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https://issues.apache.org/jira/browse/LUCENE-8041?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16841902#comment-16841902
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Huy Le commented on LUCENE-8041:
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I don't think that we can avoid paying the additional O(N) memory cost by using
Stream#sorted trick because under the hood, the stream implementation also
creates an Array, sort it then copy it into the LinkedHashMap in the terminal
stage (see SortedOps.SizedRefSortingSink). Apart from DirectFields, it is hard
to implement your approach on other cases. The patch I provided has advantage
that it is simple, clear and safe.
As we are talking about number of fields, it is unlikely that there is a index
with million fields perhaps ten of thousands is more realistic.
> All Fields.terms(fld) impls should be O(1) not O(log(N))
> --------------------------------------------------------
>
> Key: LUCENE-8041
> URL: https://issues.apache.org/jira/browse/LUCENE-8041
> Project: Lucene - Core
> Issue Type: Improvement
> Reporter: David Smiley
> Priority: Major
> Attachments: LUCENE-8041-LinkedHashMap.patch, LUCENE-8041.patch
>
>
> I've seen apps that have a good number of fields -- hundreds. The O(log(N))
> of TreeMap definitely shows up in a profiler; sometimes 20% of search time,
> if I recall. There are many Field implementations that are impacted... in
> part because Fields is the base class of FieldsProducer.
> As an aside, I hope Fields to go away some day; FieldsProducer should be
> TermsProducer and not have an iterator of fields. If DocValuesProducer
> doesn't have this then why should the terms index part of our API have it?
> If we did this then the issue here would be a simple transition to a HashMap.
> Or maybe we can switch to HashMap and relax the definition of Fields.iterator
> to not necessarily be sorted?
> Perhaps the fix can be a relatively simple conversion over to LinkedHashMap
> in many cases if we can assume when we initialize these internal maps that we
> consume them in sorted order to begin with.
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