How to access PostgreSQL Functions from Oracle through DBLink?
Have you tried creating a view in PostgreSQL and then query the view from Oracle? You need PostgreSQL to present the function like a view since Oracle doesn't really have that function.
I don't currently support PostgreSQL. However, if you create a view in PostgreSQL and test it to verify that it works, you should be able to select from that view in Oracle. It looks like the issue is selecting from a function, not selecting from PostgreSQL.