Postgresql join removal
There is an example about this feature in the PostgreSQL "Whats new in 9.0". If the optimizier detects, that a join can not add any additional rows, it will be dropped. The PostgreSQL Feature does not remove joins from views if they are not needed because the requested columns are not selected. 

Also, the feature is limited on LEFT JOINs right now ( version 9.4 ). This may change in future versions.