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From Shai Erera <ser...@gmail.com>
Subject Re: Converting HitCollector to Collector
Date Thu, 10 Dec 2009 06:51:58 GMT
Hi Max,

In 3.0.0 (actually in 2.9.0 already), Lucene moved to execute its searches
one sub-reader at a time. As a consequence, absolute docIDs are not passed
to the collect method anymore, but instead the relative docIDs of that
reader. An example, suppose you have 2 segments, with 6 documents total,
each segment has 3. Prior to 2.9, collect would receive doc IDs 0 .. 5.
Starting w/ 2.9, collect receives 0, 1, 2 and then 0, 1, 2 again from the
second segment reader. setNextReader passes the docBase for that reader, so
you can re-base the IDs passed to collect. Your implementation is correct,
by that you always add the docBase to the passed in doc. For the first
reader, docBase would be 0. For the second - 3.
BTW, some collectors may not care at all about doc IDs, for example a
Collector who simply keeps track of the max score.

As for the out-of-order, again, your implementation is right by returning
true. Using this method, the Collector declares whether it cares about the
documents passed to its collect() to be in-order or out of order. For the
example above, I could pass the documents as 0, 1, 2 or 1, 0, 2 if I have an
implementation of a query that can perform better that way. Lucene has such
query, BooleanQuery which uses BooleanScorer or BooleanScorer2. BQ performs
better if allowed to pass documents out-of-order - it uses BooleanScorer for
that.

Hope that helps,
Shai

On Wed, Dec 9, 2009 at 8:58 PM, Max Lynch <ihasmax@gmail.com> wrote:

> Hi,
> I have a HitCollector that processes all hits from a query.  I want all
> hits, not the top N hits.  I am converting my HitCollector to a Collector
> for Lucene 3.0.0, and I'm a little confused by the new interface.
>
> I assume that I can implement by new Collector much like the code on the
> API
> Docs:
>
>  Searcher searcher = new IndexSearcher(indexReader);
>  final BitSet bits = new BitSet(indexReader.maxDoc());
>  searcher.search(query, new Collector() {
>   private int docBase;
>
>   *// ignore scorer*
>   public void setScorer(Scorer scorer) {
>   }
>
>   *// accept docs out of order (for a BitSet it doesn't matter)*
>   public boolean acceptsDocsOutOfOrder() {
>     return true;
>   }
>
>   public void collect(int doc) {
>     bits.set(doc + docBase);
>   }
>
>   public void setNextReader(IndexReader reader, int docBase) {
>     this.docBase = docBase;
>   }
>  });
>
>
> But I'm confused what the docBasing is (I need to get fields from each
> document from my index searcher).  Do I need to use the doc base or
> setNextReader?  Also, what is the purpose of acceptsDocsOutOfOrder?  I
> see the docs note on it but I'm not sure how I could apply that or if
> I should care about it.
>
> Thanks,
> Max
>

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