Oracle initialization or INIT.ORA parameters with an underscore in front are hidden or unsupported parameters. One can get a list of all hidden parameters by executing this query:
SELECT * FROM SYS.x$ksppi WHERE SUBSTR (ksppinm, 1, 1) = '_';
The following query displays parameter names with their current value:
SELECT a.ksppinm "Parameter", b.ksppstvl "Session Value", c.ksppstvl "Instance Value" FROM x$ksppi a, x$ksppcv b, x$ksppsv c WHERE a.indx = b.indx AND a.indx = c.indx AND SUBSTR (ksppinm, 1, 1) = '_' ORDER BY a.ksppinm;
Remember: Thou shall not play with undocumented parameters! Using undocumented parameters without the consent of Oracle Support will make your database “un-supported”. You will be on your own if the parameters you’ve set cause problems or data corruption.