System Global Area (SGA)

System Global Area (SGA)

The System Global Area (SGA) is a group of shared memory areas that are dedicated to an Oracle instance. All Oracle processes use the SGA to hold information. The SGA is used to store incoming data and internal control information that is needed by the database. You control the amount of memory to be allocated to the SGA by setting some of the Oracle initialization parameters.

An SGA and Oracle processes constitute an Oracle instance. Oracle automatically allocates memory for an SGA when you start an instance, and the operating system reclaims the memory when you shut down the instance. Each instance has its own SGA. The SGA is read/write. All users connected to a multiple-process database instance can read information contained within the instance's SGA, and several processes write to the SGA during execution of Oracle.

The SGA contains the following data structures:

Database buffer cache
Redo log buffer
Shared pool
Java pool
Large pool
Data dictionary cache

Part of the SGA contains general information about the state of the database and the instance, which the background processes need to access; this is called the fixed SGA.