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From "Robert Muir (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (LUCENE-7081) Docvalues terms dict should sometimes prefix-compress fixed-length data.
Date Wed, 09 Mar 2016 13:00:44 GMT

     [ https://issues.apache.org/jira/browse/LUCENE-7081?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Robert Muir updated LUCENE-7081:
--------------------------------
    Attachment: LUCENE-7081.patch

initial patch after lots of experiments.

we can easily just compute how well we compress, by computing shared prefix for the first
term (count % 16 == 0) and last term (count % 16 == 15) in each block. Thats because our terms
dict here uses a single shared prefix for all terms in a block: e.g. terms are delta encoded
against the first term in the block always.

we use prefix compression when we share at least 3 bytes per term on average: thats because
prefix encoding has some overhead vs the alternative for fixed-width data (worst case 2 bytes
per term for suffix lengths).


> Docvalues terms dict should sometimes prefix-compress fixed-length data.
> ------------------------------------------------------------------------
>
>                 Key: LUCENE-7081
>                 URL: https://issues.apache.org/jira/browse/LUCENE-7081
>             Project: Lucene - Core
>          Issue Type: Improvement
>            Reporter: Robert Muir
>         Attachments: LUCENE-7081.patch
>
>
> For Sorted/SortedSet types, we encode ordinals and a term dictionary (similar to old
lucene 3 term dictionary).
> Originally we had no prefix compression, so we "save space" in the fixed-width case by
avoiding addressing, we can just use multiplication: https://github.com/apache/lucene-solr/blob/master/lucene/core/src/java/org/apache/lucene/codecs/lucene54/Lucene54DocValuesConsumer.java#L423-L425

> But it means no compression whatsoever of the actual bytes, even if values are enormous,
I don't think its necessarily a good tradeoff. The lack of prefix compression can become much
more magnified now that we have fixed width 128-bit point types in the sandbox...



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