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From "Michael McCandless (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (LUCENE-3003) Move UnInvertedField into Lucene core
Date Fri, 01 Apr 2011 16:33:05 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-3003?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13014723#comment-13014723
] 

Michael McCandless commented on LUCENE-3003:
--------------------------------------------

bq. It is inefficient - but I never saw a way around it since the lists are all being built
in parallel (due to the fact that we are uninverting).

Lucene's indexer (TermsHashPerField) has precisely this same problem
-- every unique term must point to two (well, one if omitTFAP)
growable byte arrays.  We use "slices" into a single big (paged)
byte[], where first slice is tiny and can only hold like 5 bytes, but
then points to the next slice which is a bit bigger, etc.

We could look @ refactoring that for this use too...

Though this is "just" the one-time startup cost.

bq. Another small & easy optimization I hadn't gotten around to yet was to lower the indexIntervalBits
and make it configurable.

I did make it configurable to the Lucene class (you can pass it in to
ctor), but for Solr I left it using every 128th term.

{quote}
Another small optimization would be to store an array of offsets to length-prefixed byte arrays,
rather than a BytesRef[]. At least the values are already in packed byte arrays via PagedBytes.
{quote}

Both FieldCache and docvalues (branch) store an array-of-terms like
this (the array of offsets is packed ints).

We should also look at using an FST, which'd be the most compact but
the ord -> term lookup cost goes up.

Anyway I think we can pursue these cool ideas on new [future]
issues...


> Move UnInvertedField into Lucene core
> -------------------------------------
>
>                 Key: LUCENE-3003
>                 URL: https://issues.apache.org/jira/browse/LUCENE-3003
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: Index
>            Reporter: Michael McCandless
>            Assignee: Michael McCandless
>            Priority: Minor
>             Fix For: 3.2, 4.0
>
>         Attachments: LUCENE-3003.patch, LUCENE-3003.patch, byte_size_32-bit-openjdk6.txt
>
>
> Solr's UnInvertedField lets you quickly lookup all terms ords for a
> given doc/field.
> Like, FieldCache, it inverts the index to produce this, and creates a
> RAM-resident data structure holding the bits; but, unlike FieldCache,
> it can handle multiple values per doc, and, it does not hold the term
> bytes in RAM.  Rather, it holds only term ords, and then uses
> TermsEnum to resolve ord -> term.
> This is great eg for faceting, where you want to use int ords for all
> of your counting, and then only at the end you need to resolve the
> "top N" ords to their text.
> I think this is a useful core functionality, and we should move most
> of it into Lucene's core.  It's a good complement to FieldCache.  For
> this first baby step, I just move it into core and refactor Solr's
> usage of it.
> After this, as separate issues, I think there are some things we could
> explore/improve:
>   * The first-pass that allocates lots of tiny byte[] looks like it
>     could be inefficient.  Maybe we could use the byte slices from the
>     indexer for this...
>   * We can improve the RAM efficiency of the TermIndex: if the codec
>     supports ords, and we are operating on one segment, we should just
>     use it.  If not, we can use a more RAM-efficient data structure,
>     eg an FST mapping to the ord.
>   * We may be able to improve on the main byte[] representation by
>     using packed ints instead of delta-vInt?
>   * Eventually we should fold this ability into docvalues, ie we'd
>     write the byte[] image at indexing time, and then loading would be
>     fast, instead of uninverting

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