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From "Tom Burton-West (JIRA)" <>
Subject [jira] [Commented] (LUCENE-4286) Add flag to CJKBigramFilter to allow indexing unigrams as well as bigrams
Date Wed, 08 Aug 2012 19:05:21 GMT


Tom Burton-West commented on LUCENE-4286:

Thanks Robert for all your work on non-English searching and for your quick response on this

>>If you do unigrams and bigrams in separate fields, you can bias bigrams over unigrams.
That was our original intention. 

>>The combined unigram+bigram technique is a general technique, which I think is useful
to support. ...Tom would have to do tests for his "index-time-only" approach: I can't speak
for that.

Originally I was going to use the combined unigram+bigram technique (with a boost for the
bigram fields) and wrote some custom code to implement it.  However, I started thinking about
the size of our documents. With one exception, all the literature I found that got better
results with a combination of bigrams and unigrams used newswire size documents (somewhere
in the range of a few hundred words).  Our documents are several orders of magnitude larger
(around 100,000 words).  

My understanding is that the main reason adding unigrams to bigrams increases relevance is
that often the unigram will have a related meaning to the larger word.  So using unigrams
is somewhat analogous to decompounding or stemming.  I haven't done any tests, but my guess
is that with our very large documents the additional recall added by unigrams will be offset
by a decrease in precision.

After I get a test suite set up for relevance ranking in English, I'll take a look at testing
CJK :)

> Add flag to CJKBigramFilter to allow indexing unigrams as well as bigrams
> -------------------------------------------------------------------------
>                 Key: LUCENE-4286
>                 URL:
>             Project: Lucene - Core
>          Issue Type: Improvement
>    Affects Versions: 4.0-ALPHA, 3.6.1
>            Reporter: Tom Burton-West
>            Priority: Minor
>             Fix For: 4.0-BETA, 5.0
>         Attachments: LUCENE-4286.patch, LUCENE-4286.patch
> Add an optional  flag to the CJKBigramFilter to tell it to also output unigrams.   This
would allow indexing of both bigrams and unigrams and at query time the analyzer could analyze
queries as bigrams unless the query contained a single Han unigram.
> As an example here is a configuration a Solr fieldType with the analyzer for indexing
with the "indexUnigrams" flag set and the analyzer for querying without the flag. 
> <fieldType name="CJK" autoGeneratePhraseQueries="false">
> −
> <analyzer type="index">
>    <tokenizer class="solr.ICUTokenizerFactory"/>
>    <filter class="solr.CJKBigramFilterFactory" indexUnigrams="true" han="true"/>
> </analyzer>
> <analyzer type="query">
>    <tokenizer class="solr.ICUTokenizerFactory"/>
>    <filter class="solr.CJKBigramFilterFactory" han="true"/>
> </analyzer>
> </fieldType>
> Use case: About 10% of our queries that contain Han characters are single character queries.
  The CJKBigram filter only outputs single characters when there are no adjacent bigrammable
characters in the input.  This means we have to create a separate field to index Han unigrams
in order to address single character queries and then write application code to search that
separate field if we detect a single character Han query.  This is rather kludgey.  With the
optional flag, we could configure Solr as above  
> This is somewhat analogous to the flags in LUCENE-1370 for the ShingleFilter used to
allow single word queries (although that uses word n-grams rather than character n-grams.)

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