One of the most common jobs of the database administrator is to startup or shutdown the Oracle database. Typically we hope that database shutdowns will be infrequent for a number of reasons:

* Inconvenience to the user community.

* Anytime you cycle the database, there is a risk that it will not restart.

* Starting your Oracle database flushes the Oracle memory areas, such as the database buffer cache.

Performance on a restarted database will generally be slow until the database memory areas are ?warmed? up.

Why would you shutdown your database? Some reasons include database maintenance:

* Applying a patch or an upgrade.

* Allow for certain types of application maintenance.

* Performing a cold (offline) backup of your database. (We recommend hot backups that allow you to avoid shutting down your database)

* An existing bug in your Oracle software requires you to restart the database on a regular basis.

When the time comes to ?bounce? the database (using the shutdown and startup commands), you will use SQL*Plus to issue these commands.   Let?s look at each of these commands in more detail.

The Oracle Startup Command

You start the Oracle database with the startup command. You must first be logged into an account that has sysdba or sysoper privileges such as the SYS account (we discussed connecting as SYSDBA earlier in this book). Here then is an example of a DBA connecting to his database and starting the instance:

In this example from a Windows XP server, we set the ORACLE_SID to the name of the database and we log into SQL*Plus using the ?sys as sysdba? login. This gives us the privileges we need to be able to startup the database. Finally, after we enter our password, we issue the startup command to startup the database. Oracle displays its progress as it opens the database, and then returns us to the SQL*Plus prompt once the startup has been completed.

When Oracle is trying to open your database, it goes through three distinct stages, and each of these is listed in the startup output listed previously. These stages are:

* Startup (nomount)

* Mount

* Open

Let?s look at these stages in a bit more detail.

The Startup (nomount) Stage

When you issue the startup command, the first thing the database will do is enter the nomount stage. During the nomount stage, Oracle first opens and reads the initialization parameter file (init.ora) to see how the database is configured. For example, the sizes of all of the memory areas in Oracle are defined within the parameter file.

After the parameter file is accessed, the memory areas associated with the database instance are allocated. Also, during the nomount stage, the Oracle background processes are started. Together, we call these processes and the associated allocated memory the Oracle instance. Once the instance has started successfully, the database is considered to be in the nomount stage. If you issue the startup command, then Oracle will automatically move onto the next stage of the startup, the mount stage.

Starting the Oracle Instance (Nomount Stage)

There are some types of Oracle recovery operations that require the database to be in nomount stage. When this is the case, you need to issue a special startup command: startup nomount, as seen in this example:

SQL> startup nomount

The Mount Stage

When the startup command enters the mount stage, it opens and reads the control file. The control file is a binary file that tracks important database information, such as the location of the database datafiles.

In the mount stage, Oracle determines the location of the datafiles, but does not yet open them. Once the datafile locations have been identified, the database is ready to be opened.

Mounting the Database

Some forms of recovery require that the database be opened in mount stage. To put the database in mount stage, use the startup mount command as seen here:

SQL> startup mount

If you have already started the database instance with the startup nomount command, you might change it from the nomount to mount startup stage using the alter database command:

SQL> alter database mount;

The Open Oracle startup Stage

The last startup step for an Oracle database is the open stage. When Oracle opens the database, it accesses all of the datafiles associated with the database. Once it has accessed the database datafiles, Oracle makes sure that all of the database datafiles are consistent.

Opening the Oracle Database

To open the database, you can just use the startup command as seen in this example

SQL> startup

If the database is mounted, you can open it with the alter database open command as seen in this example:

SQL> alter database open;

Opening the Database in Restricted Mode

You can also start the database in restricted mode. Restricted mode will only allow users with special privileges (we will discuss user privileges in a later chapter) to access the database (typically DBA?s), even though the database is technically open. We use the startup restrict command to open the database in restricted mode as seen in this example.

SQL> startup restrict

You can take the database in and out of restricted mode with the alter database command as seen in this example:

-- Put the database in restricted session mode.

SQL> alter system enable restricted session;

-- Take the database out of restricted session mode.

SQL> alter system disable restricted session;

Note:  Any users connected to the Oracle instance when going into restricted mode will remain connected; they must be manually disconnected from the database by exiting gracefully or by the DBA with the ?alter system kill session? command.

Problems during Oracle Startup

The typical DBA life is like that of an airline pilot, ?Long moments of boredom followed by small moments of sheer terror?, and one place for sheer terror is an error during a database startup.

The most typical reason for a database not starting up is a prior database crash, a data corruption, disk failure or some other catastrophic event from which the database cannot recover. In these cases, you have to go into database recovery mode to start your instance. There is a chapter on recovery later in this book and we will discuss what to do when Oracle will not startup.

This is an excerpt from the bestselling "Easy Oracle Jumpstart" by Robert Freeman and Steve Karam (Oracle ACE and Oracle Certified Master).  It?s only $19.95 when you buy it directly from the publisher here.

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