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https://issues.apache.org/jira/browse/LUCENE-4069?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13286717#comment-13286717
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Robert Muir commented on LUCENE-4069:
-------------------------------------

And separately, you can always contain the number of files even today by:
* using only unique instances yourself when writing (rather than waiting on 
LUCENE-4093)
* using the compound file format.

The purpose of LUCENE-4093 is just to make this simpler, but I opened it as a 
separate
issue because its really solely an optimization, and only for a pretty rare 
case where
people are customizing the index format for different fields.
                
> Segment-level Bloom filters for a 2 x speed up on rare term searches
> --------------------------------------------------------------------
>
>                 Key: LUCENE-4069
>                 URL: https://issues.apache.org/jira/browse/LUCENE-4069
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: core/index
>    Affects Versions: 3.6, 4.0
>            Reporter: Mark Harwood
>            Priority: Minor
>             Fix For: 4.0, 3.6.1
>
>         Attachments: BloomFilterPostings40.patch, 
> MHBloomFilterOn3.6Branch.patch, PrimaryKey40PerformanceTestSrc.zip
>
>
> An addition to each segment which stores a Bloom filter for selected fields 
> in order to give fast-fail to term searches, helping avoid wasted disk access.
> Best suited for low-frequency fields e.g. primary keys on big indexes with 
> many segments but also speeds up general searching in my tests.
> Overview slideshow here: 
> http://www.slideshare.net/MarkHarwood/lucene-bloomfilteredsegments
> Benchmarks based on Wikipedia content here: http://goo.gl/X7QqU
> Patch based on 3.6 codebase attached.
> There are no 3.6 API changes currently - to play just add a field with "_blm" 
> on the end of the name to invoke special indexing/querying capability. 
> Clearly a new Field or schema declaration(!) would need adding to APIs to 
> configure the service properly.
> Also, a patch for Lucene4.0 codebase introducing a new PostingsFormat

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