check hidden parameter in oracle

there are many hidden parameter in oracle database.

below  query can be used to check the value of those hidden paramters.

select a.ksppinm name,b.ksppstvl value,b.ksppstdf deflt,
decode(a.ksppity, 1,'boolean', 2,'string', 3,'number', 4,'file', a.ksppity) type,a.ksppdesc description
from sys.x$ksppi a, sys.x$ksppcv b
where a.indx = b.indx
and a.ksppinm like '_%'
order by name;

reset hidden paramter value in a session:
alter session set "_gby_hash_aggregation_enabled"= FALSE;

reference:
http://asanga-pradeep.blogspot.com/2010/08/checking-hidden-parameters.html