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Table of Contents
How to view the current global system variable?
Which variables have a greater impact on performance?
How to modify global variables?
Some easily overlooked but important details
Home Database Mysql Tutorial Understanding MySQL Server Global System Variables

Understanding MySQL Server Global System Variables

Jul 30, 2025 am 03:38 AM
mysql system variables

To view MySQL global system variables, you can use SHOW GLOBAL VARIABLES; or filter by name such as SHOW GLOBAL VARIABLES LIKE 'max_connections'; understand key performance variables such as innodb_buffer_pool_size (recommended to be set to 50%~80% physical memory), query_cache_type (only for version 5.x), max_connections (control the maximum number of connections), and wait_timeout (control the connection timeout); modifying variables can be permanently configured and restarted in my.cnf/my.ini; also pay attention to the scope of variables, version differences, and dependencies between parameters.

Understanding MySQL Server Global System Variables

The global system variables of MySQL server control the behavior of the database, and understanding them helps optimize performance, troubleshoot problems, and properly configure the server. These variables can be set through configuration files at startup or can be adjusted dynamically at runtime (some variables support). Here are some key points and practical suggestions that you may be concerned about.

Understanding MySQL Server Global System Variables

How to view the current global system variable?

You can use the following SQL command to view all or specific global variables:

 SHOW GLOBAL VARIABLES;

Or find a specific variable, such as max_connections :

Understanding MySQL Server Global System Variables
 SHOW GLOBAL VARIABLES LIKE 'max_connections';

This method is suitable for daily checking whether the configuration is effective, and is also often used to troubleshoot common problems such as connection number and timeout time.


Which variables have a greater impact on performance?

Some variables directly affect the performance of MySQL. Here are a few common key variables:

Understanding MySQL Server Global System Variables
  • innodb_buffer_pool_size
    This is the memory area used by the InnoDB storage engine to cache data and indexes. If this value is too small, it will cause frequent disk access; if it is too large, it may waste memory resources. It is usually recommended to set it to 50%~80% of physical memory (adjusted according to load conditions).

  • query_cache_type and query_cache_size
    Although MySQL 8.0 has removed the query cache, it is still available in version 5.x. If you are using an old version, be careful whether it really brings benefits after opening it, because in scenarios where more writes and less reads may become a bottleneck.

  • max_connections
    Controls the maximum number of connections allowed. If the application often experiences "Too many connections" errors, you can adjust this value appropriately, but you should also pay attention to whether the system resources can support so many connections.

  • wait_timeout and interactive_timeout
    Controls the disconnection time of non-interactive and interactive connections after inactivity, respectively. Setting too long may cause connections to accumulate, and being too short may affect the client experience.


How to modify global variables?

There are two ways to modify: temporary modification and permanent modification.

  • Temporary modification (invalid after restart)
    It can be modified directly through SQL commands, for example:

     SET GLOBAL max_connections = 1000;

    Note: Not all variables support runtime modification, some can only be set in the configuration file.

  • Permanent modification (requires editing of configuration files)
    Modify the corresponding items in my.cnf or my.ini file, for example:

     [mysqld]
    max_connections = 1000
    innodb_buffer_pool_size = 2G

    Then restart the MySQL service to make the configuration take effect.

    Reminder: It is best to test the environmental verification effect before modification to avoid unexpected problems after going online.


    Some easily overlooked but important details

    • Variable scope : Some variables have two scopes: Global (GLOBAL) and Session (SESSION). For example, autocommit can be set separately for the current session.
    • Default value changes : The default values of certain variables may change between different MySQL versions, so you need to pay attention to compatibility when upgrading.
    • Dependencies between variables : For example, innodb_log_file_size and innodb_buffer_pool_size can only be used to achieve performance advantages, and you cannot only look at a single parameter.

    Basically that's all. Mastering these basic knowledge about global system variables can help you better understand and manage MySQL behavior. Although it is not complicated, if the details are ignored, it may also cause a lot of trouble.

    The above is the detailed content of Understanding MySQL Server Global System Variables. For more information, please follow other related articles on the PHP Chinese website!

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