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https://issues.apache.org/jira/browse/SOLR-9125?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15287208#comment-15287208
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Joel Bernstein commented on SOLR-9125:
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Yeah, the CollapsingQParsePlugin can use a lot of memory. The original design
goal was to increase performance for collapsing on high cardinality fields and
large result sets, as opposed to large indexes. It was really designed to
support fast collapse queries on large e-commerce catalogs which are still
typically small compared to other data sets.
If we can find a way to maintain the performance and shrink the memory usage
this would be a great thing.
> 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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