PostgreSQL comes to mind (eg: see the query planner [0]). It is surprisingly easy to read through lots of interesting parts and see how they are working. [0] h

PostgreSQL comes to mind (eg: see the query planner [0]). It is surprisingly easy to read through lots of interesting parts and see how they are working. [0] https://github.com/postgres/postgres/blob/master/src/backend/optimizer/plan/planner.c#L283 https://github.com/postgres/postgres/blob/master/src/backend...