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https://issues.apache.org/jira/browse/JENA-999?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15059901#comment-15059901
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Andy Seaborne commented on JENA-999:
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A different and simpler approach would be a cache at the point the Lucene index
is called {{TextQueryPF.query}}. i.e. it is a Lucene cache. The cache is keys
on the arguments to the property function and the query id.
It might be necessary to do this when the arguments are easier to access in
{{concreteSubject}} and {{variableSubject}}.
The cache can be small - maybe even one slot - because the repeated call effect
happens when inputs aren't changing so a small cache will cache the majority of
possible hits without a huge memory footprint.
With a cache of "args+query" it does not matter if there are multiple
{{text:query}} in one query. By having enough of the access made, a shared
cache works.
The execution context can be used to detect when it is the same query.
QueryEngineBase can put in a unique number for each query (a good idea anyway)
although there is probably enough uniqueness at present. See
{{QueryEngineBase.constructor}} -> {{Context.setUpContext}}.
> Poor jena-text query performance when a bound subject is used
> -------------------------------------------------------------
>
> Key: JENA-999
> URL: https://issues.apache.org/jira/browse/JENA-999
> Project: Apache Jena
> Issue Type: Improvement
> Reporter: Stephen Allen
> Assignee: Stephen Allen
> Priority: Minor
> Attachments: PerformanceTester.java, jena-text benchmarks.png
>
>
> 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:
> {code}
> select *
> where {
> ?s rdf:type <http://example.org/Entity> .
> ?s text:query ( rdfs:label "test" ) .
> }
> {code}
> 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:
> {code}
> 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 . }
> {code}
> 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.
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