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https://issues.apache.org/jira/browse/LUCENE-1575?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12693474#action_12693474
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Shai Erera commented on LUCENE-1575:
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bq. We could make the change (turn off filtering), but put a setter on
IndexSearcher to have it insert the "PositiveScoresOnlyCollector" wrapper?
Then why do that at all? If I need to call searcher.setKeepOnlyPositiveScores,
then it means a change to my code. I could then just pass in the
PositiveScoresOnlyCollector to the search methods instead, right?
I guess you are referring to the methods which don't take a collector as a
parameter and instantiate a new TopScoreDocCollector internally? I tend to
think that if someone uses those, it is just because they are simple, and I
find it very hard to imagine that that someone relies on the filtering. So
perhaps we can get away with just documenting the change in behavior?
bq. I think the vast majority of users are not relying on <= 0 scoring docs to
be filtered out.
I tend to agree. This has been around for quite some time. I checked my custom
collectors, and they do the same check. I only now realize I just followed the
code practice I saw in Lucene's code, never giving it much thought of whether
this can actually happen. I believe that if I'd have extended Lucene in a way
such that it returns <=0 scores, I'd be aware of that and probably won't use
the built-in collectors. I see no reason to filter <= 0 scored docs anyway, and
if I wanted that, I'd probably write my own filtering collector ...
I think that if we don't believe people rely on the <= 0 filtering, let's just
document it. I'd hate to add a setter method to IndexSearcher, and a unit test,
and check where else it should be added (i.e., in extending searcher classes)
and introduce a new API which we might need to deprecate some day ...
People who'll need that functionality can move to use the methods that accept a
Collector, and pass in the PositiveScoresOnlyCollector. That way we also keep
the 'fast and easy' search methods really simple, fast and easy.
Is that acceptable?
> 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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