:
● Evaluate parts of the query (triple patterns)
on a continuously augmented set of data
● Look up URIs in intermediate
solutions and add retrieved data
to the query-local dataset
Query
http://bob.name
?prjName
s
ow
me
kn
na
?acq query-local
project ?prj
dataset
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 4

5. Main Idea
● Intertwine query evaluation with traversal of data links
● We alternate between:
htt
p:/ ?
● Evaluate parts of the query (triple patterns)
/bo
on a continuously augmented set of data
b.n
am
Look up URIs in intermediate
e
●
solutions and add retrieved data
to the query-local dataset
Query
http://bob.name
?prjName
s
ow
me
kn
na
?acq query-local
project ?prj
dataset
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 5

6. Main Idea
● Intertwine query evaluation with traversal of data links
● We alternate between:
htt
p:/ ?
● Evaluate parts of the query (triple patterns)
/bo
on a continuously augmented set of data
b.n
am
Look up URIs in intermediate
e
●
solutions and add retrieved data
to the query-local dataset
Query
http://bob.name
?prjName
s
ow
me
kn
na
?acq query-local
project ?prj
dataset
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 6

7. Main Idea
● Intertwine query evaluation with traversal of data links
● We alternate between:
htt
p:/ ?
● Evaluate parts of the query (triple patterns)
/bo
on a continuously augmented set of data
b.n
am
Look up URIs in intermediate
e
●
solutions and add retrieved data
to the query-local dataset
Query
http://bob.name
?prjName
s
ow
me
kn
na
?acq query-local
project ?prj
dataset
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 7

8. Main Idea
● Intertwine query evaluation with traversal of data links
● We alternate between:
htt
p:/ ?
● Evaluate parts of the query (triple patterns)
/bo
on a continuously augmented set of data
b.n
am
Look up URIs in intermediate
e
●
solutions and add retrieved data
“Descriptor object”
to the query-local dataset
Query
http://bob.name
?prjName
s
ow
me
kn
na
?acq query-local
project ?prj
dataset
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 8

9. Main Idea
● Intertwine query evaluation with traversal of data links
● We alternate between:
● Evaluate parts of the query (triple patterns)
on a continuously augmented set of data
● Look up URIs in intermediate
solutions and add retrieved data
to the query-local dataset
Query
http://bob.name
?prjName
s
ow
me
kn
na
?acq query-local
project ?prj
dataset
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 9

10. Main Idea
● Intertwine query evaluation with traversal of data links
● We alternate between:
● Evaluate parts of the query (triple patterns)
on a continuously augmented set of data
● Look up URIs in intermediate
solutions and add retrieved data
to the query-local dataset
http://bob.name
Query kno
ws
http://bob.name
http://alice.name
?prjName
s
ow
me
kn
na
?acq query-local
project ?prj
dataset
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 10

11. Main Idea
● Intertwine query evaluation with traversal of data links
?acq
● We alternate between:
http://alice.name
● Evaluate parts of the query (triple patterns)
on a continuously augmented set of data
● Look up URIs in intermediate
solutions and add retrieved data
to the query-local dataset
http://bob.name
Query kno
ws
http://bob.name
http://alice.name
?prjName
s
ow
me
kn
na
?acq query-local
project ?prj
dataset
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 11

12. Main Idea
● Intertwine query evaluation with traversal of data links
?acq
● We alternate between:
http://alice.name
● Evaluate parts of the query (triple patterns)
? me
on a continuously augmented set of data
a
e.n
a lic
://
● Look up URIs in intermediate
p
htt
solutions and add retrieved data
to the query-local dataset
Query
http://bob.name
?prjName
s
ow
me
kn
na
?acq query-local
project ?prj
dataset
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 12

13. Main Idea
● Intertwine query evaluation with traversal of data links
?acq
● We alternate between:
http://alice.name
● Evaluate parts of the query (triple patterns)
? me
on a continuously augmented set of data
a
e.n
a lic
://
● Look up URIs in intermediate
p
htt
solutions and add retrieved data
to the query-local dataset
Query
http://bob.name
?prjName
s
ow
me
kn
na
?acq query-local
project ?prj
dataset
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 13

14. Main Idea
● Intertwine query evaluation with traversal of data links
?acq
● We alternate between:
http://alice.name
● Evaluate parts of the query (triple patterns)
? me
on a continuously augmented set of data
a
e.n
a lic
://
● Look up URIs in intermediate
p
htt
solutions and add retrieved data
to the query-local dataset
Query
http://bob.name
?prjName
s
ow
me
kn
na
?acq query-local
project ?prj
dataset
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 14

