Jason Gerlowski created SOLR-15053:
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Summary: 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
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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