What is the difference between grouping and facet in lucene 3.5

Question

I found two plugins in lucene 3.5 contrib folder: one is a grouping, the other is a facet. In my version, both of them were used to separate my documents into different categories. Why does lucene have two plugins for this?

Answer 1

These are two different functions of lucene:

• Grouping was first released with Lucene 3.2, a related problem with jira LUCENE-1421 : it allows you to group search results by the specified field. For example, if you group by the author’s field, then all documents with the same value in the author’s field fall into one group. You will have some kind of tree as a weekend. If you want to use this lucene function more deeply, this blog post should be helpful.
• Faceting was first released with Lucene 3.4, a related problem with jira is LUCENE-3079 : this function does not group documents, it just tells you how many documents fall into a particular face value. For example, if you have a face based on the author’s field, you will receive a list of all your authors, and for each author you will find out how many documents belong to this particular author. After you want to view these documents, you need to ask again to add a specific filter (author = whatever). faceted search is actually based on viewing documents using multiple filters to gradually achieve documents that you are really interested in.