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https://issues.apache.org/jira/browse/JENA-999?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15090523#comment-15090523
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ASF GitHub Bot commented on JENA-999:
-------------------------------------
Github user osma commented on the pull request:
https://github.com/apache/jena/pull/119#issuecomment-170214419
I have now implemented the changes. I went for a 10-slot Atlas cache, which
is a LRU cache in my understanding though the decision is left to the
CacheFactory. I think it's big enough that it's unlikely to be ever filled by a
single SPARQL query (I can imagine having, say, 2 text queries each being fed 4
different properties, which could fill 8 slots, but not much more), but small
enough to not use huge amounts of memory - if a TextHit object within a
Multimap takes around 100-200 bytes (very rough estimate) and a text query
normally returns at most 10000 results, then the cache could take at most
around 10-20MB (1-2MB per entry) but usually a lot less than that.
In the last commit (32c1c13) I switched to `getOrFill` to handle cache
misses. I'm not sure if it improves clarity. The number of lines of code
remains the same and to me, it's not as evident when the code within the lambda
expression is actually executed as with the explicit `if (results == null)`
block. Thoughts?
Other than the style issue, I'd like to merge this soon and move on with
other jena-text things that I have in the pipeline :)
> 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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