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https://issues.apache.org/jira/browse/LUCENE-4574?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13505044#comment-13505044
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Robert Muir commented on LUCENE-4574:
-------------------------------------

Yes: thats the correct fix... already documented in this RelevanceComparator :)

                
> FunctionQuery ValueSource value computed twice per document
> -----------------------------------------------------------
>
>                 Key: LUCENE-4574
>                 URL: https://issues.apache.org/jira/browse/LUCENE-4574
>             Project: Lucene - Core
>          Issue Type: Bug
>          Components: core/search
>    Affects Versions: 4.0, 4.1
>            Reporter: David Smiley
>         Attachments: LUCENE-4574.patch, Test_for_LUCENE-4574.patch
>
>
> I was working on a custom ValueSource and did some basic profiling and 
> debugging to see if it was being used optimally.  To my surprise, the value 
> was being fetched twice per document in a row.  This computation isn't 
> exactly cheap to calculate so this is a big problem.  I was able to 
> work-around this problem trivially on my end by caching the last value with 
> corresponding docid in my FunctionValues implementation.
> Here is an excerpt of the code path to the first execution:
> {noformat}
>         at 
> org.apache.lucene.queries.function.docvalues.DoubleDocValues.floatVal(DoubleDocValues.java:48)
>         at 
> org.apache.lucene.queries.function.FunctionQuery$AllScorer.score(FunctionQuery.java:153)
>         at 
> org.apache.lucene.search.TopFieldCollector$OneComparatorScoringMaxScoreCollector.collect(TopFieldCollector.java:291)
>         at org.apache.lucene.search.Scorer.score(Scorer.java:62)
>         at 
> org.apache.lucene.search.IndexSearcher.search(IndexSearcher.java:588)
>         at 
> org.apache.lucene.search.IndexSearcher.search(IndexSearcher.java:280)
> {noformat}
> And here is the 2nd call:
> {noformat}
>         at 
> org.apache.lucene.queries.function.docvalues.DoubleDocValues.floatVal(DoubleDocValues.java:48)
>         at 
> org.apache.lucene.queries.function.FunctionQuery$AllScorer.score(FunctionQuery.java:153)
>         at 
> org.apache.lucene.search.ScoreCachingWrappingScorer.score(ScoreCachingWrappingScorer.java:56)
>         at 
> org.apache.lucene.search.FieldComparator$RelevanceComparator.copy(FieldComparator.java:951)
>         at 
> org.apache.lucene.search.TopFieldCollector$OneComparatorScoringMaxScoreCollector.collect(TopFieldCollector.java:312)
>         at org.apache.lucene.search.Scorer.score(Scorer.java:62)
>         at 
> org.apache.lucene.search.IndexSearcher.search(IndexSearcher.java:588)
>         at 
> org.apache.lucene.search.IndexSearcher.search(IndexSearcher.java:280)
> {noformat}
> The 2nd call appears to use some score caching mechanism, which is all well 
> and good, but that same mechanism wasn't used in the first call so there's no 
> cached value to retrieve.

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