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From "Cyril Barlow" <i...@fantasyfooty.org>
Subject Re: What is a Hits object?
Date Thu, 06 Oct 2005 15:28:40 GMT

----- Original Message -----
From: "J.J. Larrea" <jjl@panix.com>
To: <java-user@lucene.apache.org>
Sent: Wednesday, October 05, 2005 10:31 PM
Subject: Re: What is a Hits object?


> A Hits object is essentially a cache on query results.  It caches in 2
ways:
>
> 1. When a query returning Hits is requested, only the top 100 document IDs
and scores are requested from the scoring system, and the ID/Score pairs are
stored in a list in the Hits object.  Whenever a document ID, score, or
Document object are requested that lie beyond the end of that list, the
query is reexecuted in order to grow the list to at or beyond the request,
typically 100% beyond it.
>
> 2. Returning a Document object (rather than a score or document ID)
requires reconstituting the Document from the stored fields in the index,
which is an expensive operation.  The Hits object maintains a cache of the
200 most recently requested Document objects, so it is unlikely they will
need to be reconstituted more than once.
>
> This is all optimized around typical hitlist access patterns - navigate
forward and backwards through the results pages a small number of documents
at a time.  For applications which cannot benefit from the Hits caching, for
example which employ their own hit caching layer, one can effectively use
the so-called "low level" IndexSearcher routines which return TopDocs rather
than Hits.
>
> - J.J.
>
> At 8:26 PM +0100 10/5/05, Cyril Barlow wrote:
> >Is it an actual array of full Documents or a list of reference points to
> >Documents? And what's the typical size in memory of a Hits object with
say
> >1000 avg size docs?
>
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How long does the Hits object stay in memory for? Until the IndexSearcher
closes? And if you just use 1 IndexSearcher would there be a big collection
of Hits in memory?


	
	
		
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