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