Oracle Database

Here’s an overview of the internal architecture of an Oracle Database:

1. Database and Instance

• Database: The database consists of physical files such as data files, control files, and redo log files. These files store the actual data, metadata, and transaction logs.
• Instance: An instance is a combination of memory structures and background processes that are part of a running Oracle Database. Each instance can access only one database at a time.

2. Memory Structures

• System Global Area (SGA): The SGA is a shared memory area that contains data and control information for one Oracle instance. It includes components like the database buffer cache, redo log buffer, shared pool, large pool, and Java pool.
• Program Global Area (PGA): The PGA is a private memory area for each server process and contains session-specific data such as SQL work areas and session memory.

3. Storage Structures

• Data Files: These files contain the actual data, such as tables and indexes.
• Control Files: These files contain metadata about the database, including the database name and the locations of data files.
• Online Redo Log Files: These files record all changes made to the data, ensuring data recovery in case of a system failure.
• Archived Redo Log Files: These are copies of online redo log files that are used for data recovery and backup purposes.

4. Data Retrieval Processes

• Parsing: When a SQL statement is issued, it is parsed to check for syntax errors and to create an execution plan.
• Execution Plan: The optimizer generates an execution plan for the SQL statement, which determines the most efficient way to access the data.
• Buffer Cache: Data blocks are read into the buffer cache from disk to minimize I/O operations.
• I/O Operations: Data is read from or written to disk as needed during query execution.
• Sorting and Processing: If necessary, data is sorted and processed in memory before being returned to the user.

5. Background Processes

• Client Processes: These processes handle client requests and communicate with the server processes.
• Server Processes: These processes handle the execution of SQL statements and manage client connections.
• Background Processes: These include processes like PMON (Process Monitor), SMON (System Monitor), and RECO (Recovery), which perform various maintenance and monitoring tasks.

This architecture ensures high performance, reliability, and scalability for Oracle Databases.