Join is a relational database concept where data are typically normalized, stored in separate tables for efficiency in storage and maintaining data integrity, and then data are joined together between tables to provide a coherent view of the data. In Lucene, there are no concepts of tables because all the records are supposed to be flattened and stored as documents. Even setting up schema in advance is optional. In a document-based store such as Lucene, joins always seem like an afterthought. However, it doesn't mean that you can't do joins at all in Lucene. There are many techniques to simulate joins such as adding a document type field to identify different types (tables) of records. Then, manually combine data at runtime by issuing multiple search queries retrieving data of different types.

Lucene offers two types of join methods. One is an index-time join where documents are added in blocks; basically, parent record and child record relationships. The other type is...