[#SES-198] DISTINCT filter fails on query with optional clause

Issue Details [XML]

Key:	SES-198
Type:	Bug
Status:	Closed
Resolution:	Fixed
Priority:	Minor
Assignee:	Jeen Broekstra
Reporter:	Jeen Broekstra
Votes:	0
Watchers:	0

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Sesame

DISTINCT filter fails on query with optional clause

Created: 11/Nov/05 02:34 PM Updated: 07/Feb/06 11:58 AM

Component/s:

RDBMS Sail, SeRQL

Affects Version/s:

1.2, 1.2.1, 1.2.2

Fix Version/s:

1.2.3

Environment:

Sesame 1.2.x with MySQL/RDBMS backend. See also http://www.openrdf.org/forum/mvnforum/viewthread?thread=750#2998

Description

The SeRQL 'DISTINCT' filter sometimes fails for queries incorporating an optional clause.

Sample data:

@prefix x: <URN:com:example#>.
@prefix skos: <http://www.w3.org/2004/02/skos/core#>.
x:abc rdf:type x:MyType ;
      skos:prefLabel "aaa" ;
      skos:altLabel "bbb" ;
      skos:altLabel "ccc" .
x:def rdf:type x:MyType ;
      skos:prefLabel "bbb" ;
      skos:altLabel "aaa" .

sample query:

select distinct uri, l1
from {uri} rdf:type {x:MyType};
           skos:prefLabel {l1};
           [skos:altLabel {l2}]
where l1 = "aaa" or l2 = "aaa"
using namespace skos = <http://www.w3.org/2004/02/skos/core#>,
                x = <URN:com:example#>

obtained result:

URN:com:example#abc "aaa"
URN:com:example#abc "aaa"
URN:com:example#def "bbb"

expected result:

URN:com:example#abc "aaa"
URN:com:example#def "bbb"

The problem is likely related to the query optimizer in the MySQL SAIL that (wrongly) assumes the database can handle uniqueness issues on this type of query.

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Comment by Jeen Broekstra [14/Nov/05 11:17 AM]

It was indeed the MySQL query optimizer. I have checked in a fix in CVS where queries with optional patterns are still explicitly filtered for duplicates (instead of having the database handling duplicate filtering).