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Alter system in Oracle

Alter system archive log [start|stop|all|...]

alter system archive log all;
alter system archive log next;
alter system archive log sequence 104;
alter system archive log current;
alter system archive log current noswitch;
The following command can be used to stop arch. 
alter system archive log stop
Similarly, arch is started with 
alter system archive log start
However, changing the archiver this way doesn't last when the database is restarted. When the database is started, it consults log_archive_start in the initialization file to determine if arch is started.

Alter system archive log all

This command is used to manually archive redo logs.

alter system disconnect session

alter system kill session

alter system kill session 'session-id,session-serial'
This command kills a session. The session-id and session-serial parameters are found in the v$session view (columns sid and serial#.

alter system checkpoint

Performs a checkpoint 
alter system checkpoint

alter system dump datafile

This command can be used in order to dump one ore more blocks of a datafile. The following command dumps blocks 50 through 55 of file 5. Which file 5 is can be found out with v$datafile 
alter system dump datafile 5 block min 50 block max 55;
Note: trace files are only readable by the Oracle account. If you want to change this, set the undocumented initialization parameter _trace_files_public to true. Doing so will, however, cause a big security risk.

alter system flush buffer_cache

alter system flush buffer_cache;
This command is not available prior to 10g. It flushes the buffer cache in the SGA.
9i had an undocumented command to flush the buffer cache: 
alter session set events = 'immediate trace name flush_cache';

alter system flush shared_pool

alter system flush shared_pool;
This command flushed the shared pool.

alter system quiesce restricted

alter system suspend|resume

alter system switch logfile

Causes a redo log switch. 
alter system switch logfile;
If the database is in archive log mode, but the ARCH process hasn't been startedm, the command might hang, because it waits for the archiving of the 'next' online redo log.

alter system register

Forces the registration of database information with the listener. 
alter system register

Alter system set timed_statistics

Setting timed_statistics=true might be usefule when using tk prof.

Alter system set sql_trace

Setting sql_trace=true is a prerequisite when using tk prof.

Alter system set .... deferred

Alter system can be used to change initialization parameters on system level. However, some parameters, when changed with alter system don't affect sessions that are already opened at the time when the statement is executet; it only affects sessions started later. These parameters must be changed with alter system set <initialization parameter> DEFERRED, otherwise a ORA-02096: specified initialization parameter is not modifiable with this option error is returned.
These parameters can be identified as they have a DEFERRED in the isses_modifiable column of v$parameter.

Alter system reset <parameter_name>

Resets a parameter. 
alter system reset some_param scope=both sid='*';

scope

scope=memory

Changes the parameter's value for the running instance only. As soon as the instance is stopped and started, this change will be lost.

scope=spfile

Alters an initialization parameter in the spfile

scope=both

Alters an initialization parameter in the spfile as well as in the running instance.

Thanks

Thanks to Matt Elliot and Jamey Johnston for correcting errors on this page.