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https://issues.apache.org/jira/browse/LUCENE-4610?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13528885#comment-13528885
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Michael McCandless commented on LUCENE-4610:
--------------------------------------------
bq. I understand what you say, so let's say that we can have
NoParentsAggregator, which dedups and works properly, counting on the fly. And
if performance is not as good as NoParentsAccumulator, then we do both,
documenting the accumulator's limitations?
+1 for sure :)
I feel like we are gonna need something, somewhere, which looks at FSP (maybe
with some "additions", to include things like "these N dimensions are
single-valued"), and then picks the fastest accumulator/aggregator/collector,
that will give correct counts, for your situation.
> Implement a NoParentsAccumulator
> --------------------------------
>
> Key: LUCENE-4610
> URL: https://issues.apache.org/jira/browse/LUCENE-4610
> Project: Lucene - Core
> Issue Type: New Feature
> Components: modules/facet
> Reporter: Shai Erera
>
> Mike experimented with encoding just the exact categories ordinals on
> LUCENE-4602, and I added OrdinalPolicy.NO_PARENTS, with a comment saying that
> this requires a special FacetsAccumulator.
> The idea is to write the exact categories only for each document, and then at
> search time count up the parents chain to compute requested facets (I say
> count, but it can be any weight).
> One limitation of such accumulator is that it cannot be used when e.g. a
> document is associated with two categories who share the same parent, because
> that may result in incorrect weights computed (e.g. a document might have
> several Authors, and so counting the Author facet may yield wrong counts). So
> it can be used only when the app knows it doesn't add such facets, or that it
> always asks to aggregate a 'root' that in its path this criteria doesn't hold
> (no categories share the same parent).
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