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https://issues.apache.org/jira/browse/LUCENE-5179?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13742575#comment-13742575
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Robert Muir commented on LUCENE-5179:
-------------------------------------
Is it for real back compat or for "impersonation" ?
> Refactoring on PostingsWriterBase for delta-encoding
> ----------------------------------------------------
>
> Key: LUCENE-5179
> URL: https://issues.apache.org/jira/browse/LUCENE-5179
> Project: Lucene - Core
> Issue Type: Improvement
> Reporter: Han Jiang
> Assignee: Han Jiang
> Fix For: 5.0, 4.5
>
> Attachments: LUCENE-5179.patch
>
>
> A further step from LUCENE-5029.
> The short story is, previous API change brings two problems:
> * it somewhat breaks backward compatibility: although we can still read old
> format,
> we can no longer reproduce it;
> * pulsing codec have problem with it.
> And long story...
> With the change, current PostingsBase API will be like this:
> * term dict tells PBF we start a new term (via startTerm());
> * PBF adds docs, positions and other postings data;
> * term dict tells PBF all the data for current term is completed (via
> finishTerm()),
> then PBF returns the metadata for current term (as long[] and byte[]);
> * term dict might buffer all the metadata in an ArrayList. when all the term
> is collected,
> it then decides how those metadata will be located on disk.
> So after the API change, PBF no longer have that annoying 'flushTermBlock',
> and instead
> term dict maintains the <term, metadata> list.
> However, for each term we'll now write long[] blob before byte[], so the
> index format is not consistent with pre-4.5.
> like in Lucne41, the metadata can be written as longA,bytesA,longB, but now
> we have to write as longA,longB,bytesA.
> Another problem is, pulsing codec cannot tell wrapped PBF how the metadata is
> delta-encoded, after all
> PulsingPostingsWriter is only a PBF.
> For example, we have terms=["a", "a1", "a2", "b", "b1" "b2"] and
> itemsInBlock=2, so theoretically
> we'll finally have three blocks in BTTR: ["a" "b"] ["a1" "a2"] ["b1" "b2"],
> with this
> approach, the metadata of term "b" is delta encoded base on metadata of "a".
> but when term dict tells
> PBF to finishTerm("b"), it might silly do the delta encode base on term "a2".
> So I think maybe we can introduce a method 'encodeTerm(long[], DataOutput
> out, FieldInfo, TermState, boolean absolute)',
> so that during metadata flush, we can control how current term is written?
> And the term dict will buffer TermState, which
> implicitly holds metadata like we do in PBReader side.
> For example, if we want to reproduce old lucene41 format , we can simple set
> longsSize==0, then PBF
> writes the old format (longA,bytesA,longB) to DataOutput, and the compatible
> issue is solved.
> For pulsing codec, it will also be able to tell lower level how to encode
> metadata.
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