[jira] [Updated] (LUCENE-6863) Store sparse doc values more efficiently

["Adrien Grand (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/LUCENE-6863?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Adrien Grand updated LUCENE-6863:
---------------------------------
    Attachment: LUCENE-6863.patch

Here is a patch. It detects sparsity by checking if less than 10% documents have a value. When this happens, it stores doc IDs that have a value using a DirectMonotonicWriter and non-missing values as a regular numeric field (so values will still be compressed with table/gcd compression if applicable).

I first wanted to update the format javadocs but noticed that the low-level javadocs were very outdated (still documenting that sorted doc values were stored in a FST) so I decided to remove the low-level description in favour of the high-level one, which is much more interesting in my opinion.

This is just a first iteration, tests pass but maybe the heuristic needs to be better thought. I will also do some benchmarking.

> Store sparse doc values more efficiently
> ----------------------------------------
>
>                 Key: LUCENE-6863
>                 URL: https://issues.apache.org/jira/browse/LUCENE-6863
>             Project: Lucene - Core
>          Issue Type: Improvement
>            Reporter: Adrien Grand
>            Assignee: Adrien Grand
>         Attachments: LUCENE-6863.patch
>
>
> For both NUMERIC fields and ordinals of SORTED fields, we store data in a dense way. As a consequence, if you have only 1000 documents out of 1B that have a value, and 8 bits are required to store those 1000 numbers, we will not require 1KB of storage, but 1GB.
> I suspect this mostly happens in abuse cases, but still it's a pity that we explode storage requirements. We could try to detect sparsity and compress accordingly.



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