Join index structure

Hello,

Does anyone know the Join index structure used in the database ? (implementation) and how it is being utilized by the optimizer ? I couldn't find any documentation / papers out there other than a single sentence mentioned at the end of this page, which is not much.

https://www.monetdb.org/Documentation/Manuals/MonetDB/Optimizers/Landscape

"*Foreign Key replacements Goal: to improve multi-attribute joins over foreign key constraints Rationale: the code produced by the SQL frontend involves foreign key constraints, which provides many opportunities for speedy code using a join index. Impact: large Status: Implemented in the SQL strategic optimizer.*"

Thanks

Joseph