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https://issues.apache.org/jira/browse/LUCENE-5179?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13742792#comment-13742792
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Han Jiang edited comment on LUCENE-5179 at 8/17/13 2:05 AM:
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By the way, Mike, I think this change doesn't preclude the Simple9/16 encoding
you mentioned.
You can have a look at the changed TempFSTTermsWriter, here we always pass
'true' to encodeTerm,
so PBF will not do any delta encoding. Instead FST takes the responsibility.
When we need to block encode the long[] for a whole term block, term dict can
simply buffer all the
long[] returned by encodeTerm(...,true), then use the compressin algorithm.
Whether to do VLong/delta encode is still decided by term dict, not
PBF.'encodeTerm' only operates
'delta-encode', and provde PBF the chance to know how current term is flushed
along with other terms.
was (Author: billy):
By the way, Mike, I think this change doesn't preclude the Simple9/16
encoding you mentioned.
You can have a look at the changed TempFSTTermsWriter, here we always pass
'true' to encodeTerm,
so PBF will not do any delta encoding. Instead FST takes the responsibility.
When we need to block encode the long[] for a whole term block, term dict can
simply buffer all the
long[] returned by encodeTerm(...,true), then use the compressin algorithm.
Whether to do VLong encode is decided by term dict, not PBF. 'encodeTerm' only
operates 'delta-encode',
and provde PBF the chance to know how current term is flushed along with other
terms.
> 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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