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https://issues.apache.org/jira/browse/LUCENE-4764?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13575441#comment-13575441
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Michael McCandless commented on LUCENE-4764:
--------------------------------------------

The 30% RAM overhead I measured was for the 7 dims per doc case.

And the RAM overhead will vary greatly depending on the app: if you have fewer 
facet dims, and more docs, the overhead is higher.

I think it should just be an optional DV Format that makes the obvious 
tradeoffs, and the javadocs should say "your mileage may vary", ie the 
time/space tradeoff will be app dependent.
                
> Faster but more RAM/Disk consuming DocValuesFormat for facets
> -------------------------------------------------------------
>
>                 Key: LUCENE-4764
>                 URL: https://issues.apache.org/jira/browse/LUCENE-4764
>             Project: Lucene - Core
>          Issue Type: Improvement
>            Reporter: Michael McCandless
>            Assignee: Michael McCandless
>             Fix For: 4.2, 5.0
>
>         Attachments: LUCENE-4764.patch
>
>
> The new default DV format for binary fields has much more
> RAM-efficient encoding of the address for each document ... but it's
> also a bit slower at decode time, which affects facets because we
> decode for every collected docID.

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