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From "Michael McCandless (JIRA)" <j...@apache.org>
Subject [jira] Updated: (LUCENE-1458) Further steps towards flexible indexing
Date Tue, 18 Nov 2008 22:11:44 GMT

     [ https://issues.apache.org/jira/browse/LUCENE-1458?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Michael McCandless updated LUCENE-1458:
---------------------------------------

    Attachment: LUCENE-1458.patch


[Attached patch]

To test whether the new pluggable codec approach is flexible enough, I
coded up "pulsing" (described in detail in
http://citeseer.ist.psu.edu/cutting90optimizations.html), where
freq/prox info is inlined into the terms dict if the term freq is < N.

It was wonderfully simple :) I just had to create a reader & a writer,
and then switch the places that read (SegmentReader) and write
(SegmentMerger, FreqProxTermsWriter) to use the new pulsing codec
instead of the default one.

The pulsing codec can "wrap" any other codec, ie, when a term is
written, if the term's freq is < N, then it's inlined into the terms
dict with the pulsing writer, else it's fed to the other codec for it
to do whatever it normally would.  The two codecs are strongly
decoupled, so we can mix & match pulsing with other codecs like pfor.

All tests pass with this pulsing codec.

As a quick test I indexed first 1M docs from Wikipedia, with N=2 (ie
terms that occur only in one document are inlined into the terms
dict).  5.4M terms get inlined (only 1 doc) and 2.2M terms are not (>
1 doc).  The final size of the index (after optimizing) was a bit
smaller with pulsing (1120 MB vs 1131 MB).


> 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
>             Fix For: 2.9
>
>         Attachments: LUCENE-1458.patch, LUCENE-1458.patch, LUCENE-1458.patch
>
>
> 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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