From the post:
Recently query time joining has been added to the Lucene join module in the Lucene svn trunk. The query time joining will be included in the Lucene 4.0 release and there is a possibility that it will also be included in Lucene 3.6.
Lets say we have articles and comments. With the query time join you can store these entities as separate documents. Each comment and article can be updates without re-indexing large parts of your index. Even better would be to store articles in an article index and comments in a comment index! In both cases a comment would have a field containing the article identifier.
Joins based upon matching terms in different indexes.
Work is not finished yet so now would be the time to contribute your experiences or opinions.