Claus Stadler created JENA-1894:
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Summary: Insert-order preserving dataset
Key: JENA-1894
URL: https://issues.apache.org/jira/browse/JENA-1894
Project: Apache Jena
Issue Type: Improvement
Components: ARQ
Affects Versions: Jena 3.14.0
Reporter: Claus Stadler
To the best of my knowledge, there is no backend for datasets that retains
insert order.
This feature is particularly useful when changing RDF files in a git
repository, as it makes for nice commits. An insert-order preserving
Triple/QuadTable implementation enables:
* Writing (subject-grouped) RDF files or events from an RDF stream out in
nearly the same way they were read in - this makes it easier to compare outputs
of data transformations
* Combining ORDER BY with CONSTRUCT queries:
{code:java}
Dataset ds = DatasetFactory.createOrderPreservingDataset();
QueryExecutionFactory.create("CONSTRUCT WHERE { ?s ?p ?o } ORDER BY ?s ?p ?o",
ds);
RDFDataMgr.write(System.out, ds, RDFFormat.TURTLE_BLOCKS);
{code}
I have created an implementation for this some time ago with the main classes
of the machinery being:
*
[QuadTableFromNestedMaps.java|https://github.com/SmartDataAnalytics/jena-sparql-api/blob/a18b069e963bdef6cc9e8915f3e8f766893bab15/jena-sparql-api-rx/src/main/java/org/aksw/jena_sparql_api/rx/QuadTableFromNestedMaps.java#L26]
* In addition, is created a lazy (but adequate?) wrapper for re-using a quad
table as a triple table:
[TripleTableFromQuadTable.java|https://github.com/SmartDataAnalytics/jena-sparql-api/blob/a18b069e963bdef6cc9e8915f3e8f766893bab15/jena-sparql-api-rx/src/main/java/org/aksw/jena_sparql_api/rx/TripleTableFromQuadTable.java#L30]
* The DatasetGraph wapper:
[DatasetGraphQuadsImpl.java|https://github.com/SmartDataAnalytics/jena-sparql-api/blob/a18b069e963bdef6cc9e8915f3e8f766893bab15/jena-sparql-api-rx/src/main/java/org/aksw/jena_sparql_api/rx/DatasetGraphQuadsImpl.java#L32]
Note, that DatasetGraphQuadsImpl at present falsly claims that it is
transaction aware - because otherwise any SPARQL insert caused an exception (I
have not tried with the latest fixes for 3.15.0-SNAPSHOT yet). In any case, for
the use cases of writing out RDF transactions may not even be necessary, but if
there is an easy way to add them, then it should be done.
An example of the above code in action is here: [Git Diff based on ordered
turtle-blocks output
|https://github.com/SmartDataAnalytics/lodservatory/commit/ec50cd33230a771c557c1ed2751799401ea3fd89]
The downside of using this kind of order preserving dataset is, that
essentially it only features an gspo index. Hence, the performance
characteristics of this kind of order preserving dataset - which is intended
mostly for serialization or presentation - varies greatly form the
query-optimized implementations.
In any case, order preserving datasets are a highly useful feature for Jena and
I'd gladly contribute a PR for that. My main questions are:
* How to call the factory methods in DatasetFactory, DatasetGraphFactory etc -
createOrderPreservingDataset?
* In the approach using QuadTableFromNestedMaps needed - or can a different
implementation of QuadTable be repurposed?
* It seems that the abstract class DatasetGraphQuads does not have any
implementation at least in ARQ and the jena modules I use (according to
eclipse) - so my custom implementation of DatasetGraphQuadsImpl seems to be
needed, or is there a similar class lying around in another jena package?
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