Dynamically Modifiable Parameters
While most initialization parameters are static, taking their values from the initialization file as it exists at the time of database startup, some may be dynamically modified while the instance is up and the database is open.
Dynamic modification is different from dynamic storage of a changed value in the SPFILE. In all versions of Oracle, you can dynamically modify a setting using the ALTER SYSTEM or ALTER SESSION command. Starting with Oracle9i, these changes may also be dynamically stored in the SPFILE, depending on the value of the SCOPE clause of the ALTER command.
In the individual parameter descriptions in the following sections, we indicate which parameters may be dynamically modifiable (the Dynamic entry in the parameter description) and how they can be modified (via ALTER SYSTEM, ALTER SESSION, or both).