Oracle server is the combination of instance and database.
What is Instance?
Instance is combination of Memory structure (SGA) and (PGA) and Background process.
What is SGA?
SGA is nothing but system global area allocated for database, this memory allocated on virtual memory on server (RAM).
In initial days as per the oracle documentation SGA size specified as half of the RAM size, But it is not true in all conditions, it all depends on the database size, number of sessions accessing database and type of jobs running on database.
PGA is memory allocated for program global area.
What is Background Process?
These are mandatory demon process Running for all database, 5 background process always running for each database to function properly. If any Background session is killed will leads to database crash.
Overall database is like a whole sale shop where all items stored, application session are like customers which request for products. Background process is like shop keeper which serve customer requirement.
What is Instance?
Instance is combination of Memory structure (SGA) and (PGA) and Background process.
What is SGA?
SGA is nothing but system global area allocated for database, this memory allocated on virtual memory on server (RAM).
In initial days as per the oracle documentation SGA size specified as half of the RAM size, But it is not true in all conditions, it all depends on the database size, number of sessions accessing database and type of jobs running on database.
PGA is memory allocated for program global area.
What is Background Process?
These are mandatory demon process Running for all database, 5 background process always running for each database to function properly. If any Background session is killed will leads to database crash.
Overall database is like a whole sale shop where all items stored, application session are like customers which request for products. Background process is like shop keeper which serve customer requirement.
Over all Oracle database Architecture as found below.
