select @@global.extra_port;
@@global.extra_port
0
select @@session.extra_port;
ERROR HY000: Variable 'extra_port' is a GLOBAL variable
show global variables like 'extra_port';
Variable_name	Value
extra_port	0
show session variables like 'extra_port';
Variable_name	Value
extra_port	0
select * from information_schema.global_variables where variable_name='extra_port';
VARIABLE_NAME	VARIABLE_VALUE
EXTRA_PORT	0
select * from information_schema.session_variables where variable_name='extra_port';
VARIABLE_NAME	VARIABLE_VALUE
EXTRA_PORT	0
set global extra_port=1;
ERROR HY000: Variable 'extra_port' is a read only variable
set session extra_port=1;
ERROR HY000: Variable 'extra_port' is a read only variable