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From Michael McCandless <luc...@mikemccandless.com>
Subject Re: [jira] Commented: (LUCENE-1458) Further steps towards flexible indexing
Date Fri, 21 Nov 2008 19:57:44 GMT

We could easily add a sequence ID (ord) today, for a single segment's  
term dict; but merging them (so that MultiSegmentReader could also  
present TermEnum.ord()) is problematic.

Mike

Jason Rutherglen wrote:

> It would be nice to have btree like features such as previous(), min  
> and max.  Also a unique sequence id per term that enables faster  
> lookup if the term id is known.
>
> On Wed, Nov 19, 2008 at 1:38 PM, Michael McCandless <lucene@mikemccandless.com 
> > wrote:
>
> I think we wouldn't do any term compression for the btree, at least  
> for the parts loaded in RAM (we don't today, ie, we create the full  
> Term or String as an array).
>
> For the parts left on disk we should be able to do something similar  
> to what we do today, eg for child nodes only encode the "delta" wrt  
> the parent node?
>
> Mike
>
>
> Jason Rutherglen wrote:
>
> Michael B: Are you interested in making column stride fields  
> realtime and use the btree for the terms?  This is an idea I started  
> on I called tag index where the postings are divided into blocks.   
> The blocks can then be replaced in memory with periodic flush to  
> disk as the in ram postings grows.
>
> Michael M: How would the term compression be handled in a btree model?
>
> On Wed, Nov 19, 2008 at 2:29 AM, Michael McCandless (JIRA) <jira@apache.org 
> > wrote:
>
>   [ https://issues.apache.org/jira/browse/LUCENE-1458?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12648971

> #action_12648971 ]
>
> Michael McCandless commented on LUCENE-1458:
> --------------------------------------------
>
> bq. So something like a B+Tree would probably work better.
>
> I agree, btree is a better fit, though we don't need insertion &  
> deletion operations since each segment is write once.
>
> > 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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