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https://issues.apache.org/jira/browse/SOLR-7878?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14720548#comment-14720548
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Hoss Man commented on SOLR-7878:
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bq. I said multi-valued sorting isn't directly possible – as in you can't sort
on it normally; you have to know to use special functions you wrote recently.
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.
Because of LUCENE-6609 there's no function overhead involved in sorting on
{{field(foo,max)}} (unless, of course, you wrap that function in another
function) which is what i was worried you thought was the case because of the
word "directly" ... the implementation does in fact directly use
SortedSetSortField, which should be (unless I'm missing something) just as
efficient as SortedNumericSortField.
(and no matter what, when dealing with a multivalued field _some_ sort of new
syntax/configuration would be neccessary to define what it means to "sort" on
it.)
> 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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