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https://issues.apache.org/jira/browse/LUCENE-9536?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17212713#comment-17212713
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Julie Tibshirani commented on LUCENE-9536:
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I opened a pull request implementing the idea. It was indeed simple + fast to
detect.
> Optimize OrdinalMap when one segment contains all distinct values?
> ------------------------------------------------------------------
>
> Key: LUCENE-9536
> URL: https://issues.apache.org/jira/browse/LUCENE-9536
> Project: Lucene - Core
> Issue Type: Improvement
> Reporter: Julie Tibshirani
> Priority: Minor
> Time Spent: 0.5h
> Remaining Estimate: 0h
>
> For doc values that are not too high cardinality, it seems common to have
> some large segments that contain all distinct values (plus many small
> segments who are missing some values). In this case, we could check if the
> first segment ords map perfectly to global ords and if so store
> `globalOrdDeltas` and `firstSegments` as `LongValues.ZEROES`. This could save
> a small amount of space.
> I don’t think it would help a huge amount, especially since the optimization
> might only kick in with small/ medium cardinalities, which don’t create huge
> `OrdinalMap` instances anyways? But it is simple and seemed worth mentioning.
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