Concepts and functions of Oracle SGA

The SGA (System Global Area) of Oracle database is a key component in Oracle database. It is responsible for managing the runtime memory of the database instance. SGA stores the data and code required when the database instance is running, including buffers, shared pools, Large Pools, Java pools, etc. Because the data contained in the SGA is shared by all users and sessions, it is considered the "global data area" within the database instance.

In the Oracle database, the role of SGA mainly includes four aspects:

1. Improve access performance. SGA stores a large amount of data, such as data cache, SQL block cache and shared pool, which can help Oracle process database access requests more efficiently.
2. Improve data consistency. Data stored in SGA is shared by all database sessions, so all sessions can process the same data set. This significantly improves data consistency and reliability.
3. Improve database security. SGA can filter illegal access requests and provide appropriate protection and control of data.
4. Improve the speed of query and analysis. SGA can preprocess a large number of requests, which can significantly reduce response time during query and analysis.

How to modify Oracle SGA

The size of Oracle SGA and the proportion of its components have a key impact on database performance. The size of SGAs is controlled by parameters such as DB_CACHE_SIZE, SHARED_POOL_SIZE, LARGE_POOL_SIZE, JAVA_POOL_SIZE and STREAMS_POOL_SIZE. In order to optimize database performance to the greatest extent, adjust the size of SGA and set parameters to achieve better control of database performance goals.

1. Determine the maximum value of the SGA size that can be modified
Before modifying the SGA, first determine the maximum value of the SGA size that can be adjusted. This value is determined by the size of the operating system's memory. If the size of the SGA exceeds the capacity of available memory, it will cause operating system problems and the Oracle instance will not start, so the size of the SGA must be set very carefully.

For example, you can determine the upper limit of the memory size supported by the system through the following query:

SELECT * FROM V$SGA_DYNAMIC_COMPONENTS WHERE MEMORY_MAX_TARGET > 0;

2. Adjust the SGA size as needed
By modifying the parameters of the SGA size, you can adjust the size of the SGA size. The following is how to adjust the SGA size:

a. Query the current SGA size
Enter the following command in SQL*Plus to display the current SGA size:

SHOW SGA;

b. Update the size parameters of SGA
The SGA size can be updated in the following ways:

ALTER SYSTEM SET SGA_MAX_SIZE=100M SCOPE=SPFILE;

c. Restart the Oracle instance
Restart the Oracle instance for the updated SGA parameters to take effect:

SHUTDOWN IMMEDIATE;
STARTUP;

Summary

In the Oracle database, SGA, as a global data area, is an important component to improve database performance and reliability. Through the correct configuration and adjustment of SGA, the performance and reliability of Oracle database can be significantly improved. However, you must be very careful when adjusting the SGA, because an SGA that is too large or too small may cause system crashes or poor database performance. Therefore, you must have an in-depth understanding of SGA before operating it.

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