15. Main Idea
● Intertwine query evaluation with traversal of data links
?acq
● We alternate between:
http://alice.name
● Evaluate parts of the query (triple patterns)
on a continuously augmented set of data
● Look up URIs in intermediate
solutions and add retrieved data
to the query-local dataset
Query
http://bob.name
?prjName
s
ow
me
kn
na
?acq query-local
project ?prj
dataset
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 15

16. Main Idea
● Intertwine query evaluation with traversal of data links
?acq
● We alternate between:
http://alice.name
● Evaluate parts of the query (triple patterns)
on a continuously augmented set of data
● Look up URIs in intermediate
solutions and add retrieved data
to the query-local dataset
Query
http://bob.name
?prjName
s
ow
me
kn
na
?acq query-local
project ?prj
dataset
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 16

17. Main Idea
● Intertwine query evaluation with traversal of data links
?acq
● We alternate between:
http://alice.name
● Evaluate parts of the query (triple patterns)
on a continuously augmented set of data
● Look up URIs in intermediate
solutions and add retrieved data
to the query-local dataset
http://alice.name
Query pr o
http://bob.name jec
t
?prjName http://.../AlicesPrj
s
ow
me
kn
na
?acq query-local
project ?prj
dataset
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 17

18. Main Idea
● Intertwine query evaluation with traversal of data links
?acq
● We alternate between:
http://alice.name
● Evaluate parts of the query (triple patterns)
on a continuously augmented set of data
● Look up URIs in intermediate ?acq ?prj
http://alice.name http://.../AlicesPrj
solutions and add retrieved data
to the query-local dataset
http://alice.name
Query pr o
http://bob.name jec
t
?prjName http://.../AlicesPrj
s
ow
me
kn
na
?acq query-local
project ?prj
dataset
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 18

19. Main Idea
● Intertwine query evaluation with traversal of data links
?acq
● We alternate between:
http://alice.name
● Evaluate parts of the query (triple patterns)
on a continuously augmented set of data
● Look up URIs in intermediate ?acq ?prj
http://alice.name http://.../AlicesPrj
solutions and add retrieved data
to the query-local dataset
Query
http://bob.name
?prjName
s
ow
me
kn
na
?acq query-local
project ?prj
dataset
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 19

20. Main Idea
● Intertwine query evaluation with traversal of data links
?acq
● We alternate between:
http://alice.name
● Evaluate parts of the query (triple patterns)
on a continuously augmented set of data
● Look up URIs in intermediate ?acq ?prj
http://alice.name http://.../AlicesPrj
solutions and add retrieved data
to the query-local dataset ?prj ?prjName
http://.../AlicesPrj “…“
Query
http://bob.name
?prjName
s
ow
me
kn
na
?acq query-local
project ?prj
dataset
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 20

21. Main Idea
● Intertwine query evaluation with traversal of data links
?acq
● We alternate between:
http://alice.name
● Evaluate parts of the query (triple patterns)
on a continuously augmented set of data
● Look up URIs in intermediate ?acq ?prj
http://alice.name http://.../AlicesPrj
solutions and add retrieved data
to the query-local dataset ?prj ?prjName
http://.../AlicesPrj “…“
Query ?acq ?prj ?prjName
http://bob.name
?prjName http://alice.name http://.../AlicesPrj “…“
s
ow
me
kn
na
?acq query-local
project ?prj
dataset
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 21

22. Characteristics
● Link traversal based query execution:
● Evaluation on a continuously augmented dataset
● Discovery of potentially relevant data during execution
● Discovery driven by intermediate solutions
● Main advantage:
● No need to know all data sources in advance
● Limitations:
● Query has to contain a URI as a starting point
●
Ignores data that is not reachable* by the query execution
*
formal definition in the paper
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 22

23. The Issue
Query
?acq interest
?i
s
ow
label
kn
http://bob.name
?iLabel
query-local
dataset
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 23

24. The Issue
Query
?acq interest
?i
s
ow
label
kn
http://bob.name
?iLabel
htt query-local
p: //b
ob dataset
? .nam
e
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 24

25. The Issue
Query
?acq interest http://bob.name
?i
kno
s
ow
w s
label
kn
http://alice.name
http://bob.name
?iLabel
query-local
dataset
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 25

26. The Issue
Query
?acq interest http://bob.name
?i
kno
s
ow
w s
label
kn
http://alice.name
http://bob.name
?iLabel
query-local
dataset
?acq ?i ?iLabel
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 26

