Details
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Improvement
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Status: Closed
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Minor
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Resolution: Fixed
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Jena 3.0.1
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None
Description
When executing a jena-text query, the performance is terrible if the subject is already bound to a variable. This is because the current code will execute a new lucene query that does not have the subject/entity bound on every iteration and then iterate through the lucene results to join against the subject. This is quite inefficient.
Example query:
select *
where {
?s rdf:type <http://example.org/Entity> .
?s text:query ( rdfs:label "test" ) .
}
This would be quite slow if there were a lot of entities in the system.
Two potential solutions present themselves:
- Craft a more explicit lucene query that specifies the entity URI, so that the results coming back from lucene are much smaller. However, this would cause problems with the score not being correct across multiple iterations. Additionally we are still potentially running a lot of lucene queries, each of which has a probably non-negligble constant cost (parsing the query string, etc).
- Execute the more general lucene query the first time it is encountered, then caching the results somewhere. From there, we can then perform a hash table lookup against those cached results.
I would like to pursue option 2, but there is a problem. Because jena-text is implemented as a property function instead of a query op in and of itself (like QueryIterMinus is for example), we have to find a place to stash the lucene results. I believe this can be done by placing it in the ExecutionContext object, using the lucene query as a cache key. Updates provide a slightly troubling case because you could have an update request like:
insert data { <urn:test1> rdf:type <http://example.org/Entity> ; rdfs:label "test" } ;
delete { ?s ?p ?o }
where { ?s rdf:type <http://example.org/Entity> ; text:query ( rdfs:label "test" ) . ?p ?o . } ;
insert data { <urn:test2> rdf:type <http://example.org/Entity> ; rdfs:label "test" } ;
delete { ?s ?p ?o }
where { ?s rdf:type <http://example.org/Entity> ; text:query ( rdfs:label "test" ) ; ?p ?o . }
And then the end result should be an empty database. But if the ExecutionContext was the same for both delete queries, you would be using the cached results from the first delete query in the second delete query, which would result in <urn:test2> not being deleted properly.
If the ExecutionContext is indeed shared between the two update queries in the situation above, I think this can be solved by making the cache key for the lucene resultset be a combination of both the lucene query and the QueryIterRoot or BindingRoot. I need to investigate this. An alternative, if there was a way to be notified when a query has finished executing, we could clear the cache in the ExecutionContext.
