If collections in the FROM clause are not related to each other, you can use the WHERE clause to join them.
The statement below returns all the persons from the /Persons region with the same name as a flower in the /Flowers region.
SELECT DISTINCT p FROM /Persons p, /Flowers f WHERE p.name = f.name
Indexes are supported for region joins. To create indexes for region joins, you create single-region indexes for both sides of the join condition. These are used during query execution for the join condition.