27. The Issue
Query
?acq interest
?i
s
ow
label
kn
http://bob.name
?iLabel
query-local
dataset
Query
http://bob.name
?prjName
s
ow
me
kn
na
?acq query-local
project ?prj
dataset
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 27

28. Reusing the Query-Local Dataset
Query
?acq interest
?i
s
ow
label
kn
http://bob.name
?iLabel
query-local
dataset
Query
http://bob.name
?prjName
s
ow
me
kn
na
?acq query-local
project ?prj
dataset
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 28

29. Reusing the Query-Local Dataset
Query
?acq interest
?i
s
ow
label
kn
http://bob.name
?iLabel
http://alice.name
o ws
Query kn
http://bob.name
http://bob.name
?prjName
s
ow
me
kn
na
?acq query-local
project ?prj
dataset
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 29

30. Reusing the Query-Local Dataset
Query
?acq interest
?i ?acq
s
ow
http://alice.name
label
kn
http://bob.name
?iLabel
http://alice.name
o ws
Query kn
http://bob.name
http://bob.name
?prjName
s
ow
me
kn
na
?acq query-local
project ?prj
dataset
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 30

31. Hypothesis
Re-using the query-local dataset (a.k.a. data caching)
may benefit
query performance + result completeness
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 31

32. Contributions
● Systematic analysis of the impact of data caching
●
Theoretical foundation*
●
Conceptual analysis*
● Empirical evaluation of the potential impact
*
see paper
● Out of scope: Caching strategies (replacement, invalidation)
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 32

33. Experiment – Scenario
● Information about the
distributed social
network of FOAF
profiles
● 5 types of queries
● Experiment Setup:
● 23 persons
● Sequential use
➔ 115 queries
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 33

34. Experiment – Complete Sequence
no reuse given 0 0,2 0,4 0,6 0,8 1 ● no reuse experiment:
order
ContactInfoPhillipe ● No data caching
(Query No. 36)
● given order experiment
UnsetPropsPhillipe ● Reuse of the query-local
(Query No. 37) dataset for the complete
sequence of all 115 queries
2ndDegree1Phillipe
(Query No. 38)
2ndDegree2Phillipe
(Query No. 39) ● Hit rate:
IncomingPhillipe look-ups answered from cache
(Query No. 40) all look-up requests
0 0,2 0,4 0,6 0,8 1
hit rate
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 34

35. Experiment – Complete Sequence
no reuse given 0 0,2 0,4 0,6 0,8 1 ● no reuse experiment:
order
ContactInfoPhillipe ● No data caching
(Query No. 36)
● given order experiment
UnsetPropsPhillipe ● Reuse of the query-local
(Query No. 37) dataset for the complete
sequence of all 115 queries
2ndDegree1Phillipe
(Query No. 38)
2ndDegree2Phillipe
(Query No. 39) ● Hit rate:
IncomingPhillipe look-ups answered from cache
(Query No. 40) all look-up requests
0 0,2 0,4 0,6 0,8 1
hit rate
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 35

36. Experiment – Complete Sequence
no reuse given 0 0,2 0,4 0,6 0,8 1 0 5 10 15 20 25 30 0 20 40 60 80
order
ContactInfoPhillipe
(Query No. 36)
UnsetPropsPhillipe
(Query No. 37)
2ndDegree1Phillipe
(Query No. 38)
2ndDegree2Phillipe
(Query No. 39)
IncomingPhillipe
(Query No. 40)
0 0,2 0,4 0,6 0,8 1 0 5 10 15 20 25 30 0 20 40 60 80
hit rate number of query results query execution time
(in seconds)
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 36

37. Experiment – Complete Sequence
no reuse given 0 0,2 0,4 0,6 0,8 1 0 5 10 15 20 25 30 0 20 40 60 80
order
ContactInfoPhillipe
(Query No. 36)
UnsetPropsPhillipe
(Query No. 37)
2ndDegree1Phillipe
(Query No. 38)
2ndDegree2Phillipe
(Query No. 39)
IncomingPhillipe
(Query No. 40)
0 0,2 0,4 0,6 0,8 1 0 5 10 15 20 25 30 0 20 40 60 80
hit rate number of query results query execution time
(in seconds)
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 37

38. Summary
● Contributions:
● Theoretical foundation
● Conceptual analysis
● Empirical evaluation
● Main findings:
● Additional results possible (for semantically similar queries)
● Impact on performance may be positive but also negative
● Future work:
● Analysis of caching strategies in our context
● Main issue: invalidation
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 38

