last_query does not work for me with Oracle

How can I print the last query using CodeIgniter 3 and Oracle?


With same code, but with MySql, I write 

PHP Code:

print_r($this->db->last_query()); 


and I get successfully the print of the last query...


But if I move to Oracle db, It doesn't works for me...  no query are printed.

PS: I have grant on oracle V$SQL view.

Thank you