SGA（System Global Area）的定义：
The SGA is a read/write memory area that, along with the Oracle background processes, form a database instance. All server processes that execute on behalf of users can read information in the instance SGA. Several processes write to the SGA during database operation.
Note: The server and background processes do not reside within the SGA, but exist in a separate memory space.
Each database instance has its own SGA. Oracle Database automatically allocates memory for an SGA at instance startup and reclaims the memory at instance shutdown.
✓ Shared pool ✓ Database buffer cache ✓ Redo log buffer ✓ Large pool ✓ Java pool ✓ Streams pool