39. Backup Slides
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 39

40. Contributions
● Theoretical foundation (extension of the original definition)
● Reachability by a Dseed-initialized execution of a BGP query b
● Dseed-dependent solution for a BGP query b
● Reachability R(B) for a serial execution of B = b1 , … , bn
➔ Each solution for bcur is also R(B)-dependent solution for bcur
● Conceptual analysis of the impact of data caching
● Performance factor: p( bcur , B ) = c( bcur , [ ] ) – c( bcur , B )
● Serendipity factor: s( bcur , B ) = b( bcur , B ) – b( bcur , [ ] )
● Empirical verification of the potential impact
● Out of scope: Caching strategies (replacement, invalidation)
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 40

41. Query Template Contact
SELECT * WHERE { <PERSON> foaf:knows ?p .
OPTIONAL { ?p foaf:name ?name }
OPTIONAL { ?p foaf:firstName ?firstName }
OPTIONAL { ?p foaf:givenName ?givenName }
OPTIONAL { ?p foaf:givenname ?givenname }
OPTIONAL { ?p foaf:familyName ?familyName }
OPTIONAL { ?p foaf:family_name ?family_name }
OPTIONAL { ?p foaf:lastName ?lastName }
OPTIONAL { ?p foaf:surname ?surname }
OPTIONAL { ?p foaf:birthday ?birthday }
OPTIONAL { ?p foaf:img ?img }
OPTIONAL { ?p foaf:phone ?phone }
OPTIONAL { ?p foaf:aimChatID ?aimChatID }
OPTIONAL { ?p foaf:icqChatID ?icqChatID }
OPTIONAL { ?p foaf:jabberID ?jabberID }
OPTIONAL { ?p foaf:msnChatID ?msnChatID }
OPTIONAL { ?p foaf:skypeID ?skypeID }
OPTIONAL { ?p foaf:yahooChatID ?yahooChatID }
}
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 41

42. Query Template UnsetProps
SELECT DISTINCT ?result ?resultLabel WHERE
{
?result rdfs:isDefinedBy <http://xmlns.com/foaf/0.1/> .
?result rdfs:domain foaf:Person .
OPTIONAL { <PERSON> ?result ?var0 }
FILTER ( !bound(?var0) )
<PERSON> foaf:knows ?var2 .
?var2 ?result ?var3 .
?result rdfs:label ?resultLabel .
?result vs:term_status ?var1 .
}
ORDER BY ?var1
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 42

43. Query Template Incoming
SELECT DISTINCT ?result WHERE
{
?result foaf:knows <PERSON> .
OPTIONAL
{
?result foaf:knows ?var1 .
FILTER ( <PERSON> = ?var1 )
<PERSON> foaf:knows ?result .
}
FILTER ( !bound(?var1) )
}
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 43

44. Query Template 2ndDegree1
SELECT DISTINCT ?result WHERE
{
<PERSON> foaf:knows ?p1 .
<PERSON> foaf:knows ?p2 .
FILTER ( ?p1 != ?p2 )
?p1 foaf:knows ?result .
FILTER ( <PERSON> != ?result )
?p2 foaf:knows ?result .
OPTIONAL {
<PERSON> ?knows ?result .
FILTER ( ?knows = foaf:knows )
}
FILTER ( !bound(?knows) )
}
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 44

45. Query Template 2ndDegree2
SELECT DISTINCT ?result WHERE
{
<PERSON> foaf:knows ?p1 .
<PERSON> foaf:knows ?p2 .
FILTER ( ?p1 != ?p2 )
?result foaf:knows ?p1 .
FILTER ( <PERSON> != ?result )
?result foaf:knows ?p2 .
OPTIONAL {
<PERSON> ?knows ?result .
FILTER ( ?knows = foaf:knows )
}
FILTER ( !bound(?knows) )
}
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 45

46. Experiment – Single Query
no reuse upper 0 0,2 0,4 0,6 0,8 1 ● no reuse experiment:
bound
ContactInfoPhillipe ● No data caching
(Query No. 36)
● upper bound experiment
UnsetPropsPhillipe ● Reuse of query-local dataset
(Query No. 37) for 3 executions of each query
2ndDegree1Phillipe ● Third execution measured
(Query No. 38)
2ndDegree2Phillipe
(Query No. 39) ● Hit rate:
IncomingPhillipe look-ups answered from cache
(Query No. 40) all look-up requests
0 0,2 0,4 0,6 0,8 1
hit rate
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 46

