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https://issues.apache.org/jira/browse/LUCENE-4610?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13528879#comment-13528879
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Michael McCandless commented on LUCENE-4610:
--------------------------------------------
I think counting the parents ordinals on the fly is going to be much more
costly than aggregating up only in the end?
I suspect that was a big part of the gains I saw, since it means we only count
1 int not 3 in my test (but we should separately test it). I realize that
means the NoParentsAggregator would not be "general purpose", because you
couldn't use it on multi-valued fields, but I suspect in the common case many
facet dimensions are single-valued.
Also, for the multi-valued case, having NoParentsAccumulator that must dedup
on-the-fly is likely to be expensive? Ie I think it's likely for the
multi-valued case that you will want to dedup at indexing time and store the
full-path ords in the index (ie what we do today by default)?
> 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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