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https://issues.apache.org/jira/browse/LUCENE-6863?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14979533#comment-14979533
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Robert Muir commented on LUCENE-6863:
-------------------------------------

I think this needs to be a very low number, if its going to be "automatic", 
like 3%, pure abuse case detection, just like how sparse norms worked.

Otherwise, it needs to be a separate format in codecs/ that is explicit opt-in 
by the user.

> 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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