47. Experiment – Single Query
no reuse upper 0 0,2 0,4 0,6 0,8 1 ● no reuse experiment:
bound
ContactInfoPhillipe ● No data caching
(Query No. 36)
● upper bound experiment
UnsetPropsPhillipe ● Reuse of query-local dataset
(Query No. 37) for 3 executions of each query
2ndDegree1Phillipe ● Third execution measured
(Query No. 38)
2ndDegree2Phillipe
(Query No. 39) ● Hit rate:
IncomingPhillipe look-ups answered from cache
(Query No. 40) all look-up requests
0 0,2 0,4 0,6 0,8 1
hit rate
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 47

48. Experiment – Single Query
no reuse upper 0 0,2 0,4 0,6 0,8 1 0 5 10 15 20 25 30 0 20 40 60 80
bound
ContactInfoPhillipe
(Query No. 36)
UnsetPropsPhillipe
(Query No. 37)
2ndDegree1Phillipe
(Query No. 38)
2ndDegree2Phillipe
(Query No. 39)
IncomingPhillipe
(Query No. 40)
0 0,2 0,4 0,6 0,8 1 0 5 10 15 20 25 30 0 20 40 60 80
hit rate number of query results query execution time
(in seconds)
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 48

49. Experiment – Single Query
no reuse upper 0 0,2 0,4 0,6 0,8 1 0 5 10 15 20 25 30 0 20 40 60 80
bound
ContactInfoPhillipe
(Query No. 36)
UnsetPropsPhillipe
(Query No. 37)
2ndDegree1Phillipe
(Query No. 38)
2ndDegree2Phillipe
(Query No. 39)
IncomingPhillipe
(Query No. 40)
0 0,2 0,4 0,6 0,8 1 0 5 10 15 20 25 30 0 20 40 60 80
hit rate number of query results query execution time
(in seconds)
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 49

50. Experiment – Single Query
Experiment Avg.1 number of Average1 Avg.1 query
Query Results Hit Rate Execution Time
(std.dev.) (std.dev.) (std.dev.)
4.983 0.576 30.036 s
no reuse
(11.658) (0.182) (46.708)
5.070 0.996 1.943 s
upper bound
(11.813) (0.017) (11.375)
1
Averaged over all 115 queries
● In the ideal case for Bupper= [ bcur , bcur ] :
● pupper( bcur , Bupper ) = c( bcur , [ ] ) – c( bcur , Bupper ) = c( bcur , [ ] )
● supper( bcur , Bupper ) = b( bcur , Bupper ) – b( bcur , [ ] ) = 0
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 50

51. Experiment – Single Query
Experiment Avg.1 number of Average1 Avg.1 query
Query Results Hit Rate Execution Time
(std.dev.) (std.dev.) (std.dev.)
4.983 0.576 30.036 s
no reuse
(11.658) (0.182) (46.708)
5.070 0.996 1.943 s
upper bound
(11.813) (0.017) (11.375)
1
Averaged over all 115 queries
● Summary (measurement errors aside):
● Same number of query results
● Significant improvements in query performance
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 51

52. Experiment – Complete Sequence
no reuse upper 0 given0,4 0,6 0,8
0,2 1 ●
0
given15 20 25 experiment:
5 10
order 30 0 20 40 60 80
bound order
ContactInfoPhillipe ● Reuse of the query-local
(Query No. 36) dataset for the complete
sequence of all 115 queries
UnsetPropsPhillipe
(Query No. 37)
2ndDegree1Phillipe
(Query No. 38)
2ndDegree2Phillipe
(Query No. 39)
IncomingPhillipe
(Query No. 40)
0 0,2 0,4 0,6 0,8 1 0 5 10 15 20 25 30 0 20 40 60 80
hit rate number of query results query execution time
(in seconds)
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 52

53. Experiment – Complete Sequence
no reuse upper 0 given0,4 0,6 0,8
0,2 1 ●
0
given15 20 25 experiment:
5 10
order 30 0 20 40 60 80
bound order
ContactInfoPhillipe ● Reuse of the query-local
(Query No. 36) dataset for the complete
sequence of all 115 queries
UnsetPropsPhillipe
(Query No. 37)
2ndDegree1Phillipe
(Query No. 38)
2ndDegree2Phillipe
(Query No. 39)
IncomingPhillipe
(Query No. 40)
0 0,2 0,4 0,6 0,8 1 0 5 10 15 20 25 30 0 20 40 60 80
hit rate number of query results query execution time
(in seconds)
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 53

