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An Oracle database stores and retrieves data. Each database consists of one or more data
files. An Oracle database server consists of an Oracle database and an Oracle instance.
Every time a server is started, a shared memory region called the system global area (SGA) is
allocated and the Oracle background processes are started. The combination of the
background processes and SGA is called an Oracle instance. On some operating systems, like
Windows, there are no separate background processes. Instead threads run within the Oracle
image.