Define all system variables in go type.

[SunRunAway]

Feature Request

Is your feature request related to a problem? Please describe:

For now, all system variables are defined in a map with type string.

	// systems variables, don't modify it directly, use GetSystemVar/SetSystemVar method.
	systems map[string]string

If we need a system variable in which the real type is an integer, for example, we must do string conversation. It's hard to code and bad to performance.

	valStr, _ := ctx.GetSessionVars().GetSystemVar(variable.MaxAllowedPacket)
	maxAllowedPacket, err := strconv.ParseUint(valStr, 10, 64)
	if err != nil {
		return nil, err
	}

Describe the feature you'd like:

We need to define all system variables with their real type in go code. So that all of them can be called directly without performance loss.

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy:

All the system variables defined in MySQL are located in https://github.com/mysql/mysql-server/blob/8.0/sql/system_variables.h#L174

[kilgaloon]

I guess this can be mapped to interface{} then type asserted later for usage?

[ngaut]

It's not a good idea, we should set system variables explicitly.

[ghost]

This is required for fixing compatibility issues. i.e. When you run SHOW VARIABLES LIKE 'x' in MySQL you should get a string type, but SELECT @@x should return the actual type. I am tracking a request for typed variables in #15131 , so I am going to close this issue to reduce duplication. Thanks!