Parallel hint can be used for enforcing direct path read, but there's no hint for serial direct path read.
Serial direct path read is hard to control; it depends on several factors such as;
- FULL SCAN
- segment size and buffer cache size
- the num of blocks cached on buffer cache
Besides, the above logic is not written in manual and different from version to version.
So I would like to propose adding a hint like "FULL_DIRECT_PATH( emp )" to enforce FULL SCAN with direct path read.
This could be not only used to enforce optimizer serial direct path read but also ensuring smart scan on Exadata.