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with Oracle. This section discusses the many ways that you can run Oracle. Some of these methods will be important to administrators, while others will allow for full use. This feature is important when you need to perform both critical and noncritical activities, and not interfere with your users or your data.
Oracle has several modes of operation. In most cases, when you start Oracle, you will simply issue the command:
> Startup;
This command actually takes Oracle through three distinct startup phases automatically, or you could also choose to explicitly step through these phases:
1. In the nomount phase, the database reads the spfile or the init.ora parameter file and starts up the Oracle memory structures as well as the background processes. The instance is started, but the database is not yet associated with the newly started instance. This is usually used in cases where you need to re-create the controlfile. The command to perform this is
> startup nomount;
2. In order to associate a database with the instance, the instance ''mounts" the database. This is done in the mount phase. The previously read parameter file is used to find those controlfiles, which contain the name of the data files and redo logs. The database is then mounted to allow some maintenance activities to be performed. Datafiles and redo logs are not opened when the database is in mount mode, so the database is not yet accessible by end users for normal tasks. Commands to mount a database are
> startup mount;
> alter database mount;
3. When Oracle opens the database in the open phase, it opens the data files and redo logs, making the database available for normal operations. Your redo logs must exist in order for the database to open. If they do not, the resetlogs command must be used to create new redo logs in the location specified in the control files.
> Startup {open} {resetlogs};
> alter database open;
There are some other options for opening a database. For example, you may want to open it in Read-Only mode so that no database changes (inserts, updates, or deletes) can be performed. There are also the upgrade/downgrade options that allow a database to be opened to perform a downgrade or upgrade to another version of Oracle.
> alter database open read only;
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