When Oracle starts an instance, it reads the server parameter file (SPFILE) or
initialization parameter file to determine the values of initialization parameters. Then,
it allocates an SGA, which is a shared area of memory used for database information,
and creates background processes. At this point, no database is associated with these
memory structures and processes.

实例是如何启动的
当 Oracle 启动实例时，首先需要从服务器参数文件或初始化参数文件中读取初始化参数，
之后创建存储数据库信息的共享内存区SGA，和后台进程 .