mysql 动态变量，可以在线修改

Many server system variables are dynamic and can be set at runtime. See Section 15.7.6.1, “SET Syntax for Variable Assignment”. For a description of the privilege requirements for setting system variables, see Section 7.1.9.1, “System Variable Privileges”

The following table lists all dynamic system variables applicable within mysqld.

The table lists each variable's data type and scope. The last column indicates whether the scope for each variable is Global, Session, or both. Please see the corresponding item descriptions for details on setting and using the variables. Where appropriate, direct links to further information about the items are provided.

Variables that have a type of “string” take a string value. Variables that have a type of “numeric” take a numeric value. Variables that have a type of “boolean” can be set to 0, 1, ON or OFF. Variables that are marked as “enumeration” normally should be set to one of the available values for the variable, but can also be set to the number that corresponds to the desired enumeration value. For enumerated system variables, the first enumeration value corresponds to 0. This differs from the ENUM data type used for table columns, for which the first enumeration value corresponds to 1.

Table 7.5 Dynamic System Variable Summary