What will never be included and why it cannot be done:
Oracle version 7.0 on AIX 3 uses OS-supported locks in shared memory that allow an application designer to link an application in a "single tasking" way. Instead of starting one client process per application process, both the database part and the application part run in the same process. In later versions of Oracle this is no longer supported.
This would require a total redesign of the PostgreSQL access model and the performance gain does not justify the effort.
Read the upstream documentation. For corrections, first check the current manual.