4.3.4. Using Options to Set Program Variables
Many MySQL programs have internal variables that can be set at
runtime. 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 16MB, use either of the
following commands:
shell> mysql --max_allowed_packet=16777216
shell> mysql --max_allowed_packet=16M
The first command specifies the value in bytes. The second
specifies the value in megabytes. For variables that take a
numeric value, the value can be given with a suffix of
K, M, or
G (either uppercase or lowercase) to indicate
a multiplier of 1024, 10242 or
10243. (For example, when used to set
max_allowed_packet, the suffixes indicate
units of kilobytes, megabytes, or gigabygtes.)
In an option file, variable settings are given without the
leading dashes:
[mysql]
max_allowed_packet=16777216
Or:
[mysql]
max_allowed_packet=16M
If you like, underscores in a variable name can be specified as
dashes. The following option groups are equivalent. Both set the
size of the server's key buffer to 512MB:
[mysqld]
key_buffer_size=512M
[mysqld]
key-buffer-size=512M
Note: Before MySQL 4.0.2, the
only syntax for setting program variables was
--set-variable=option=value
(or
set-variable=option=value
in option files). This syntax still is recognized, but is
deprecated as of MySQL 4.0.2.
Many server system variables can also be set at runtime. For
details, see Section 5.2.3.2, “Dynamic System Variables”.
