PostgreSQL can do all of the same query optimizations. The main difference is that for primary key scans PostgreSQL will have an additional layer of indirection meaning more work needs to be done when scanning on primary key and potentially more disk seeks.

On the flip side PostgreSQL's approach is good when you query secondary indexes since these can point directly to the heap rather than to a primary key, removing the cost of long primary keys and allowing for scanning the heap in physical order after some kinds of secondary key lookups. It is also cheaper to sequentially scan heap compared to sequentially scanning a clustered index.

There are also some difference s on the write side too, but the gist of it is that both models have their own strengths and weaknesses.