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https://issues.apache.org/jira/browse/LUCENE-2091?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12785569#action_12785569
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Uwe Schindler commented on LUCENE-2091:
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Thanks for the explanation!
About the IDF: The problem with a per-document IDF in lucene would be that most
uses also add fields that are e.g. catch-all fields (which would be the IDF you
want to have) but in addition they add special fields like numeric field (which
would not produce a good IDF, at the moment this IDF is ignored). Some users
also add fileds simply for sorting. So a IDF for documents is impossible with
Lucene. You can only use e.g. catch all fields (which are always a godd idea
for non-fielded searches, because oring all fields together is slower that just
indexing the same terms a second time in a catch-all field), e.g. "contents"
contains all terms from "title", "subject", "mailtext" as an example for
emails. But the IDF for BM25F could be taken from the "contents" field even
when searching only for a title.
> Add BM25 Scoring to Lucene
> --------------------------
>
> Key: LUCENE-2091
> URL: https://issues.apache.org/jira/browse/LUCENE-2091
> Project: Lucene - Java
> Issue Type: New Feature
> Components: contrib/*
> Reporter: Yuval Feinstein
> Priority: Minor
> Fix For: 3.1
>
> Attachments: LUCENE-2091.patch, persianlucene.jpg
>
> Original Estimate: 48h
> Remaining Estimate: 48h
>
> http://nlp.uned.es/~jperezi/Lucene-BM25/ describes an implementation of
> Okapi-BM25 scoring in the Lucene framework,
> as an alternative to the standard Lucene scoring (which is a version of mixed
> boolean/TFIDF).
> I have refactored this a bit, added unit tests and improved the runtime
> somewhat.
> I would like to contribute the code to Lucene under contrib.
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