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https://issues.apache.org/jira/browse/LUCENE-1458?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12783471#action_12783471
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Michael McCandless commented on LUCENE-1458:
--------------------------------------------
OK I finally worked out a solution for the UTF16 sort order problem
(just committed).
I added a TermRef.Comparator class, for comparing TermRefs, and I
removed TermRef.compareTo, and fixed all low-level places in Lucene
that rely on sort order of terms to use this new API instead.
I changed the Terms/TermsEnum/TermsConsumer API, adding a
getTermComparator(), ie, the codec now determines the sort order for
terms in each field. For the core codecs (standard, pulsing,
intblock) I default to UTF16 sort order, for back compat, but you
could easily instantiate it yourself and use a different term sort.
I changed TestExternalCodecs to test this new capability, by sorting 2
of its fields in reversed unicode code point order.
While this means your codec is now completely free to define the
term sort order per field, in general Lucene queries will not behave
right if you do this, so it's obviously a very advanced use case.
I also changed (yet again!) how DocumentsWriter encodes the terms
bytes, to record the length (in bytes) of the term, up front, followed by the
term bytes (vs the trailing 0xff that I had switched to). The length
is a 1 or 2 byte vInt, ie if it's < 128 it's 1 byte, else 2 bytes.
This approach means the TermRef.Collector doesn't have to deal with
0xff's (which was messy).
I think this also means that, to the flex API, a term is actually
opaque -- it's just a series of bytes. It need not be UTF8 bytes.
However, all of analysis, and then how TermsHash builds up these
byte[]s, and what queries do with these bytes, is clearly still very
much Unicode/UTF8. But one could, in theory (I haven't tested this!)
separately use the flex API to build up a segment whose terms are
arbitrary byte[]'s, eg maybe you want to use 4 bytes to encode int
values, and then interact with those terms at search time
using the flex API.
> 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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