MySQL :: MySQL 8.4 Reference Manual :: 7.1.8 Server System Variables

The MySQL server maintains many system variables that affect its operation. Most system variables can be set at server startup using options on the command line or in an option file. Most of them can be changed dynamically at runtime using the SET statement, which enables you to modify operation of the server without having to stop and restart it. Some variables are read-only, and their values are determined by the system environment, by how MySQL is installed on the system, or possibly by the options used to compile MySQL. Most system variables have a default value, but there are exceptions, including read-only variables. You can also use system variable values in expressions.