Oracle has three shutdown modes: normal, immediate, and abort. A normal shutdown waits for all sessions to complete before shutting down, which can take hours. An immediate shutdown prevents new logins, rolls back uncommitted transactions, and shuts down. An abort shutdown forcibly terminates all sessions and immediately shuts down, providing the fastest shutdown but potentially leaving transactions needing rollback.

1. Oracle shutdown
By Burleson Consulting
During a normal shutdown, it's my understanding that Oracle will close all sessions (see modes
below), close the database, un-mount the data files and then shut down the instance in two steps,
first issuing a "free" the SGA RAM heap and finally, terminating the background processes.
Oracle has three shutdown modes:
·
Normal (default) - waits for in-flight work to complete. This could take houes.
·
Immediate - terminates all sessions and does a rollback on all
uncommitted transactions.
·
Abort - aborts all sessions, leaving current DML in need of rollback, de-allocates the
SGA and terminates the background processes. Note that a shutdown abort is NOT evil
or dangerous. The "abort" simply means that Oracle will terminate all active work,
which is what most people want during a shutdown!
The "normal" and "immediate" modes can take a long time in you have in-flight transactions,
and many Oracle DBA's ensure a swift clean shutdown this way, aborting the sessions, restarting to allow warmstart rollback of the aborted transactions, and a shutdown immediate to
close cleanly:
SQL> shutdown abort
SQL> startup
SQL> shutdown immediate
Normal Shutdown
A normal shutdown of an Oracle database is actually rarely used. This is because the normal
shutdown waits for everyone to complete their work and then logoff in an orderly fashion. When
a normal shutdown occurs, the database is closed in a normal manner, and all changes made in
the database are flushed to the database datafiles . This is known as a “clean shutdown.”
Most of the time this is not practical… there always seems to be someone who has left for
vacation and who forgot to log out, or there are times that Oracle processes become “zombied”
(this is where Oracle thinks someone is connected to the database but they really are not). In
these cases, the database will never come down.

2. It will simply wait forever until you manually kill those sessions. Because of this, we often
recommend the shutdown immediate or shutdown abort commands, which we will discuss in the
next sections. Here is an example of the use of the normal shutdown command.
SQL> shutdown
When you execute a shutdown , Oracle will flush all the changes in memory out to the database
datafiles . This makes database startup quicker because the database is in a consistent state.
Think of it this way: if you jump into the air and land on your feet, you have landed in a way that
prepares you to make another jump. If, instead, you jump and land on your back, you are in no
position to make another jump; instead, you must perform a recovery by taking the actions
required to stand again. A clean shutdown is one that is prepared to come back up without
delay. A dirty shutdown is one that lands on its back; it can not come back up without first
recovering itself.
Shutdown Immediate
Perhaps the best way to initially shutdown the database is the shutdown immediate command.
This command will prevent any new logins, then rollback any uncommitted transactions, and
then bring down the database. In the process of bringing down the database, Oracle will flush all
the changes in memory out to the database datafiles too, just like a regular shutdown does. This
makes database startup quicker. Here is an example of shutting down a database with the
shutdown immediate command:
SQL> shutdown immediate
The shutdown immediate command will work most of the time, but there are times when it can
hang and fail to shutdown the database. In these cases, the shutdown abort command is called
for.
Shutdown Abort
The shutdown abort command is pretty much a guaranteed way to get your database to
shutdown. It’s a “hard crash” of the database, and this can result in a longer time to start the
database back up. Still, you can’t really hurt the database using the shutdown abort command,
and during your DBA years you will find more than a few occasions to use the shutdown abort
command.
A shutdown abort can be your first shutdown method of choice, since there may be times when
you must force the database down. Here is an example using the shutdown abort command:
SQL> shutdown abort