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What is Program Global Area (PGA)?
Program Global Area (PGA) is a memory structure in Oracle database systems that is allocated by the server process for each connected user session. It stores data and control information specific to that user session. The PGA is private to each session and is used to meet the memory requirements of various database operations performed by the user.
The Purpose of PGA
The PGA is responsible for managing session-specific data and resources in Oracle databases. It plays a crucial role in optimizing the performance and memory usage of individual user sessions.
Here are some key components and functions of Program Global Area:
1. Stack Area: Each session in Oracle has a private stack area within the PGA. This stack is used to manage session-specific variables, such as session state and local variables used in PL/SQL programs.
2. Session memory: PGA holds session-level memory structures, including private SQL areas, result cache, and session cursor information. These components are used to process SQL statements and store execution plans.
3. Sorting and Hashing: When a session performs sorting or hashing operations, PGA provides temporary space for these operations. This temporary space is used to store intermediate results during sort or join operations, ensuring efficient data processing.
4. Data buffers: PGA also includes data buffers that hold data blocks during read and write operations. These buffers help in reducing disk I/O and improving data access performance.
PGA Aggregates
In addition to individual session PGAs, Oracle also maintains PGA aggregates at the system level. PGA aggregates are used to monitor and manage the overall memory usage of user sessions.
Some key aspects of PGA aggregates are:
1. PGA Aggregate Target: Oracle provides a configurable parameter called PGA Aggregate Target, which specifies the overall memory limit for all PGAs combined. This allows administrators to efficiently allocate and manage memory resources based on the system’s needs.
2. Automatic Memory Management: Oracle Database can dynamically distribute and manage memory resources among different components, including PGAs, using Automatic Memory Management feature. This helps optimize the memory usage and performance of the entire database system.
In summary, Program Global Area (PGA) is a dedicated memory area within Oracle databases that manages session-specific data and resources. It plays a crucial role in optimizing performance and memory utilization at the individual session level. By efficiently managing PGA, Oracle ensures a smooth and efficient execution of database operations.
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