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From "Michael McCandless (JIRA)" <>
Subject [jira] Commented: (LUCENE-1575) Refactoring Lucene collectors (HitCollector and extensions)
Date Mon, 30 Mar 2009 17:16:50 GMT


Michael McCandless commented on LUCENE-1575:

bq. And have STFC extend NSTFC? I see no reason to create an abstract TopFieldCollector.


bq. Although this sounds like a nice utility class, now that we have a setScorer on Collector
- others may find it useful too.

OK I agree, it would be useful to have for general usage (eg chaining collectors).

But what is the plan now for the FieldComparator API?  We no longer pass score all around,
but expose access to scorer, which only RelevanceComparator (in core) will use?

bq. Did you mean continue to use STFC? The current behavior is that scoring is tracked, I

Sorry, yes, STFC.  Beginning to lose mind...

bq. I actually prefer not to expose any more methods. IndexSearcher already has plenty of
them. Instead, one can use the very generic, simple and useful method search(Query, Collector)
and pass in a NSTFC instance. Otherwise we'll end up adding many search() methods to IndexSearcher,
if we continue with that approach going forward.

I think that'd be OK... the only thing that bothers me is I think the natural default when
sorting by field is to not gather the score.  Ie I don't want someone evaluating Lucene in
the future to say our field sort is too slow when they didn't realize they had to go use this
advanced API that turns off scoring.

What if we add a new method, and deprecate the old one?  This way come 3.0 we will not have
added any methods, and then when sorting by field you see that you have to choose with or
without scores.

> Refactoring Lucene collectors (HitCollector and extensions)
> -----------------------------------------------------------
>                 Key: LUCENE-1575
>                 URL:
>             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|].
> 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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