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https://issues.apache.org/jira/browse/LUCENE-1458?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12762576#action_12762576
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Michael McCandless commented on LUCENE-1458:
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bq. One of the common statistics one needs to implement some more advanced
scoring approaches is the average document length. Is this patch far enough
along that I could take a look at it and think about how one might do this?
Well, thinking through how you'd do this... likely you'd want to store
the avg length (in tokens), eg as a single float per field per
segment, right? The natural place to store this would be in the
FieldInfos, I think?. Unfortunately, this patch doesn't yet add
extensibility to FieldInfos.
And you'd need a small customization to the indexing chain to
compute this when indexing new docs, which is already doable today
(though, package private).
But then on merging segments, you'd need an extensions point, which we
don't have today, to recompute the avg. Hmm: how would you handle
deleted docs? Would you want to go back to the field length for every
doc & recompute the average? (Which'd mean you need to per doc per
field length, not just the averages).
Unfortunately, this patch doesn't yet address things like customizing
what's stored in FieldInfo or SegmentInfo, nor customizing what
happens during merging (though it takes us a big step closer to this).
I think we need both of these to "finish" flexible indexing, but I'm
thinking at this point that these should really be tackled in followon
issue(s). This issue is already ridiculously massive.
> Further steps towards flexible indexing
> ---------------------------------------
>
> Key: LUCENE-1458
> URL: https://issues.apache.org/jira/browse/LUCENE-1458
> Project: Lucene - Java
> Issue Type: New Feature
> Components: Index
> Affects Versions: 2.9
> Reporter: Michael McCandless
> Assignee: Michael McCandless
> Priority: Minor
> Attachments: LUCENE-1458-back-compat.patch,
> LUCENE-1458-back-compat.patch, LUCENE-1458-back-compat.patch,
> LUCENE-1458-back-compat.patch, LUCENE-1458-back-compat.patch,
> LUCENE-1458-back-compat.patch, LUCENE-1458.patch, LUCENE-1458.patch,
> LUCENE-1458.patch, LUCENE-1458.patch, LUCENE-1458.patch, LUCENE-1458.patch,
> LUCENE-1458.patch, LUCENE-1458.patch, LUCENE-1458.tar.bz2,
> LUCENE-1458.tar.bz2, LUCENE-1458.tar.bz2, LUCENE-1458.tar.bz2,
> LUCENE-1458.tar.bz2, LUCENE-1458.tar.bz2, LUCENE-1458.tar.bz2
>
>
> I attached a very rough checkpoint of my current patch, to get early
> feedback. All tests pass, though back compat tests don't pass due to
> changes to package-private APIs plus certain bugs in tests that
> happened to work (eg call TermPostions.nextPosition() too many times,
> which the new API asserts against).
> [Aside: I think, when we commit changes to package-private APIs such
> that back-compat tests don't pass, we could go back, make a branch on
> the back-compat tag, commit changes to the tests to use the new
> package private APIs on that branch, then fix nightly build to use the
> tip of that branch?o]
> There's still plenty to do before this is committable! This is a
> rather large change:
> * Switches to a new more efficient terms dict format. This still
> uses tii/tis files, but the tii only stores term & long offset
> (not a TermInfo). At seek points, tis encodes term & freq/prox
> offsets absolutely instead of with deltas delta. Also, tis/tii
> are structured by field, so we don't have to record field number
> in every term.
> .
> On first 1 M docs of Wikipedia, tii file is 36% smaller (0.99 MB
> -> 0.64 MB) and tis file is 9% smaller (75.5 MB -> 68.5 MB).
> .
> RAM usage when loading terms dict index is significantly less
> since we only load an array of offsets and an array of String (no
> more TermInfo array). It should be faster to init too.
> .
> This part is basically done.
> * Introduces modular reader codec that strongly decouples terms dict
> from docs/positions readers. EG there is no more TermInfo used
> when reading the new format.
> .
> There's nice symmetry now between reading & writing in the codec
> chain -- the current docs/prox format is captured in:
> {code}
> FormatPostingsTermsDictWriter/Reader
> FormatPostingsDocsWriter/Reader (.frq file) and
> FormatPostingsPositionsWriter/Reader (.prx file).
> {code}
> This part is basically done.
> * Introduces a new "flex" API for iterating through the fields,
> terms, docs and positions:
> {code}
> FieldProducer -> TermsEnum -> DocsEnum -> PostingsEnum
> {code}
> This replaces TermEnum/Docs/Positions. SegmentReader emulates the
> old API on top of the new API to keep back-compat.
>
> Next steps:
> * Plug in new codecs (pulsing, pfor) to exercise the modularity /
> fix any hidden assumptions.
> * Expose new API out of IndexReader, deprecate old API but emulate
> old API on top of new one, switch all core/contrib users to the
> new API.
> * Maybe switch to AttributeSources as the base class for TermsEnum,
> DocsEnum, PostingsEnum -- this would give readers API flexibility
> (not just index-file-format flexibility). EG if someone wanted
> to store payload at the term-doc level instead of
> term-doc-position level, you could just add a new attribute.
> * Test performance & iterate.
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