Types Of System Variables

mysqld maintains two kinds of system variables.
- Global variables, that affect the overall operation of the server.
- Session variables, that affect its operation for individual client connections.

System variable values can be set globally at server startup by using options on the command line or in an option file.
- A system variable can have both a global value and a session value.

Global and session system variables are related as follows:
- When the server starts, it initializes all global variables to their default values.
- These defaults can be changed by options specified on the command line or in an option file.
- The server also maintains a set of session variables for each client that connects.
- The client's session variables are initialized at connect time using the current values of the corresponding global variables.

The SUPER privilege is required to set global variables.
- To indicate explicitly that a variable is a global variable, precede its name by GLOBAL or @@global.

Setting a session variable requires no special privilege.
- A client can change only its own session variables.
  - It not change those of any other client.
- To indicate explicitly that a variable is a global variable, precede its name by SESSION or @@session.
- LOCAL and @@local. are synonyms for SESSION and @@session.