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https://issues.apache.org/jira/browse/SOLR-7878?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14721373#comment-14721373
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David Smiley commented on SOLR-7878:
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bq. Ok, cool – but just to clarify, you need to know that new syntax to sort on
a multivalued field, but the implementation does in fact "directly" use
SortedSetSortField.
Oh yes I read your code and observed that.
bq. the implementation does in fact directly use SortedSetSortField, which
should be (unless I'm missing something) just as efficient as
SortedNumericSortField
Ah, no; not that I've benchmarked it but I can't imagine SortedSetSortField
would be faster than SortedNumericSortField. SortedSetSortField works on byte
array terms which must be decoded to a number, as opposed to numeric docValues
which are intrinsically numbers -- 'long' being the one type in particular for
which the other number types are mapped to. I see this in the call to
{{NumericUtils.prefixCodedToLong(bytes)}} in
TrieLongField.getSingleValueSource.longVal.
> Use SortedNumericDocValues (efficient sort & facet on multi-valued numeric
> fields)
> ----------------------------------------------------------------------------------
>
> Key: SOLR-7878
> URL: https://issues.apache.org/jira/browse/SOLR-7878
> Project: Solr
> Issue Type: Improvement
> Components: Facet Module
> Reporter: David Smiley
>
> Lucene has a SortedNumericDocValues (i.e. multi-valued numeric DocValues),
> ever since late in the 4x versions. Solr's TrieField.createFields
> unfortunately still uses SortedSetDocValues for the multi-valued case.
> SortedNumericDocValues is more efficient than SortedSetDocValues; for example
> there is no 'ordinal' mapping for sorting/faceting needed.
> Unfortunately, updating Solr here would be quite a bit of work, since there
> are backwards-compatibility concerns, and faceting code would need a new code
> path implementation just for this. Sorting is relatively simple thanks to
> SortedNumericSortField, and today multi-valued sorting isn't directly
> possible.
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