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https://issues.apache.org/jira/browse/LUCENE-1575?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12693796#action_12693796
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Michael McCandless commented on LUCENE-1575:
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bq. Or introducing a new ctor which does not require maxScore, and defaults to
Float.NaN?
I think that's a good approach, though for TopDocs the ctor should be package
private I think (only called from TopFieldDocs' ctor)? And the javadocs should
clearly spell out that this could happen (so people don't get scared on seeing
Float.NaN coming back).
bq. Turning it off does not necessarily improve anything .. Might be, and might
not. In setScorer() I'd still need to register Scorer for passing on to
FieldComparator. In collect() I'd still need to check whether score tracking is
on, and if so, call scorer.score() and track maxScore. Note that if
ScoreCacheScorer is used, then calling scorer.score() in collect does not have
too much overhead.
I think this is an improvement? (Scorer.score() will not have been called...
that's the goal here).
I guess we could also consider making a separate TopFieldCollector
(NonScoringTopFieldCollector or some such), instead of sprinkling if statements
all over the place.
bq. Also, what will be the default from a Lucene perspective? true - i.e.,
always keep track of scores?
Good question... we have the freedom to choose. Perhaps default to off? But
say clearly in the migration javadocs that you have to set that to true to get
same behavior as TSDC?
> Refactoring Lucene collectors (HitCollector and extensions)
> -----------------------------------------------------------
>
> Key: LUCENE-1575
> URL: https://issues.apache.org/jira/browse/LUCENE-1575
> Project: Lucene - Java
> Issue Type: Improvement
> Components: Search
> Reporter: Shai Erera
> Fix For: 2.9
>
>
> This issue is a result of a recent discussion we've had on the mailing list.
> You can read the thread
> [here|http://www.nabble.com/Is-TopDocCollector%27s-collect()-implementation-correct--td22557419.html].
> We have agreed to do the following refactoring:
> * Rename MultiReaderHitCollector to Collector, with the purpose that it will
> be the base class for all Collector implementations.
> * Deprecate HitCollector in favor of the new Collector.
> * Introduce new methods in IndexSearcher that accept Collector, and deprecate
> those that accept HitCollector.
> ** Create a final class HitCollectorWrapper, and use it in the deprecated
> methods in IndexSearcher, wrapping the given HitCollector.
> ** HitCollectorWrapper will be marked deprecated, so we can remove it in 3.0,
> when we remove HitCollector.
> ** It will remove any instanceof checks that currently exist in IndexSearcher
> code.
> * Create a new (abstract) TopDocsCollector, which will:
> ** Leave collect and setNextReader unimplemented.
> ** Introduce protected members PriorityQueue and totalHits.
> ** Introduce a single protected constructor which accepts a PriorityQueue.
> ** Implement topDocs() and getTotalHits() using the PQ and totalHits members.
> These can be used as-are by extending classes, as well as be overridden.
> ** Introduce a new topDocs(start, howMany) method which will be used a
> convenience method when implementing a search application which allows paging
> through search results. It will also attempt to improve the memory
> allocation, by allocating a ScoreDoc[] of the requested size only.
> * Change TopScoreDocCollector to extend TopDocsCollector, use the topDocs()
> and getTotalHits() implementations as they are from TopDocsCollector. The
> class will also be made final.
> * Change TopFieldCollector to extend TopDocsCollector, and make the class
> final. Also implement topDocs(start, howMany).
> * Change TopFieldDocCollector (deprecated) to extend TopDocsCollector,
> instead of TopScoreDocCollector. Implement topDocs(start, howMany)
> * Review other places where HitCollector is used, such as in Scorer,
> deprecate those places and use Collector instead.
> Additionally, the following proposal was made w.r.t. decoupling score from
> collect():
> * Change collect to accecpt only a doc Id (unbased).
> * Introduce a setScorer(Scorer) method.
> * If during collect the implementation needs the score, it can call
> scorer.score().
> If we do this, then we need to review all places in the code where
> collect(doc, score) is called, and assert whether Scorer can be passed. Also
> this raises few questions:
> * What if during collect() Scorer is null? (i.e., not set) - is it even
> possible?
> * I noticed that many (if not all) of the collect() implementations discard
> the document if its score is not greater than 0. Doesn't it mean that score
> is needed in collect() always?
> Open issues:
> * The name for Collector
> * TopDocsCollector was mentioned on the thread as TopResultsCollector, but
> that was when we thought to call Colletor ResultsColletor. Since we decided
> (so far) on Collector, I think TopDocsCollector makes sense, because of its
> TopDocs output.
> * Decoupling score from collect().
> I will post a patch a bit later, as this is expected to be a very large
> patch. I will split it into 2: (1) code patch (2) test cases (moving to use
> Collector instead of HitCollector, as well as testing the new topDocs(start,
> howMany) method.
> There might be even a 3rd patch which handles the setScorer thing in
> Collector (maybe even a different issue?)
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