Starting and Altering Database

Starting Up a Database

There are different states in which database can start up. The following scenarios describe different stages of starting up an instance starting with shutdown.







Starting the Instance (NOMOUNT)

An instance is started in the NOMOUNT stage only during database creation or the re-creation
of control files.
Starting an instance includes the following tasks:

• Reading the initialization file from $ORACLE_HOME/dbs in the following order:
        – First spfileSID.ora
        – If not found then, spfile.ora
        – If not found then, initSID.ora
• Allocating the SGA
• Starting the background processes
• Opening the alertSID.log file and the trace files


Specifying the PFILE parameter with STARTUP overrides the default behavior

STARTUP NOMOUNT PFILE = /u01/oracle/dbs/init.ora

Mounting the Database (MOUNT)

To perform specific maintenance operations, you start an instance and mount a database but
do not open the database.

For example, the database must be mounted but not open during the following tasks:
• Renaming data files (data files for an offline tablespace can be renamed when the
database is open)
• Enabling and disabling online redo log file archiving options
• Performing full database recovery
Mounting a database includes the following tasks:
• Associating a database with a previously started instance
• Locating and opening the control files specified in the parameter file
• Reading the control files to obtain the names and status of the data files and online
redo log files. However, no checks are performed to verify the existence of the data
files and online redo log files at this time.



Opening the Database (OPEN)

Normal database operation means that an instance is started and the database is mounted and
open. Opening the database includes the following tasks:

• Opening the online data files
• Opening the online redo log files

If any of the data files or online redo log files are not present when you attempt to open the
database, the Oracle server returns an error. Oracle server verifies that all the data files and online redo log
files can be opened and checks the consistency of the database. If necessary, the SMON
background process initiates instance recovery.



Restricting Access to an Instance at Startup

You can start an instance, and optionally mount and open a database, in restricted mode so that the instance is available only to administrative personnel (not general database users). Use this mode of instance startup when you need to accomplish one of the following tasks:

  • Perform an export or import of data

  • Perform a data load (with SQL*Loader)

  • Temporarily prevent typical users from using data

  • Perform certain migration or upgrade operations



Forcing Instance to Start / Instance with Media Recovery

         You cannot shut down the current instance with the SHUTDOWN NORMAL, SHUTDOWN IMMEDIATE, or SHUTDOWN TRANSACTIONAL commands. Use the STARTUP command with the FORCE clause:


If media recovery is required, start by using the STARTUP command with the RECOVER clause:


Altering Database Availability

You can alter the availability of a database. You may want to do this in order to restrict access for maintenance reasons or to make the database read only. The following sections explain how to alter the availability of a database:


Database can be altered to mount state from nomount state but cannot be altered directly to open state.


To open a mounted database, use the ALTER DATABASE statement with the OPEN clause:



Opening a database in read-only mode enables you to query an open database while eliminating any potential for online data content changes.


You can also open a database in read/write mode as follows which is the default mode:


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