54. Experiment – Complete Sequence
no reuse upper 0 given0,4 0,6 0,8
0,2 1 0 5 10 15 20 25 30 0 20 40 60 80
bound order
ContactInfoPhillipe
(Query No. 36)
UnsetPropsPhillipe
(Query No. 37)
2ndDegree1Phillipe
(Query No. 38)
2ndDegree2Phillipe
(Query No. 39)
IncomingPhillipe
(Query No. 40)
0 0,2 0,4 0,6 0,8 1 0 5 10 15 20 25 30 0 20 40 60 80
hit rate number of query results query execution time
(in seconds)
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 54

55. Experiment – Complete Sequence
no reuse upper 0 given0,4 0,6 0,8
0,2 1 0 5 10 15 20 25 30 0 20 40 60 80
bound order
ContactInfoPhillipe
(Query No. 36)
UnsetPropsPhillipe
(Query No. 37)
2ndDegree1Phillipe
(Query No. 38)
2ndDegree2Phillipe
(Query No. 39)
IncomingPhillipe
(Query No. 40)
0 0,2 0,4 0,6 0,8 1 0 5 10 15 20 25 30 0 20 40 60 80
hit rate number of query results query execution time
(in seconds)
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 55

56. Experiment – Complete Sequence
Bgiven order= [ q1 , … , q38 ]
no reuse upper 0 given0,4 0,6 0,8
0,2 1 0 5 10 15 20 25 30 0 20 40 60 80
bound order s( q , Bgiven order ) = b( q39 , Bgiven order ) – b( q39 , [ ] )
39
ContactInfoPhillipe =9–1
(Query No. 36)
=8
UnsetPropsPhillipe
(Query No. 37)
2ndDegree1Phillipe
(Query No. 38)
2ndDegree2Phillipe
(Query No. 39)
IncomingPhillipe
(Query No. 40)
0 0,2 0,4 0,6 0,8 1 0 5 10 15 20 25 30 0 20 40 60 80
hit rate number of query results query execution time
(in seconds)
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 56

57. Experiment – Complete Sequence
Bgiven order= [ q1 , … , q38 ]
no reuse upper 0 given0,4 0,6 0,8
0,2 1 0 5 10 15 20 25 30 0 20 40 60 80
p'( q , B
bound order given order
39
) = c'( q39 , [ ] ) – c'( q39 , Bgiven order )
ContactInfoPhillipe = 31.48 s – 68.64 s
(Query No. 36)
= – 37.16 s
UnsetPropsPhillipe
(Query No. 37)
2ndDegree1Phillipe
(Query No. 38)
2ndDegree2Phillipe
(Query No. 39)
IncomingPhillipe
(Query No. 40)
0 0,2 0,4 0,6 0,8 1 0 5 10 15 20 25 30 0 20 40 60 80
hit rate number of query results query execution time
(in seconds)
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 57

58. Experiment – Complete Sequence
Experiment Avg.1 number of Average1 Avg.1 query
Query Results Hit Rate Execution Time
(std.dev.) (std.dev.) (std.dev.)
4.983 0.576 30.036 s
no reuse
(11.658) (0.182) (46.708)
5.070 0.996 1.943 s
upper bound
(11.813) (0.017) (11.375)
6.878 0.932 39.845 s
given order
(12.158) (0.139) (145.898)
1
Averaged over all 115 queries
● Summary:
● Data cache may provide for additional query results
● Impact on performance may be positive but also negative
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 58

59. Experiment – Complete Sequence
Experiment Avg.1 number of Average1 Avg.1 query
Query Results Hit Rate Execution Time
(std.dev.) (std.dev.) (std.dev.)
4.983 0.576 30.036 s
no reuse
(11.658) (0.182) (46.708)
5.070 0.996 1.943 s
upper bound
(11.813) (0.017) (11.375)
6.878 0.932 39.845 s
given order
(12.158) (0.139) (145.898)
6.652 0.954 36.994 s
random orders
(11.966) (0.036) (118.700)
● Executing the query sequence in a random order results in
measurements similar to the given order.
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 59

60. These slides have been created by
Olaf Hartig
http://olafhartig.de
This work is licensed under a
Creative Commons Attribution-Share Alike 3.0 License
(http://creativecommons.org/licenses/by-sa/3.0/)
Olaf Hartig - How Caching Improves Efficiency and Result Completeness for Querying Linked Data 60