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From "Dawid Weiss (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (LUCENE-3297) FST doesn't fully share common prefix across all outputs
Date Tue, 09 Aug 2011 11:27:27 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-3297?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13081571#comment-13081571
] 

Dawid Weiss commented on LUCENE-3297:
-------------------------------------

I guess I just don't like marker values from the domain range (like END_LABEL)... they make
me nervous. I'll experiment.

> FST doesn't fully share common prefix across all outputs
> --------------------------------------------------------
>
>                 Key: LUCENE-3297
>                 URL: https://issues.apache.org/jira/browse/LUCENE-3297
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: core/FSTs
>            Reporter: Michael McCandless
>            Priority: Minor
>
> FST will try to share prefixes of outputs when possible, however in the [I think unusual
in practice] case where all outputs share a common prefix, FST really ought to store this
just once, on the root arc, but instead it's only able to push back to the N root arcs.  It's
sort of an off-by-one on how far back the pushing goes...
> One [synthetic] example where this makes a big difference is the new Test2BPostings test,
when it uses MemoryCodec, because this test has 26 terms (letters of alphabet) and each term
has exactly the same long (~85 MB) all 1s byte[] as the postings.  If we fixed this issue,
then the resulting FST would only be ~85 MB but now instead it needs to be ~85 * 26 MB.

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