无法在 MySQL 上设置全局变量

Question

I'm using MySQL in localhost (in ubuntu and also in windows). I want to set a global variable, and I have tried in all ways but even though I get an "ok" message from mysql, then when I do the "select @var" it allways says "NULL". I've tried:

set global var=17;
set @global.var=17;
set @@var=17;

Can anyone help me?

Thanks in advance.

ps: I have the SUPER privilege.

Answer 1

The variable name var does not reference a valid system variable. The GLOBAL and SESSION keywords in the SET statement are used for specifying the scope when setting MySQL system variables, not MySQL user variables.

Try for example:

SELECT @@global.net_read_timeout ;

SET GLOBAL net_read_timeout = 45 ;

SELECT @@global.net_read_timeout ;

---

http://dev.mysql.com/doc/refman/8.0/en/set-statement.html

http://dev.mysql.com/doc/refman/5.5/en/set-statement.html

Answer 2

According to the MySQL 5.0 Reference Manual :

User-defined variables are session-specific. That is, a user variable defined by one client cannot be seen or used by other clients. All variables for a given client session are automatically freed when that client exits.

You could consider using an extension like MySQL Global User Variables UDF (old archived link) to use global (persistent shared) variables.

Answer 3

On MySQL , you cannot create custom global or session system variables but can change existed global or session system variables as shown below:

SET GLOBAL max_connections = 1000;    -- Existed global system variable
SET SESSION sql_mode = 'TRADITIONAL'; -- Existed session system variable

And, you can create user-defined(custom) variables which are removed when you exit(log out) a session as shown below:

SET @first_name = 'John', @last_name = 'Smith';

User-defined variables are session specific. A user variable defined by one client cannot be seen or used by other clients. (Exception: A user with access to the Performance Schema user_variables_by_thread table can see all user variables for all sessions.) All variables for a given client session are automatically freed when that client exits.

User variable names are not case-sensitive. Names have a maximum length of 64 characters.