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https://issues.apache.org/jira/browse/SOLR-9125?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15287339#comment-15287339
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Jeff Wartes commented on SOLR-9125:
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Isn't there a chicken-and-egg situation there? You need the set of matching
docs to figure out the HLL.cardinality to specify the initial size of the map
you're going to save the set of matching docs in?
Or maybe collect() would just throw every doc in the FBS, and finish() would do
all the finding group heads and collapsing?
> CollapseQParserPlugin allocations are index based, not query based
> ------------------------------------------------------------------
>
> Key: SOLR-9125
> URL: https://issues.apache.org/jira/browse/SOLR-9125
> Project: Solr
> Issue Type: Improvement
> Components: query parsers
> Reporter: Jeff Wartes
> Priority: Minor
> Labels: collapsingQParserPlugin
>
> Among other things, CollapsingQParserPlugin’s OrdScoreCollector allocates
> space per-query for:
> 1 int (doc id) per ordinal
> 1 float (score) per ordinal
> 1 bit (FixedBitSet) per document in the index
>
> So the higher the cardinality of the thing you’re grouping on, and the more
> documents in the index, the more memory gets consumed per query. Since high
> cardinality and large indexes are the use-cases CollapseQParserPlugin was
> designed for, I thought I'd point this out.
> My real issue is that this does not vary based on the number of results in
> the query, either before or after collapsing, so a query that results in one
> doc consumes the same amount of memory as one that returns all of them. All
> of the Collectors suffer from this to some degree, but I think OrdScore is
> the worst offender.
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