[Koha-bugs] [Bug 28781] New: Elasticsearch facets may be inaccurate

[bugzilla-daemon at bugs.koha-community.org]

https://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=28781

            Bug ID: 28781
           Summary: Elasticsearch facets may be inaccurate
 Change sponsored?: ---
           Product: Koha
           Version: master
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P5 - low
         Component: Searching - Elasticsearch
          Assignee: koha-bugs at lists.koha-community.org
          Reporter: nick at bywatersolutions.com

To recreate:
1 - Set system preferences:
    SearchEngine: Elasticsearch
    FacetMaxCount: 20
    displayFacetCount: Show
2 - Using koha testing docker default data search for '*'
3 - Note the author facets like
 Beckett, Samuel (5)
 Aristophanes (3)
 De Niese, Danielle (3)
 Euripides (3)
 Aristotle (2) 
4 - Set FacetMaxCount:400
5 - Repeat search
6 - Note facets like:
 Beckett, Samuel (5)
 Stevens, W. Richard (5)
 Plato (4)
 Aristophanes (3)
 De Niese, Danielle (3) 
7 - Some top terms are missing when FacetMaxCount is passed to ES as 'size'

>From the docs:
The size parameter can be set to define how many term buckets should be
returned out of the overall terms list. By default, the node coordinating the
search process will request each shard to provide its own top size term buckets
and once all shards respond, it will reduce the results to the final list that
will then be returned to the client. This means that if the number of unique
terms is greater than size, the returned list is slightly off and not accurate
(it could be that the term counts are slightly off and it could even be that a
term that should have been in the top size buckets was not returned).
https://www.elastic.co/guide/en/elasticsearch/reference/6.8/search-aggregations-bucket-terms-aggregation.html

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