Oracle Database Architecture

Understanding Oracle Database architecture is essential for effective database administration, performance tuning, and troubleshooting. Oracle separates the instance (memory + processes) from the database (physical files), and this distinction is central to how everything works.

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Instance vs Database

Component	What It Is
Instance	Memory structures (SGA) + background processes — exists in RAM and CPU
Database	Physical files on disk — datafiles, redo logs, control files, archived logs

An instance can only open one database at a time (in traditional architecture). Multiple instances can open the same database simultaneously using Real Application Clusters (RAC).

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System Global Area (SGA)

The SGA is a shared memory region that all server and background processes access. Key components:

Database Buffer Cache

• Caches data blocks read from disk
• Uses an LRU (Least Recently Used) algorithm to manage memory
• Larger buffer cache = fewer physical disk reads = better performance

Shared Pool

• Library Cache — stores parsed SQL statements and execution plans
• Data Dictionary Cache — caches metadata (table definitions, user privileges)
• Reduces the cost of re-parsing identical SQL statements