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From "Michael McCandless (JIRA)" <j...@apache.org>
Subject [jira] Commented: (LUCENE-1458) Further steps towards flexible indexing
Date Tue, 24 Nov 2009 10:38:39 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-1458?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12781859#action_12781859
] 

Michael McCandless commented on LUCENE-1458:
--------------------------------------------

{quote}
in trunk, things sort in UTF-16 binary order.
in branch, things sort in UTF-8 binary order.
these are different...
{quote}

Ugh!  In the back of my mind I almost remembered this... I think this
was one reason why I didn't do this back in LUCENE-843 (I think we had
discussed this already, then... though maybe I'm suffering from déjà
vu).  I could swear at one point I had that fixup logic implemented in
a UTF-8/16 comparison method...

UTF-8 sort order (what flex branch has switched to) is true unicode
codepoint sort order, while UTF-16 is not when there are surrogate
pairs as well as high (>= U+E000) unicode chars.  Sigh....

So this is definitely a back compat problem.  And, unfortunately, even
if we like the true codepoint sort order, it's not easy to switch to
in a back-compat manner because if we write new segments into an old
index, SegmentMerger will be in big trouble when it tries to merge two
segments that had sorted the terms differently.

I would also prefer true codepoint sort order... but we can't break
back compat.

Though it would be nice to let the codec control the sort order -- eg
then (I think?) the ICU/CollationKeyFilter workaround wouldn't be
needed.

Fortunately the problem is isolated to how we sort the buffered
postings when it's time to flush a new segment, so I think w/ the
appropriate fixup logic (eg your comment at
https://issues.apache.org/jira/browse/LUCENE-1606?focusedCommentId=12781746&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#action_12781746)
when comparing terms in oal.index.TermsHashPerField.comparePostings
during that sort, we can get back to UTF-16 sort order.


> 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.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,
LUCENE-1458_sortorder_bwcompat.patch, LUCENE-1458_termenum_bwcompat.patch, UnicodeTestCase.patch,
UnicodeTestCase.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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