Of collapsing in Solr by Paul Masurel.
From the post:
This post is about the innerworkings of one of the two most popular open source search engines : Solr. I noticed that many questions (one or two everyday) on solr-user’s mailing list were about Solr’s collapsing functionality.
I thought it would be a good idea to explain how Solr’s collapsing is working. Because its documentation is very sparse, and because a search engine is the kind of car you to take a peek under the hood to make sure you’ll drive it right.
The Solr documentation at Apache refers to field collapsing and result grouping being “different ways to think about the same Solr feature.”
I read the post along with the Solr documentation.
BTW, note from “Known Limitations” in the Solr documentation:
Support for grouping on a multi-valued field has not yet been implemented.
That would be really nice with subjectIdentifier and subjectLocator having the potential to be sets of values.