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https://issues.apache.org/jira/browse/SOLR-13132?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17071103#comment-17071103
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Michael Gibney commented on SOLR-13132:
---------------------------------------

Quick prose follow-up to March 16 patch version, pushed to [PR 
#751|https://github.com/apache/lucene-solr/pull/751]. As I re-read the comment 
above, I realize that a lot of the feeback applies to the caching change (now 
excised and introduced separately under SOLR-13807).

The changes presented here now consist of:
 # Refactoring facet collection in {{FacetFieldProcessorByArrayDV}} and 
{{FacetFieldProcessorByArrayUIF/UnInvertedField}} to perform facet count 
collection via the new {{SweepDISI}} abstraction. This refactoring is exercised 
by every instance of facet count collection over these implementations, and 
should indeed be well-covered by existing tests.
 # A change in the implementation of {{RelatednessAgg}} which, for 
{{FacetFieldProcessorByArray}} bulk collection over fields of cardinality > 1 
(i.e., not refinement requests), leverages the new {{SweepDISI}} abstraction to 
collect over multiple DocSet domains in a single sweep. No caching or 
heuristics are involved, so this collection always takes place in exactly the 
same way for a given input.

> Improve JSON "terms" facet performance when sorted by relatedness 
> ------------------------------------------------------------------
>
>                 Key: SOLR-13132
>                 URL: https://issues.apache.org/jira/browse/SOLR-13132
>             Project: Solr
>          Issue Type: Improvement
>          Components: Facet Module
>    Affects Versions: 7.4, master (9.0)
>            Reporter: Michael Gibney
>            Priority: Major
>         Attachments: SOLR-13132-with-cache-01.patch, 
> SOLR-13132-with-cache.patch, SOLR-13132.patch
>
>          Time Spent: 1.5h
>  Remaining Estimate: 0h
>
> When sorting buckets by {{relatedness}}, JSON "terms" facet must calculate 
> {{relatedness}} for every term. 
> The current implementation uses a standard uninverted approach (either 
> {{docValues}} or {{UnInvertedField}}) to get facet counts over the domain 
> base docSet, and then uses that initial pass as a pre-filter for a 
> second-pass, inverted approach of fetching docSets for each relevant term 
> (i.e., {{count > minCount}}?) and calculating intersection size of those sets 
> with the domain base docSet.
> Over high-cardinality fields, the overhead of per-term docSet creation and 
> set intersection operations increases request latency to the point where 
> relatedness sort may not be usable in practice (for my use case, even after 
> applying the patch for SOLR-13108, for a field with ~220k unique terms per 
> core, QTime for high-cardinality domain docSets were, e.g.: cardinality 
> 1816684=9000ms, cardinality 5032902=18000ms).
> The attached patch brings the above example QTimes down to a manageable 
> ~300ms and ~250ms respectively. The approach calculates uninverted facet 
> counts over domain base, foreground, and background docSets in parallel in a 
> single pass. This allows us to take advantage of the efficiencies built into 
> the standard uninverted {{FacetFieldProcessorByArray[DV|UIF]}}), and avoids 
> the per-term docSet creation and set intersection overhead.



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