Opening a mounted database makes it available for normal database operations. Any
valid user can connect to an open database and access its information. Usually, a
database administrator opens the database to make it available for general use.

When you open the database, Oracle opens the online datafiles and redo log files. If a
tablespace was offline when the database was previously shut down, the tablespace
and its corresponding datafiles will still be offline when you reopen the database.

If any of the datafiles or redo log files are not present when you attempt to open the
database, then Oracle returns an error. You must perform recovery on a backup of any
damaged or missing files before you can open the database.

打开数据库时发生了什么
1. 打开数据库时,Oracle 将打开所有联机的数据文件及重做日志文件
2. 在数据库之前offline的表空间,在数据库重新打开之后该表空间及他关联的数据文件依然为offline状态
3. 如果 Oracle 在打开数据库时找不到所需的数据文件或重做日志文件,系统将返回错误信息.
管理员必须从备份中恢复损坏或缺失的文件后,才能打开数据库.