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https://issues.apache.org/jira/browse/LUCENE-2369?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12853884#action_12853884
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Toke Eskildsen commented on LUCENE-2369:
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The current implementation accepts Comparator<Object> (which must accept
Strings) as well as a Locale (which is converted to
Collator.getInstance(locale) under the hoo)d as arguments. Plugging in the ICU
collator directly should be trivial. If/when it gets possible to use byte[] for
sorters in general, I'll add support for that.
Indexing ICU collator keys and using them in combination with LUCENE-2369 is an
interesting idea, as it would speed up the building process quite a lot, while
keeping the memory usage down. As long as fillFields=false, the two methods are
independent as should work well with each other. Fairly easy to try.
For fillFields=true, it gets a bit trickier and requires a special
FieldComparatorSource that keeps two maps from docID: One to the ICU collator
key, one to the original term. Still, it should not be that hard to implement
and I'll be happy to do it if the fillFields=false-case turns out to work well.
> Locale-based sort by field with low memory overhead
> ---------------------------------------------------
>
> Key: LUCENE-2369
> URL: https://issues.apache.org/jira/browse/LUCENE-2369
> Project: Lucene - Java
> Issue Type: New Feature
> Components: Search
> Reporter: Toke Eskildsen
> Priority: Minor
>
> The current implementation of locale-based sort in Lucene uses the FieldCache
> which keeps all sort terms in memory. Beside the huge memory overhead,
> searching requires comparison of terms with collator.compare every time,
> making searches with millions of hits fairly expensive.
> This proposed alternative implementation is to create a packed list of
> pre-sorted ordinals for the sort terms and a map from document-IDs to entries
> in the sorted ordinals list. This results in very low memory overhead and
> faster sorted searches, at the cost of increased startup-time. As the
> ordinals can be resolved to terms after the sorting has been performed, this
> approach supports fillFields=true.
> This issue is related to https://issues.apache.org/jira/browse/LUCENE-2335
> which contain previous discussions on the subject.
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