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https://issues.apache.org/jira/browse/SOLR-15053?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17270570#comment-17270570
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David Smiley commented on SOLR-15053:
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The preference is there due to my modifications some time ago. Yes, Collapse &
Expand may not satisfy all use cases but if it does then please use it (IMO).
Another motivation of mine is that I think grouping is a bit of a mess in terms
of code organization / concerns spilling out into QueryComponent and elsewhere.
I wish we didn't have it in the first place, honestly.
> Remove ref-guide preference for collapse over grouping
> ------------------------------------------------------
>
> Key: SOLR-15053
> URL: https://issues.apache.org/jira/browse/SOLR-15053
> Project: Solr
> Issue Type: Task
> Security Level: Public(Default Security Level. Issues are Public)
> Reporter: Jason Gerlowski
> Priority: Major
>
> Currently, the ref-guide states a clear preference for collapse over grouping.
> bq. Generally, you should prefer Collapse & Expand.
> But the reality is more complicated. Collapse grouping has a lot of
> limitations on when it can be used (single shard environments only, with the
> exception that multi-shard environments can be made to work by ensuring that
> each value in the grouping field is colocated within the same shard.
> Further, it's not necessarily more or less performant than traditional
> grouping:
> As Joel Bernstein put it in a recent mailing list thread:
> bq. There is a very specific use case where collapse performs better and in
> these scenarios collapse might be the only option that would work. The use
> case where collapse works better is: (1) High cardinality grouping field,
> like product id, (2) Larger result sets, (3) The need to know the full number
> of groups that match the result set. In grouping this is group.ngroups. At a
> certain point grouping will become too slow under the scenario
> described above. It will all depend on the scale of #1 and #2 above.
> We should correct the ref-guide wording here, as it's misleading for novice's
> and experts alike.
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