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https://issues.apache.org/jira/browse/SOLR-16144?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17518434#comment-17518434
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Michael Gibney commented on SOLR-16144:
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Even if the current implementation is left as-is, we should at least throw an
error if a client tries to explicitly set a {{min_popularity}} value less than
{{0.00001}} (which would currently effectively exclude _all_ buckets).
However, I think it would be preferable to _not_ round these values internally.
For a relatively high-cardinality field, perfect correlation for a
{{background_popularity}} of 9/2,000,000 feels meaningful, and in any case well
above any threshold that I might intuitively consider to indicate "noise". My
sense is that different use cases would have different "noise" thresholds, and
that the purpose of the {{min_popularity}} param is to allow clients to specify
their own "noise" threshold. AFAICT it's cleaner and there's no real downside
to deferring pop-value-rounding until the response is externalized to be sent
back to the client.
[PR #790|https://github.com/apache/solr/pull/790] makes concrete the above
"preferred" proposal.
> Don't internally round [foreground|background]_popularity values in
> RelatednessAgg
> ----------------------------------------------------------------------------------
>
> Key: SOLR-16144
> URL: https://issues.apache.org/jira/browse/SOLR-16144
> Project: Solr
> Issue Type: Improvement
> Security Level: Public(Default Security Level. Issues are Public)
> Components: Facet Module
> Affects Versions: main (10.0)
> Reporter: Michael Gibney
> Priority: Trivial
> Time Spent: 10m
> Remaining Estimate: 0h
>
> The "relatedness" facet function supports the concept of
> {{foreground_popularity}} and {{background_popularity}} -- i.e., the
> cardinality of the intersection of bucket domain with the foreground and
> background sets (respectively), each normalized with respect to background
> set cardinality.
> The logic appears to be:
> # To provide clients with context of computed relatedness values
> # To preemptively (optionally) screen out "noise" from low-frequency terms
> via the {{min_popularity}} function parameter.
> For both purposes, popularity values are currently rounded to 5 digits.
> This issue proposes that although rounding to 5 digits makes sense for the
> _first_ case (providing context to clients), this arbitrary truncation does
> not make sense as currently implemented for internally evaluating threshold
> pop values for bucket inclusion.
> Consider the case of a high-cardinality field with a relatively large
> background set and a selective foreground set. For {{|background_set| =
> 2,000,000}} and a foreground set of cardinality 9, even a bucket with a
> domain that exactly matches the foreground set would be screened out, for
> _any_ explicit setting of {{min_popularity}}.
> This behavior is due to where the rounding takes place (internally, upon
> initial {{computeDerivedValues()}}). It is further problematic that
> {{RelatednessAgg}} will currently accept {{min_popularity < 0.00001}}, which
> would be guaranteed to exclude _all_ buckets.
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