I third it. We've found that D2R grinds to a halt due to seemingly
unnecessary joins. It's a shame, because D2R could be a really useful
piece of technology.
dlb
On Thu, Apr 4, 2013 at 11:25 AM, Diogo FC Patrao <djogopat...@gmail.com>wrote:
> Hi Hendrik
>
> I second that; And I also add that sometimes stuff should not be joined at
> all, but resolved in two different queries. I know, D2R isn't suited for
> connecting at two databases at once, but if it could tell when to JOIN,
> when to use the same FROM and when to join things from memory, it would be
> great.
>
> Cheers,
>
>
> --
> diogo patrão
>
>
>
>
> On Wed, Apr 3, 2013 at 10:17 PM, Hendrik Adler <hen.ad...@gmail.com>wrote:
>
>> Hi,
>>
>> I am new to this forum and run into a problem. I would like to map a rdf
>> schema to a view in a PostgreSQL-database.
>> Problem is that the query that d2rq generates is extremely slow.
>>
>> This is the sparql-query:
>> select ?time ?value where
>> {
>> ?a <uri:sensor:value> ?value.
>> ?a <uri:sensor:timestamp> ?time.
>> FILTER (?time < '2012-01-06T08:00:00'^^xsd:dateTime && ?time >
>> '2012-01-06T00:00:00'^^xsd:dateTime) }
>>
>> And this the generated SQL Code:
>> DISTINCT SELECT ...... FROM "v1" AS "T1_v1", "v1" AS "T2_v1" WHERE (....)
>>
>> FROM "v1" AS "T1_v1", "v1" AS "T2_v1" leads to a really bad performance,
>> because PostgreSQL joins view v1 with itself by nesting loops.
>> I think it happens because the view generates the measured values from a
>> denormalized table, where 3600 values are stored per one column using
>> arrays.
>> (for better performance and saving storage space)
>>
>>
>> That is why it would great to have a simple query in the form:
>> DISTINCT SELECT ...... FROM "v1" WHERE (....)
>>
>> Is there a way to use just one Table after the "from" to avoid selfjoins
>> or must I adapt the code? I start the server (version 0.8.1) with the
>> "fast" option.
>>
>> Many Thanks
>> Hendrik
>>
>> That is my ttl-config:
>>
>> # D2RQ Namespace
>> @prefix d2rq: <http://www.wiwiss.fu-berlin.de/suhl/bizer/D2RQ/0.1#>
>> .
>> # Namespace of the ontology
>> @prefix : <uri:sensor:measure> .
>> #@prefix : <:measurement> .
>> # Namespace of the mapping file; does not appear in mapped data
>> @prefix map: <file:///Users/d2r/example.ttl#> .
>>
>>
>> # Other namespaces
>> @prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
>> @prefix xsd: <http://www.w3.org/2001/XMLSchema#> .
>> @prefix jdbc: <http://d2rq.org/terms/jdbc/> .
>> @prefix sens: <urn:measurement:> .
>>
>> @prefix db: <> .
>> @prefix vocab: <vocab/> .
>> @prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
>> @prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
>>
>> map:Configuration a d2rq:Configuration;
>> d2rq:useAllOptimizations true.
>>
>> map:database a d2rq:Database;
>> d2rq:jdbcDSN "jdbc:postgresql://192.168.0.181/measured-data";
>> d2rq:jdbcDriver "org.postgresql.Driver";
>> d2rq:username "abc";
>> d2rq:password "123";
>> jdbc:autoReconnect "true";
>> jdbc:zeroDateTimeBehavior "convertToNull";
>> .
>>
>> # Table measuredvalues
>> map:measuredvalues a d2rq:ClassMap;
>> d2rq:dataStorage map:database;
>> d2rq:class :measuredvalues;
>> d2rq:uriPattern "
>> http://example.org/lineItems/sensorid@@v1.sensorid@@-@@v1.timestamp@@";;
>> .
>>
>> map:measuredvaluetimestamp a d2rq:PropertyBridge;
>> d2rq:belongsToClassMap map:measuredvalues;
>> d2rq:column "v1.timestamp";
>> d2rq:datatype xsd:dateTime;
>> d2rq:property :timestamp;
>> .
>>
>> map:measuredvalue_2_value a d2rq:PropertyBridge;
>> d2rq:belongsToClassMap map:measuredvalues;
>> d2rq:column "v1.value";
>> d2rq:datatype xsd:double;
>> d2rq:property :value;
>> .
>>
>> map:measuredvaluesensorid a d2rq:PropertyBridge;
>> d2rq:belongsToClassMap map:measuredvalues;
>> d2rq:column "v1.sensorid";
>> d2rq:datatype xsd:integer;
>> d2rq:property :sensorid;
>> .
>>
>>
>>
>>
>>
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