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https://issues.apache.org/jira/browse/LUCENE-8551?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16673119#comment-16673119
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David Smiley commented on LUCENE-8551:
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Agreed -- if this does not wind up happening automatically, it could be added
to some other mechanism like SOLR-12259. I'm not sure yet how much complexity
it would add to the regular merge. I'm also not yet sure how much performance
degradation this is causing my employer... it remains to be measured. Even
then, it's a YMMV thing.
> Purge unused FieldInfo on segment merge
> ---------------------------------------
>
> Key: LUCENE-8551
> URL: https://issues.apache.org/jira/browse/LUCENE-8551
> Project: Lucene - Core
> Issue Type: Improvement
> Components: core/index
> Reporter: David Smiley
> Priority: Major
>
> If a field is effectively unused (no norms, terms index, term vectors,
> docValues, stored value, points index), it will nonetheless hang around in
> FieldInfos indefinitely. It would be nice to be able to recognize an unused
> FieldInfo and allow it to disappear after a merge (or two).
> SegmentMerger merges FieldInfo (from each segment) as nearly the first thing
> it does. After that, the different index parts, before it's known what's
> "used" or not. After writing, we theoretically know which fields are used or
> not, though we're not doing any bookkeeping to track it. Maybe we should
> track the fields used during writing so we write a filtered merged fieldInfo
> at the end instead of unfiltered up front? Or perhaps upon reading a
> segment, we make it cheap/easy for each index type (e.g. terms index, stored
> fields, ...) to know which fields have data for the corresponding type.
> Then, on a subsequent merge, we know up front to filter the FieldInfos.
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