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https://issues.apache.org/jira/browse/LUCENE-5780?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14038674#comment-14038674
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Robert Muir commented on LUCENE-5780:
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This looks good (+1 to commit to trunk/4.10) but i think we can do better, by
explicitly sorting? E.g. take a long[] sizes parameter (can be optional and
zeros would give us what we have today if we use a stable sort), that the user
could populate either with valueCount or number of docs in the segment (both
are probably a fine heuristic).
I know this means we will need an array to remap lookups, but this only happens
once per segment with the new LongValues api so it won't impact performance.
> OrdinalMap's mapping from global ords to segment ords is sometimes wasteful
> ---------------------------------------------------------------------------
>
> Key: LUCENE-5780
> URL: https://issues.apache.org/jira/browse/LUCENE-5780
> Project: Lucene - Core
> Issue Type: Bug
> Reporter: Adrien Grand
> Assignee: Adrien Grand
> Fix For: 5.0, 4.10
>
> Attachments: LUCENE-5780.patch, LUCENE-5780.patch
>
>
> Robert found a case when the ordinal map can be quite wasteful in terms of
> memory usage: in order to be able to resolve values given a global ordinals,
> it stores two things:
> - an identifier of the segment where the value is
> - the difference between the ordinal on the segment and the global ordinal
> The issue is that OrdinalMap currently picks any of the segments that contain
> the value but we can do better: we can pick the first segment that has the
> value. This will help for two reasons:
> - it will potentially require fewer bits per value to store the segment ids
> if NRT segments don't introduce new values
> - if all values happen to appear in the first segment, then the map from
> global ords to deltas only stores zeros.
> I just tested on an index where all values are in the first segment and this
> helped reduce memory usage of the ordinal map by 4x (from 3.5MB to 800KB).
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