MySQL Reference Manual for version 4.0.18.

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4.3.4 Using Options to Set Program Variables

Many MySQL programs have internal variables that can be set at runtime. As of MySQL 4.0.2, program variables are set the same way as any other long option that takes a value. For example, mysql has a max_allowed_packet variable that controls the maximum size of its communication buffer. To set the max_allowed_packet variable for mysql to a value of 64 MB, use either of the following commands:

 
shell> mysql --max_allowed_packet=6710740
shell> mysql --max_allowed_packet=64M

The first command specifies the value in bytes. The second specifies the value in megabytes. Variable values can have a suffix of K, M, or G (either uppercase or lowercase) to indicate units of kilobytes, megabytes, or gigabytes.

In an option file, the variable setting is given without the leading dashes:

 
[mysql]
max_allowed_packet=6710740

Or:

 
[mysql]
max_allowed_packet=64M

If you like, underscores in a variable name can be specified as dashes.

Prior to MySQL 4.0.2, program variable names are not recognized as option names. Instead, use the --set-variable option to assign a value to a variable:

 
shell> mysql --set-variable=max_allowed_packet=6710740
shell> mysql --set-variable=max_allowed_packet=64M

In an option file, omit the leading dashes:

 
[mysql]
set-variable = max_allowed_packet=6710740

Or:

 
[mysql]
set-variable = max_allowed_packet=64M

With --set-variable, underscores in variable names may not be given as dashes for versions of MySQL older than 4.0.2.

The --set-variable option is still recognized in MySQL 4.0.2 and up, but is deprecated.



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