Let's take a look at the example:
Cayenne model: Team and User entities, many-to-many relationship between them.
Data: There are 3 teams and a user, who is linked to all these teams.
We're trying to fetch Teams and their related Users (via the prefetching) by using any of disjoint semantics (perhaps it doesn't matter what semantic is being used in this case).
Thus Cayenne engine performs 2 queries:
1. To fetch Team records;
2. To fetch User records:
SELECT t0.*, t2.id FROM user t0
JOIN user_has_team t1 ON (t0.id = t1.user_id)
JOIN team t2 ON (t1.team_id = t2.id);
3 records are resulted from the 2nd query execution. The records are differs by "t2.id" column (it's a Team id), so they are not distinct.
And as a result DistinctResultIterator should take this fact into account during processing the results.