Originally posted by marist89
Some parameters can be changed while the database is up, some can not. For a list of the parameters that can be changed:
select name from v$parameter
where issys_modifiable = 'IMMEDIATE'

Otherwise, you will have to bounce your database for the parameter to become effective.
The above query should be changed to:

select name from v$parameter
where issys_modifiable in ('IMMEDIATE', 'DEFERRED')

Changes to parameters with value of 'IMMEDIATE' are visible immediately to all sessions. Changes to parameters with value of 'DEFERRED' are vissible to future sessions, meaning the currently opened sessions will not see the changes until they reconnect.